CW
Apr 25, 2022
5.1 Electric fields
What are the key terms to do with electricity
and magnetism.
Define some of these key terms.
Why should we try to avoid using the word
‘electricity’?
Explaining
electrostatics
Experiments also show that positively charged objects
are attracted to negatively charged objects and vice
versa.
The distance between them also affects the force.
+
-
+
+
-
-
Add arrows to show the direction of the forces.
Adjust the arrows length to show the magnitude of the forces.
Why must there be two forces for each pair of charges?
Explaining
electrostatics
We now know that simple electrostatic effects are due
only to the movement of the negatively charged electrons.
An object with no observed charge (net) has an exact
balance between e- and p+.
Some electrons in conducting materials are loosely
attached to their respective atoms and can move.
The object that loses electrons becomes positive and the
one that gains them becomes negative.
What causes electrostatic effects?
What can we say about neutral objects in terms of charges?
Why are some materials easier to charge than others?
How do we exchange charges?
Which charged particles move?
Why do only these particles move?
Charging by induction
What is the charge of the sphere initially?
How does the sphere become positively charged?
Why does the sphere need to be insulated?
Measuring and defining
charge
The unit of charge is the coulomb, C.
Charge is a scalar quantity.
The coulomb is defined as the
charge transferred by a current of
one ampere in one second.
What is the unit of charge?
If charge can be positive and negative, why is it not a vector?
How is the coulomb defined?
Measuring and defining
charge
Measurements show that all electrons are
identical, with each one having a charge equal
to -1.6 x 10-19 C, called the elementary charge e.
Charges smaller than this are not observed
directly in nature.
The coulomb is a large unit so we often use pC
and nC to describe quantities of charge.
What is the relative charge of the electron?
What is the charge of the electron?
Why have you learnt more about relative charge in previous years?
What happens to force as distance increases?
What is the relationship between the force between two stationary
charges and the distance between them?
How would you write this mathematically?
What is the relationship between the force between two stationary
charges and the inverse square of the distance between them?
How would you write this mathematically?
Why is this graph more useful than the previous one?
Force between two
stationary charges,
N
Coulomb constant
= 8.99 x 109 N m2
C-2
Charge on second
object, C
Charge on first
object, C
Write down the formula and definitions.
Distance between
centres of two
charged objects, m
Write down the different versions of the formula.
Explain what the equation means physically.
Time period for one
complete oscillation, s
Permittivity of free space =
8.85 x 10-12 C2 N-1 m-2
Write down the formula and definitions.
Write down the different versions of the formula.
Explain what the equation means physically.
Force between two
stationary charges,
N
Permittivity of free
space = 8.85 x 10-12
C2 N-1 m-2
Charge on second
object, C
Charge on first
object, C
Write down the formula and definitions.
Distance between
centres of two
charged objects, m
Write down the different versions of the formula.
Explain what the equation means physically.
The permittivity of free
space, ε0
Permittivity is a measure of the
polarisation of a medium.
A material with high permittivity polarises
more and therefore can store more energy.
Free space, or a vacuum has the lowest
permittivity = 8.854 x 10-12 C2 N-1 m-2.
What is the value of the permitivity of free space?
What is permittivity?
What is the permittivity of free space?
Permittivity
Which substance has the highest permittiivty?
How much greater is the permittivity of water compared with air?
Why is the permittivity of water so great?
Electric fields
Sometimes the origin of a force between two object
is obvious, such as the friction pad in a brake
rubbing on the rim of a bicycle wheel to slow down.
In other cases there is no physical contact yet a
force still exists.
Example of this include the magnetic force between
two magnets and the electrostatic force between two
charged objects: this is action at a distance.
What is a contact force? Give examples.
What is a non-contact force? Give examples.
How does a force reach across space?
Electric fields
The term field is used in physics for cases where two
separated objects exert forces on each other.
We say that in the case of the comb picking up paper, the
paper is sitting in the electric field due to the comb.
The concept of the field is an extremely powerful one in
physics not least because there are many ideas common to
all fields.
As well as the magnetic and electrostatic fields already
mentioned, gravity fields also obey the same rules.
Give three examples of fields in physics.
What is a field?
Does a field exist?
What do the lines on the map represent?
What do close lines mean?
How does this correspond to electric field lines?
+
-
+
-
+
-
+
-
+
-
The rules
The lines start and end on charges with opposite signs
An arrow is essential to show the direction in which a
positive charge would move.
Where the field is strong the lines are close together:
the lines act to repel each other.
The lines never cross.
The lines meet a conducting surface at 90o.
What are the rules for drawing electric fields?
Which rule is the most important?
How would gravitational fields be different?
Electric field strength
This is defined using the concept of the
positive test charge.
We place a small positive test charging in a
field and it will experience a force in the
direction of the field.
An electron would feel a force in the
opposite direction.
What direction do electric fields lines go?
What direction does a positive test charge go when dropped on a field
line?
Why does an electron experience a force in the opposite direction?
Electric field strength
What direction do electric fields lines go?
What direction does a positive test charge go when dropped on a field
line?
Why does an electron experience a force in the opposite direction?
Electric field strength, N C-1
Force, N
Charge, C
Voltage, V
Distance, m
Write down the formula and definitions.
Write down the three versions of the formula.
Explain what the equation means physically.
Gravitational field strength, N kg-1
Force, N
F
g=
m
Mass, kg
Write down the formula and definitions.
Write down the three versions of the formula.
Explain what the equation means physically.
Radial field
The field shape for a point charge is
known as a radial field.
The field lines radiate away
(positive) or towards (negative) the
point charge
What shape is the field around a point charge?
Sketch the field for a positive and negative charge.
Add arrows to show the direction of the field.
Radial field
What shape is the field around a point charge?
Sketch the field for a positive and negative charge.
Add arrows to show the direction of the field.
Combining fields
We can add electric field strengths using
either a calculation or scale diagrams.
If the fields are parallel we can simply
add.
If they are not we need to resolve
vertically and horizontally before adding.
How can we add electric field strengths?
How do we add parallel fields?
How do we add non-parallel fields?
Adding fields
E1
E2
+
+
+
Q2
Q1
How can we add electric field strengths?
How do we add parallel fields?
How do we add non-parallel fields?
Close to a conductor
Conducting sphere
This can be taken a step further for a conducting
sphere, whether it is hollow or solid.
The free electrons at the surface are equally spaced
and all the field lines at the surface of the sphere
are at 90° to it.
The consequence is that the field must be radial
There is no electric field inside a sphere, hollow or
solid
Why are the electrons at the surface equally spaced?
What direction are the field lines?
What is the electric field inside a sphere like?
Shuttling ball
Watch the video.
What happens?
Explain why the ball moves as it does.
Shuttling ball
Electrons move easily along the connecting wires.
When the supply is turned on, the electrons soon
distribute themselves so that the plate connected to
the negative supply has an excess of electrons and
the other plate has a deficit.
When the ball touches a plate it will gain the same
charge and repel: a force acts on the ball as it is
in an electric field.
What happens when the supply is turned on?
Why does the ball initially move in terms of charge?
Why does the ball initially move in terms of a field?
Key points
An electric current results when
charge moves.
The charge is moved by the presence
of an electric field.
What occurs when charge moves?
Why does charge move?
What else links to this phenomena?
Metallic structure
The metal atoms in a solid are bound by
the metallic bond.
When a metal solidifies, its atoms form a
regular lattice arrangement.
Electrons are donated from the outer shell
electrons to a common sea of electrons.
What is a lattice?
What is a metallic bond?
Why are metals much better electrical conductors than other
materials?
Metallic structure
What does the - particles represent?
What do the + particles represent?
Why is there no current?
Metallic structure
The positive ions sit in a fixed position
on the lattice.
There are ions at each lattice site as
each atom has lost an electron.
In temperatures above absolute zero,
they vibrate in fixed positions.
What charge are these ions?
Why are the atoms ions?
What is a superconductor?
Electrical resistance
The electrons can interact with the metal
ions, causing them to change direction
or slow down.
This transfers kinetic energy from the
electron to the ion.
This is what we call electrical resistance.
What happens when electrons interact with the metal ions?
What is this phenomena called?
Why is this temperature dependent?
Conduction in gases
and liquids
Electrical conduction is possible when ionic
substances are molten or dissolved
When an electric field is applied, the ions
will move and a current is observed.
If the field is strong enough the material
itself can undergo electrical breakdown
leading to the creation of ions: lighting.
How can conduction occur in a fluid?
Describe these two methods.
Explain electrical breakdown.
Electrical breakdown
Electric current
When charge flows in a conductor we
say that there is an electric current.
Current is measured in amperes, A
using an ammeter.
Current does not flow, charge does.
What is electric current?
What is the formula for electric current?
Why is describing current as flowing incorrect?
Sketch the circuit.
What happens as the voltage increases?
How could we measure current?
current, A
ΔQ
I=
Δt
Write down the formula and definitions.
Write down the three versions of the formula.
Explain what the equation means physically.
charge, C
time, s
AC/DC
AC is alternating current: the current
(and voltage) changes from positive
to negative: Mains electricity.
DC is direct current: the current
(and voltage are constant): Batteries
and cells.
What is DC?
What is AC?
Compare AC/DC with electron flow and conventional current.
What is this an image of?
What are the red and white lights?
What are the yellow lights?
t = 1/16 s
What do you notice about the red and white lights?
What do you notice about the yellow lights?
Explain the difference.
t = 1/16 s
What do the dotted lights mean?
Calculate the frequency of electricity in California.
Repeat this experiment at home. Compare your value.
Drift speed
The slow speed at which the ions move along the conductor
is known as the drift speed.
Imagine a cylindrical conductor that is carrying an
electric current I.
The cross-sectional area of the conductor is A and it
contains charge carriers of charge q.
We assume that each carrier has a speed v and that there
are n charge carriers in 1 m3 of conductor: the charge
density.
What is drift speed?
Follow the derivation carefully.
Is it safe to assume the drift speeds are equal?
Drift speed
What is drift speed?
Follow the derivation carefully.
Is it safe to assume the drift speeds are equal?
Drift speed
The previous diagram shows charge carriers of charge q,
moving past a point P at a speed v.
In one second, a volume of Av of charge carriers passes P.
The total number of charge carriers is nAv so the total
charge is nAvq
This is the charge that passes a point P in one second.
Follow the derivation closely.
What is the equation for current outline above?
What is the equation for drift speed?
Cross-sectional area, m2
current, A
Speed of charges, m s-1
I = nAvq
charge, C
Number of charge carriers per unit
volume, m-3
Write down the formula and definitions.
Write down the three versions of the formula.
Explain what the equation means physically.
Bicycle wheel
The electrons move slowly
but they all move at the same
time.
This is like braking on a
bicycle wheel: when you apply
the brakes to one part of the
wheel, the whole wheel slows
down.
How can the light switch on so quickly if the drift velocity is so
low?
What does the spinning wheel represent?
Create another analogy for this phenomena.
Potential difference
Free electrons move in a conductor when an
electric field acts on the conductor
A power supply transfers energy to electrons
and provide the electric field
As the electrons move through the
conductors, they collide with the positive
ions in the lattice and transfer energy
What does a power supply do?
What causes free electrons to move?
Why do electrons transfer energy?
Potential difference
When fields act, we use ideas of potential (will come later)
and potential difference to help us understand.
Potential difference (pd) is a measure of the electrical
potential energy transferred from an electron when it is
moving between two points in a circuit.
However, given the very small amount of charge possessed
by each electron this amount of energy is also very small.
It is better to use the much larger quantity represented by
one coulomb of charge.
What is a coulomb?
Describe potential difference.
Why are joules a problematic unit for the energy of an electron?
Work done, J
Potential
difference, V
W
V=
Q
Charge
transferred, C
Write down the formula and definitions.
Write down the three versions of the formula.
Explain what the equation means physically.
Sketch the circuit.
Add in at least three voltmeters and show the voltages.
Why is the voltage 0 V at either end of a wire?
What are the key terms to do with electricity and magnetism.
Define some of these key terms.
Why should we try to avoid using the word ‘electricity’?
Electromotive
force (emf)
Not a force.
When energy is transferred to the
electrons (e.g. cell).
pd will be used when energy is
transferred from electrons (e.g. lamp).
Is electromotive force a force?
What is electromotive force?
How is emf different to pd?
Device
Input energy
Output energy
pd or emf
Cell
Chemical
Elecrtrical
emf
Resistor
Electrical
Internal
pd
Microphone
Sound
Electrical
emf
Loudspeaker
Electrical
Sound
pd
Lamp
Electrical
Light
pd
PVT cell
Light
Electrical
emf
Dynamo
Kinetic
Electrical
emf
Electric motor
Electrical
Kinetic
pd
Add in the input energies.
Add in the output energies.
Label the voltage as either pd or emf.
Power, current and
pd
Suppose there is a conductor with a pd V between
its ends when a current I is in the conductor.
In time Δt the charge Q that moves through the
conductor is equal to IΔt.
The energy W transferred to the conductor is
IΔtV
The power supplied is W/Δt to give P = IV
What is power?
What is electrical power?
What is the formula for power?
Current, A
Power, W
Voltage, V
Write down the formula and definitions.
Write down the three versions of the formula.
Explain what the equation means physically.
How do I remember the difference
between the units?
Weight
W = mg
W = 0.1 kg x 10N/kg
W = 1 N
m = 0.1 kg
W = 1 N
1 m
Power
Work
P = W / t
W = Fd
P = 1 J / 1 s
W = 1 N x 1 m
P = 1 J/s = 1 W
W = 1 Nm = 1 J
The electron-volt,
eV
Energies involved with individual
particles are very small.
We use the electron-volt as a unit
of energy (not voltage).
It is 1.6 x 10-19 J.
What is an electron-volt a measure of?
What is the value of 1 eV in joules?
Why do we use electron-volts?
5.1 Electric fields
Write down as much as you
can about this lesson.
Include any key words or
diagrams you think are
necessary.
CW
Apr 25, 2022
5.2 Heating affects of an electric current.
Identify the sign and nature of charge carriers
in a metal.
Identify two forms of charge and the
direction of forces between them.
Describe Coulomb’s Law.
Effects of electric
current
Heating effect.
Chemical effect.
Magnetic effect.
What is an electric current?
What are the three effects of an electric current?
Why can electric current not flow?
Heating effect
When energy is transferred to a resistor, internal energy is
generated.
What is generated when energy is transferred to a resistor?
How does a light bulb work?
Why is argon used?
Chemical effect
When chemical react together to alter the energy of electrons and to cause
them to move, or when electric current in a material causes chemical
changes.
What energy is stored in a battery?
What direction do the electrons flow?
Why is there a barrier between the two electrodes?
Magnetic effect
When a current produces a magnetic field, or when magnetic
changes near conductors and induce an emf in the conductor.
What is an alternating electric current?
How can a magnetic field be generated?
What is the difference between an emf and a pd?
Fill in the gaps of the first two columns
And match the descriptions for five components.
And match the description for all the components.
AC/DC (again)
Some components are intended for direct
current, such as mobile phones and
flashlights.
Others are for alternating current which
require high voltages and large amounts of
energy, such as kettles and washing machines.
Common frequencies for AC are 50 or 60 Hz.
What are the two types of current?
Differentiate the two types.
Why are both types required?
Practical measurements of
current and potential
difference
We often need to measure the current in a
circuit and the pd across components in the
circuit.
This can be achieved with the use of meters or
sensors connected to computers.
You will use both on this course but the key
thing to remember is that meters can be either
analogue or digital.
What is a meter?
What are the two most common quantities to measure?
What is the difference between analogue and digital?
Analogue meters
Have a mechanical system of a coil and a
magnet.
When charge flows through the coil, a magnetic
field is produced that interacts with the field of
the magnet and the coil swings around against a
spring.
The position reached by the pointer attached to
the coil is a measure of the current in the meter.
What is an analogue meter?
What are the fundamental components of an analogue meter?
Explain how an analogue meter works.
Digital meter
Sample the potential difference
across the terminals of the meter
(or, for current, the pd across a
known resistor) and then convert
the answers into a form suitable for
display on the meter.
What is a digital meter?
What are the fundamental components of a digital meter?
Explain how a digital meter works.
Ammeters
We want to know the size of the current in
a component which must be the same as the
ammeter.
Therefore it must be in series with the
circuit or component.
An ideal ammeter will not take any energy
from the electrons as they flow through it
What is an ammeter?
Describe how it is connected in a circuit?
Why is an ideal ammeter preferred?
Voltmeters
Measure the energy converted per unit
charge that flows in a component.
You can think of a voltmeter as needing to
compare energy in the electrons before and
after a component.
The voltmeter must be placed across the
terminals of the component: in parallel.
What is a voltmeter?
Describe how it is connected in a circuit?
Why is an ideal voltmeter preferred?
Resistance
We have already seen that when electrons
move through a metal they can transfer
energy.
The amount of energy transferred depends
on various factors, such as the type of metal.
We call the overall variable electrical
resistance.
What is resistance?
What happens, in terms of energy, when electrons move through
metal?
What factors influence this energy transfer?
Potential
difference, V
Electrical
resistance, Ω
Current, A
Write down the formula and definitions.
Write down the different versions of the formula.
Explain what the equation means physically.
A graph to show the affect of current on the pd across a piece of resistance wire
6
5
pd / V
4
3
2
1
0
0.0
0.2
0.4
0.6
0.8
1.0
Current / A
Plot the data.
Add the line of best fit.
Calculate the gradient.
1.2
1.4
1.6
1.8
Ohmic resistor
For this wire, the resistance is the same for
all values of current.
Such a resistor is known as an ohmic
resistor.
The current and pd are proportional,
assuming the temperature of the wire did
not change.
What is an Ohmic resistor?
What is the relationship between the current and voltage?
What assumption must we make for this relationship to be true?
Ohm’s law
The behaviour of metallic wires was first
observed by Ohm in 1826.
It leads to a rule known as Ohm’s law.
This states that the pd across a metallic
conductor is directly proportional to the
current (assuming the physical conditions,
e.g. temperature, do not change).
What is Ohm’s law?
What assumptions must we make?
Why was this not discovered earlier?
Plot the data.
Add the line of best fit.
Explain why the gradient increases as current increases.
Non-ohmic resistors
The graph is not straight so V and I are
not proportional.
The filament lamp does not obey Ohm’s
law: it is non-ohmic.
This is not a fair test as the filament’s
temperature changes.
What is the relationship between V and I for the lamp?
What does non-Ohmic mean?
Why is the experiment not a fair test?
Calculating resistance
We do not use the graph to calculate the
individual resistance using the tangent
We use the individual data point using
V/I.
This is because the definition of
resistance is V/I not ΔV/ΔI.
How do we normally calculate the gradient of a graph?
How do we calculate the resistance for an I-V graph?
Why do we not calculate the gradient in this way for I-V graphs?
What is happening?
The resistance of the lamp increases as the
current increases.
At larger currents, it takes a greater change in
pd to change the current by a fixed amount.
As the current increases, more energy is
transferred from the electrons every second
as more electrons flow at higher currents.
What happens to the resistance as the current increases?
Describe what happens at larger currents.
Explain this relationship.
What is happening?
The energy goes into increasing the kinetic energy of
the lattice ions and therefore the temperature of the
bulk material.
But the more the ions vibrate in the lattice, the more the
electrons can collide with them so at higher
temperatures even more energy is transferred to the
lattice by the moving charges.
Other non-ohmic conductors are diodes and thermistors:
made from semiconductors.
Name three non-Ohmic conductors.
What happens to the temperature of the material?
Explain why this happens.
Which are the Ohmic conductors?
Which are the non-Ohmic conductors?
Label the I-V graphs.
Semiconducting diodes
Semiconducting diodes are designed only to allow to
allow charges to flow through them in one direction.
This is seen clearly in the graph.
For negative values of V there is actually a very small
current flowing in the negative direction.
The nature of semiconductor material also means
that there is no significant current in the forward
direction until a certain forward pd is exceeded.
What are diodes designed to do?
How can you tell from the graphs that this is the case?
How does a semi-conductor differ from a conductor?
Thermistors
Thermistors are made from one of the two
elements that are electrical semiconductors: silicon
and germanium.
There are several types of thermistors but we will
only consider the negative temperature coefficient
type (NTC).
As the temperature of an NTC thermistors increases,
its resistance falls: the opposite to that of a metal.
What is a thermistor?
How is this different from a regular conductor?
Explain the mechanism behind this.
Thermistors
Semiconductors have many fewer free electrons per
cubic metre compared with metals.
Their resistances are typically 105 times greater than
similar metal samples.
Unlike in a metal, the charge density in semiconductors
depends strongly on the temperature.
The higher the temperature of the semiconductor, the
more charge carriers are made available to the material.
What is a semiconductor in terms of electrons?
How does the resistance of a semiconductor compare with a similar
metal.
What happens to a semiconductor as the temperature increases?
As the temperature rises
in the germanium:
The lattice ions vibrate more and impede the movement of
the charge carriers, the same as a metal, which leads to an
increase in resistance.
More and more charge carriers become available to
conduct because the increase in temperature provides them
with enough energy to break away from their atoms, which
leads to a large decrease in resistance.
The second effect is much greater than the first and some
the net effect is that conduction increases (resistance falls)
as the temperature of the semiconductor rises.
Name a semiconductor.
What two effects happens as temperature increases?
Why does the resistance increase with temperature?
Resistivity
The resistance of a sample of a material depends not
only on what it is made of, but also on the physical
dimensions of the sample.
The graphs you obtained should give straight lines
that go through the origin:
Resistance is proportional to its length
Resistance is inversely proportional to its crosssectional area
What is resistance proportional to?
Write these proportionalities down.
Convert this into an equation.
Resistance, Ω
Cross-sectional
area, m2
Resistivity, Ω m
Length, m
Write down the formula and definitions.
Write down the different versions of the formula.
Explain what the equation means physically.
Resistivity
The unit of resistivity is the ohmmeter (Ω m).
For a specific substance, resistivity
is independent of size and shape,
like density or specific latent heat.
What is the unit of resistivity?
What does resistivity depend upon?
What is resistivity?
Copy and complete the sheet.
Attach crocodile clips to either end.
Record the resistance and plot the graph.
Practical resistors
Resistors are of great importance in the
electronics and electrical industries.
They can be a single value (fixed)
devices or they can be variable.
They can be manufactured in bulk and
are readily and cheaply available.
What are the two types of resistor?
What is a resistor?
How can resistors combine?
Practical resistors
Resistors come in different sizes.
Small resistors can have a large resistance but only be able
to dissipate a modest amount of energy each second.
If the power that is being generated in the resistor is too
large, then its temperature will increase and could burn.
Resistors are rated by their manufacturers so that, for
example, a resistor could have a resistance of 270 Ω with a
power rating of 0.5 W: P = I2R therefore I = (P/R)1/2 =
(0.5/270)1/2 = 43 mA.
What is resistance?
What is power?
How can this apply to resistors?
Combining resistors
Electrical components can be linked
together in two ways:
Series: one after another
Parallel: connected across each
other
How can resistors be combined?
Describe these methods.
Describe these methods mathematically?
Series circuits
The current is the same in two series-connected
components.
The number of free electrons leaving the first
component must equal the number entering the second
component.
If electrons were to stay in the first component then it
would become negatively charged would repel further
electrons and prevent them from entering it: the flow of
charges would rapidly grind to a halt.
How does the current compare between two series components?
Describe this in terms of free electrons.
Explain why this must be the case.
Series circuits
The potential differences add.
The total energy lost is equal to the sum of
the two separate amounts of energy in the
components.
Because the charge is the same in both cases,
the sum of the pds is equal to the total pd
dropped across them.
How do potential differences combine in a series circuit?
What is the total energy lost equal to?
Compare the sum of the pds to the voltage drop.
RT
Total resistance
R1
R3
R2
Resistance of resistor 2
Resistance of resistor 1
Resistance of resistor 3
What is the formula for combining resistors in series?
How does RT compare with R1, R2 and R3?
If R1 = 2 Ω, R2 = 5 Ω and R3 = 8 Ω, what is RT.
R1
R2
RT
R3
Total resistance
Resistance of resistor 2
Resistance of resistor 1
Resistance of resistor 3
What is the formula for combining resistors in parallel?
How does RT compare with R1, R2 and R3?
If R1 = 2 Ω, R2 = 5 Ω and R3 = 8 Ω, what is RT.
Resistance of resistor 1
Resistance of resistor 2
Total resistance
Resistance of resistor 2
Resistance of resistor 1
What is the formula for combining two resistors in parallel?
How does RT compare with R1, R2 and R3?
If R1 = 2 Ω, R2 = 5 Ω, what is RT.
In series
In parallel
Currents…
Potential
differences
…
Total
resistance…
… are the
same
… add
… is larger
… add
… are the
same
… is smaller
Compare current in series and parallel.
Compare voltage in series and parallel.
Compare resistance in series and parallel.
More complicated
networks
When the networks of resistors are more complicated,
then the individual parts of the network need to be
broken down into the simplest form.
What does a potential divider do?
What is a potential divider used for?
What is the major disadvantage to using a potential divider?
Potential divider
This component is commonly used with
sensors to produce variable potential
differences.
It has some advantages over the
simpler variable resistor circuit even
though it is more complicated to set up.
What does a potential divider do?
What is a potential divider used for?
What is the major disadvantage to using a potential divider?
Potential divider
What does a potential divider do?
What is a potential divider used for?
What is the major disadvantage to using a potential divider?
Potential divider
The most basic potential divider
consists of two resistors with resistances
R1 and R2 in series with a power supply.
This arrangement is used to provide a
fixed pd at a value somewhere between
zero and the emf of the power supply.
What does a potential divider consist of?
What does this arrangement provide?
Why can the potential divider not provide a pd greater than the
emf?
What is Vin equal to?
What changes the resistance of the second circuit?
What changes the resistance of the third circuit?
Potential divider
The two resistors have the same current
in them, and the sum of the pds across
the resistors is equal to the source emf.
What is Ohm’s law?
What is the total resistance of two resistors in series?
What is the voltage across a resistor with two equal resistors in
series?
Worked example: a potential divider consists of two
resistors in series with a battery of 18 V. The resistors
have resistances 3.0 Ω and 6.0 Ω. Calculate, for each
resistor: the pd across it, the current in it.
Using a potential
divider with sensors
It is a simple matter to extend the fixed pd
arrangement to a circuit that will respond to
changes in the external conditions.
Such an arrangement might be used by a
computer that can sense changes in pd and
respond accordingly (for example by turning
on a warning siren if a refrigerator becomes
too warm)
How can a potential divider be modified to respond to external
conditions?
What components could be used to do this?
Give an example of this.
Using a potential
divider with sensors
Recall that when a thermistor is at a high temperature its
resistance is small, and that the resistance increases when the
temperature falls.
Rather than calculating the values, for this example we will use
our knowledge of how pd and current are related to work out
the behaviour of the circuit from first principles.
Suppose the temperature is low and that the thermistor resistance
is high relative to that of the fixed-value resistor.
Most of the pd will be dropped across the thermistor and very
little across the fixed resistor.
What physical quantity makes a thermistor change resistance?
What is the relationship between temperature and resistance for a
thermistor?
Why does a thermistor change resistance based upon temperature?
Using a potential
divider with sensors
If you cannot see this straight away, remember
the equations from the previous section.
So the larger resistance (the thermistor at low
temperatures) has the larger pd across it.
What physical quantity makes a thermistor change resistance?
What is the relationship between temperature and resistance for a
thermistor?
Why does a thermistor change resistance based upon temperature?
Using a potential
divider with sensors
If the thermistor temperature now increases, then the
thermistor resistance will fall.
Now the fixed-value resistor will have the larger
resistance and the pds will be reversed with the
thermistor having the small voltage drop across it.
A voltage sensor connected to a computer can be set
to detect this voltage change and can activate an
alarm if the thermistor has too high a temperature.
What happens to the resistance of a thermistor when its
temperature rises?
How does this affect its pd share?
How could this be used in real life?
Using a potential
divider with sensors
You may be asking what the resistance of the
fixed resistor should be.
The answer is that it is normally set equal to that
of the thermistor when it is at its optimum
(average) temperature.
Then any deviation from the average will change
the potential difference and trigger the
appropriate change in the sensing circuit.
What values should the fixed resistor have?
How does this affect the pd across it?
Why is the value set as this?
Using a potential
divider with sensors
The same principle can be applied to another sensor
device, an LDR.
This is made of semiconducting material but this
time it is sensitive to photons incident on it.
When the light intensity is large, charge carriers are
released in the LDR and this the resistance falls.
When the intensity is low, the resistance is high as
the charge carriers now recombine with their atoms.
What is an LDR?
How does an LDR’s resistance change with light?
How could this be used?
Using a potential divider
give a variable pd
A variable resistor consists of a power
supply, an ammeter, a variable resistor and
a resistor.
The value of each component is given on
the diagram.
We can predict the way this circuit will
behave.
What is a rheostat?
How can this be used in a potential divider?
What affect will this have?
Using a potential divider
give a variable pd
When the variable resistor is set to its minimum, 0 Ω, then
there will be a pd of 2 V across the resistor and a current of
0.2 A in the circuit.
When the variable resistor is set to its maximum value, 10 Ω,
then the total resistance in the circuit is 20 Ω, and the
current is 0.1 A.
This means that with 0.1 A in the 10 Ω fixed resistor, only 1
V is dropped across it. Therefore the range of pd across the
fixed resistor can only vary from 1 V to 2 V - half of the
available pd that the power supply can in principle provide.
What is the maximum voltage across a resistor?
What is the maximum total resistance of the circuit?
What is the minimum current of this circuit?
What is the maximum voltage across a resistor?
What is the maximum total resistance of the circuit?
What is the minimum current of this circuit?
Using a potential divider
give a variable pd
The limited range is a significant limitation in the use of
the variable resistor.
To achieve a better range, we could use a variable resistor
with a much higher range of resistance.
To get a pd of 0.1 V across the fixed resistor the resistance
of the variable resistor has to be about 200 Ω.
If the fixed resistor has a much greater resistance, then the
variable resistor would need an even higher value too and
this would limit the current.
What is the significant limitation of this potential divider?
How can this be fixed?
How would a pd of 0.1 V across the fixed resistor be achieved?
Using a potential divider
give a variable pd
The potential divider arrangement allows a much
greater range of pd to the component under test than
does a variable resistor in series with the component.
In a potential divider, the same variable resistor can be
used but the set-up is different and involves the use of
the three terminals on the variable resistor (or rheostat).
One terminal is connected to one side of the cell, and
the other end of the rheostat resistor is connected to
the other terminal of the cell.
What are the benefits of using a variable resistor?
How is the circuit set-up?
Why are three terminal necessary for the rheostat?
Using a potential divider
give a variable pd
The potential at any point along the resistance winding depends on
the position of the slider that can be swept across the windings
form one end to the other.
The component that is under rest is connected in a secondary
circuit between one terminal of the resistance winding and the
slider.
When the slider is positioned at one end, the full 2 V from the cell
is available to the resistor under test.
When at the other end, the pd between the ends of the resistor is 0 V
(the two leads to the resistor are effectively connected directly to
each other at the variable resistor).
How does a variable resistor work?
Why is the maximum voltage achievable across the fixed resistor the
emf?
Why is the minimum voltage achievable 0 V?
Worked example: a light sensor consists of
a 6.0 V battery, a 1800 Ω resistor and a an
LDR in series. When the LDR is in darkness
the pd across the resistor is 1.2 V.
Calculate the resistance of the LDR when it is in darkness.
When the sensor is in the light, its resistance falls to 2400 Ω. Calculate the pd across the LDR
Answer the question without any help.
Use a different colour if you receive any help.
Explain what other questions you could be asked in your exam.
Heating effect
equations
We saw earlier that the power P dissipated in a
component is related to the pd V across the
component and the current I in it:
What is the equation for power?
What is the equation for electric power?
What is Ohm’s law?
Kirchoff’s first and
second laws
We have seen that the charge carriers in a conductor move
into and out of the conductor at equal rates.
If 106 flow into a conductor in one second, then 106 must
flow out during the same time to avoid the buildup of a
static charge.
We also considered what happens when current splits into two
or more parts at the junction where a parallel circuit begins.
We can take this one step further to a situation where there
is more than one incoming current at the junction too.
Why is a phone never out of charge?
What can we say about the current in a series circuit?
Explain this.
Kirchoff’s first law
The diagram shows a junction with three
incoming currents and two outgoing ones.
Our rule about the incoming charge
equating to the outgoing charge applies
here:
I1 + I2 + I3 = I4 + I5,
Sketch the diagram.
What is Kirchhoff’s first law?
𝛴
Write this mathematically.
I = 0
𝛴
I = 0
The sum of the currents into a junction equals
the sum of the currents away from a junction
Total charge flowing into a junction equals
the total charge flowing away from the
junction.
This is known as Kirchoff’s first law or the
conservation of charge.
Sketch the diagram.
What is Kirchhoff’s first law?
Write this mathematically.
Proving Kirchoff’s 1st law
Sketch the diagram.
What is Kirchhoff’s first law?
Write this mathematically.
Kirchoff’s second law
In any electric circuit there are sources of emf and
sinks of pd.
A general rule in physics is that energy is conserved.
Electrical components have to obey this too.
In any electrical circuit, the energy being converted
into electrical energy must be equal to the energy
being transferred from electrical to internal, by the
sinks of pd.
What is the law of energy conservation?
What is the difference between a pd and an emf?
What is the general rule about voltages?
IR
This is Kirchoff’s second law, equivalent to conservation of
energy.
This second law applies to all closed circuits - both simple
and complex.
In a complete circuit loop, the sum of the emfs in the loop
is equal to the sum of the potential differences in the loop.
The sum of all variations of potential in a closed loop
equals zero: ε = IR.
What is Kirchhoff’s second law?
Write this mathematically.
𝛴
𝛴
What is the cause of this law?
𝛴
𝛴
ε =
Proving Kirchoff’s 2nd law
Sketch the diagram.
What is Kirchhoff’s first law?
Write this mathematically.
Sketch a loop around the circuit.
Sketch two loops around the circuit.
Sketch three loops around the circuit.
Loop GABCDEG travelling
anticlockwise round the
loop
This loop begins at the cell and
goes around the circuit, through
resistor R1 and resistor R2,
finally ending at the cell again.
In this loop there is one source
of emf and two sinks of pd
(ignoring the wires, which we
assume have zero resistance).
How many sources of emf are there?
How many sinks of pd are there?
Why do we ignore there wires?
Loop GABCDEG travelling
anticlockwise round the
loop
So ε = I1R1 + I2R2
The direction of the loop travel and the current
direction are in all cases the same.
We give a positive sign to the currents when this is
the case.
The emf of the cell is driving in the same direction
as the loop travel direction; it gets a positive sign as
well.
If the loop direction and the current or emf were to
be opposed then they would be given a negative sign.
How many sources of emf are there?
How many sinks of pd are there?
Why do we ignore there wires?
Loop EFGE travelling
clockwise around the loop
This loop goes first through resistor R3
and the loop direction is in the same
direction as the conventional current.
Next the loop goes through resistor R1
but this time the current direction and
the loop are different so there has to be
a negative sign.
There is no source of emf in the loop so
the Kirchoff equation becomes: 0 = I3R3
- I2R2.
How many sources of emf are there?
How many sinks of pd are there?
Why do we ignore there wires?
Loop EFGE travelling
clockwise around the loop
Kirchoff’s first law can be applied at point G.
The total current into point G is I2 + I3; the total out
is I1. The application of the law is I1 = I2 + I3.
There are now three separate equations with three
unknowns and these equations can be solved to work
out the currents in each part of the circuit assuming
that we know the value for the emf of the cell and the
values of the resistances in the circuit.
By setting up a series of loops it is possible to work
out the currents and pds for complicated resistor
networks, more complicated than could be done using
the resistor series and parallel rules alone.
How many sources of emf are there?
How many sinks of pd are there?
Why do we ignore there wires?
Ideal and non-deal
meters
We have assumed that the meters used
in the circuit were ideal.
This means they have no effect on the
circuit they are measuring.
We would always want this to be true,
but real meters are not ideal.
What is an ideal meter?
Why do we assume meters are ideal?
Why are meters never ideal?
Real ammeters
Ammeters are placed in series with components
so the ammeter has the same current as the
components.
It is undesirable for the ammeter to change the
current in a circuit but, if the ammeter has a
resistance of its own, then this is what will
happen.
An ideal ammeter has zero resistance.
What is the resistance of an ideal ammeter?
What is the resistance of an ideal voltmeter?
Why are both these not possible?
Real voltmeters
Voltmeters are placed in parallel with the
device or parts of a circuit they are measuring.
In an ideal world, the voltmeter will not
require any energy for its coil to move or for
its analogue to digital conversion.
The way to avoid current in the voltmeter is
for the meter to have infinite resistance.
What is the resistance of an ideal ammeter?
What is the resistance of an ideal voltmeter?
Why are both these not possible?
Worked example: calculate
the currents in the circuit
shown.
Worked example: calculate
the currents in the circuit
shown.
5.2 Heating affects of an electric
current.
Write down as much as you
can about this lesson.
Include any key words or
diagrams you think are
necessary.
CW
Apr 25, 2022
5.3 Electric cells
What is the equation for resistivity?
Describe Kirchhoff’s second law?
Explain resistivity.
Introduction
Electric currents can produce a chemical effect.
This has great importance in chemical
industries as it can be a method for extracting
ores or purifying materials.
We will emphasise the use of an electric cell to
store energy in chemical form and then
released as electric energy to perform work.
What can electric currents produce?
Give an example of the importance of this effect.
How is the uncertainty of a repeated measurement calculated?
Cells
Cells operate as direct-current (dc) devices meaning that the
cell drives charge in one direction.
The electron charge carriers leave the negative terminal of the
cell.
After passing around the circuit, the electrons re-enter the
cell as the positive terminal which has a higher potential than
the negative terminal: so electrons ‘gain’ energy.
The chemicals in the cell are reacting while current flows and
as a result the electrons gain energy and continue their
journey.
What type of current does a cell generate?
What side of a cell does the conventional current leave?
How do electrons gain energy?
Primary cells
Many of the portable devices we use today can
operate with internal cells.
Some cells are used until they are ‘dead’ and
are thrown away: primary cells.
The original chemicals have completely reacted
and been used up and they cannot be
recharged: e.g. AA cells or dry cells, button
mercury cells.
List five objects which use cells.
What is a primary cell?
Give examples of primary cells.
Secondary cells
Some devices use rechargeable cells so when the
chemicals are no longer producing voltage they
can be connected to a charger.
Then the chemical reaction is reversed and the
original chemicals form again.
When as much of the re-conversion as is possible
has been achieved, the cells is available as a
chemical energy store.
What is a secondary cell?
How is a secondary cell recharged?
What type of energy does a cell store?
Capacity of a cell
Two cells with the same chemistry will
generate the same electromotive force (emf)
as each other.
However if one of the cells has larger plates
than the other and contains larger volumes
of chemicals, then it will be able to supply
energy for longer when both cells carry the
same current.
What is the symbol for emf?
What is emf?
What happens if a cell has larger plates than another?
Capacity of a cell
The capacity of a cell is the quantity used to measure the
ability of a cell to release charge.
If a cell is discharged at a high rate then it will not be long
before the cells is exhausted or needs recharging, if the
discharge current is low then the cell will supply energy for
longer times.
The capacity of a cell or battery is the constant current that it
can supply for a given discharge.
If a cell can supply a constant current of 2 A for two hour
then it has a capacity of 40 A h.
What is the capacity of a cell?
How can this be affected?
If a cell can supply a constant current of 2 A for two hours, what
is its capacity?
Discharge of a cell
Complete the experiment and obtain the data.
Construct a graph to show this data.
Annotate the final graph to show what various regions mean.
What are the three major parts of the graph?
Why is emf greater than the pd?
What is the working terminal of this cell?
Recharging secondary
cells
The chemicals produce an excess of electrons at
the negative terminal.
During discharge these electrons move through
the circuit transferring energy.
When the electron arrives at the positive
terminal, all of its energy will have been
transferred to other forms and it will need to
gain more from the chemical store.
What do the chemicals produce at the negative terminal?
What to these products do?
What happens to the energy involved?
Recharging secondary
cells
To reverse this process we need to return
energy to the cell using electrons, so that
the chemical reaction can be reversed.
When charging, the electrons need to travel
in the reverse direction to that of the
discharge current and you can imagine that
the charger has to force the electrons the
‘wrong way’ through the cell.
What type of cell is a rechargeable one?
How is this process reversed?
Why does the current change direction when it is reverse?
Charging circuit for a
cell
The charging current is in the
opposite direction to that of the
cell.
An input pd of 14 V is needed
with the polarity of the current
shown using the diode.
The ammeter will show a large
current initially but this will
grown smaller as the cell
charges until it eventually
reaches zero.
How does the current change before charging?
How does the current change while charging?
What is the purpose of the LED?
Internal resistance and
emf of a cell
The materials the cells is
constructed from has its own
resistance: internal resistance.
We model this has if it has a small
resistor in series with the ideal
cell, inside a box.
We assume the emf (ε) and internal
resistance (r) are both constant.
Sketch the diagram fully, and annotate.
Use Kirchhoff’s second law to generate an equation for this set-up.
What is the difference between R and r?
Calculating internal
resistance
Sketch the diagram fully, and annotate.
Use Kirchhoff’s second law to generate an equation for this set-up.
What is the difference between R and r?
Electromotive
force, V
Resistance of load, Ω
Current through
load, A
Internal resistance
of cell, Ω
Write down the formula and definitions.
Write down the different versions of the formula.
Explain what the equation means physically.
Remember
The emf is the open circuit pd across the
terminal when no current is supplied.
The output voltage is always less than the
emf when in operation.
The difference is the ‘lost volts’ which are
required to push the electrons through the
cell.
What is emf? What is terminal pd?
Compare the voltage before and after the circuit is in operation.
Describe ‘lost volts’.
Annotate your graph to show emf.
Calculate the gradient of your graph.
Determine a value for the internal resistance of the cell.
Power supplied by a
cell
The total power supplied by a non-ideal cell
is equal to the power delivered to the
external circuit plus the power wasted in the
cell.
We can write this algebraically:
What is the general power equation?
List three versions of the electrical power equation.
Where does the equation on the right appear?
Power supplied by a
cell
What does this graph show?
What does the peak correspond to?
What does this tell us?
Answer the question without any help.
Use a different colour if you receive any help.
Explain what other questions you could be asked in your exam.
5.3 Electric cells
Write down as much as you
can about this lesson.
Include any key words or
diagrams you think are
necessary.
CW
Apr 25, 2022
5.4 Magnetic effects of electric current
Which elements are magnetic?
Describe, in terms of a domain, what a magnet
is.
Explain why only some materials are magnetic.
Effects observed when charge
moves in a circuit:
Heating effect (when energy is transferred to a
resistor as internal energy)
Chemical effect (when chemicals react together
to alter the energy of electrons or when electric
current causes chemical changes)
Electromagnetism (when a current produces a
magnetic field, or when a magnetic field change
induces an emf)
What are the three effects observed when charge moves in a
circuit?
Which subsections do these fall under?
Describe these effects.
Nature of science: talking
about poles
When we write magnetic north pole, what we really
mean is the “magnetic pole that seeks the
geographic north pole”.
A compass does not point towards the north
magnetic pole.
A north pole in IB, means a north-seeking pole
which is attracted to a south magnetic pole.
The north geographic pole is a south magnetic pole.
How many poles does the Earth have?
What is the difference between the Earth’s geographical north pole
and its magnetic north pole?
Why is this the case?
Show that your magnetic field pattern matches this.
What direction do the magnetic fields point.
Explain why this is concerning for D of E students.
What is the name of this magnetic field?
How is this magnetic achieved?
What direction do the field lines point?
What shape is the magnetic field of the Earth?
How is the diagram different from reality?
Why has Father Christmas’ Work Shop still not been found?
Why is the shape of the field different from theory?
How does this protect Earth’s organisms?
How does this explain the aurorae?
What is this phenomena called?
Where is this likely to have been photographed?
What specifically caused the aurorae?
Electron flow versus conventional current
Electrons are
negatively charged
particles so are
repelled by the -ve
terminal of a battery
and attracted to the
+ve terminal
Conventional current
is the flow of
imaginary +ve charges
so go from +ve to -ve.
What are the two definitions of current?
Describe the two types of current.
Explain which one is used most often.
Whenever you see ‘I’ think conventional current
What happens to the field if the current went down into the page.
Describe methods of checking the presence of the field lines.
Describe methods of checking the direction of the field lines.
x
O
What type of magnetic field exists between the two poles?
Draw in this magnetic field.
Why are the field lines around the wire at different distances?
Which hand do we use?
Describe the use of the right-hand rule.
What rule would we use if Franklin was right about charge
carriers?
The solenoid
Turn
N = 15 turns
What is a solenoid?
What is a turn?
What is the etymology of the word solenoid?
What shape is the magnetic field of a solenoid?
Compare this with that of the magnetic field of a bar magnet.
How does the magnetic field of a long conductor change into
that of a conductor?
How many poles does a solenoid have?
How are these poles distinguished?
Why can this rule be confusing!
What happens to the field when two opposite fields come together?
What happens to the field when two like fields come together?
What is the overall effect of this addition?
The strength of the magnetic
field in a solenoid can be
increased by:
Increasing the current in the wire
Increasing the number of turns per
unit length of the solenoid
Adding an iron core inside the
solenoid.
What factors affect the strength of the magnetic field.
Explain why one of these factors has the effect it has.
Explain why these factors affect the strength of the magnetic
field.
Forces on moving charges:
forces between two currentcarrying wires
What is the direction of the force when the current is in series?
What is the direction of the force when the current is in parallel?
Sketch magnetic field patterns around the wires to show both
scenarios.
Forces on moving charges:
forces between two currentcarrying wires
What is the direction of the force when the current is in series?
What is the direction of the force when the current is in parallel?
Sketch magnetic field patterns around the wires to show both
scenarios.
Wires in parallel
x
x
What is the direction of the force when the current is in series?
What is the direction of the force when the current is in parallel?
Sketch magnetic field patterns around the wires to show both
scenarios.
Wires in parallel
x
x
What is the direction of the force when the current is in series?
What is the direction of the force when the current is in parallel?
Sketch magnetic field patterns around the wires to show both
scenarios.
Wires in series
O
x
What is the direction of the force when the current is in series?
What is the direction of the force when the current is in parallel?
Sketch magnetic field patterns around the wires to show both
scenarios.
Wires in parallel
O
x
What is the direction of the force when the current is in series?
What is the direction of the force when the current is in parallel?
Sketch magnetic field patterns around the wires to show both
scenarios.
The motor effect
When a currentcarrying wire is placed
in a magnetic field, it
will experience a force
(provided the current
is not parallel to the
field). This is called
the motor effect.
Copy down the definition.
What direction does the magnetic field go?
What direction does the current go?
What happens if the current is reduced?
What happens if the current is reversed?
What happens if the magnetic field is reversed?
What happens if the current is reduced?
What happens if the current is reversed?
What happens if the magnetic field is reversed?
Fleming’s Left-Hand Rule
Force
Magnetic field
Current
What type of current is this?
What direction does the magnetic field go?
Which hand must you use?
Which hand do we use?
Describe the use of the left-hand rule.
What rule would we use if Franklin was right about charge
carriers?
Imagine a wire with a
current going into the page.
x
Draw in the magnetic field of the wire including arrows.
What direction do the electrons flow?
What happens to the field if the current is reversed?
Now imagine this wire is
located in a magnetic field
x
What type of magnetic field exists between the two poles?
Draw in this magnetic field.
Why are the field lines around the wire at different distances?
Now imagine this wire is
located in a magnetic field
N
x
S
What type of magnetic field exists between the two poles?
Draw in this magnetic field.
Why are the field lines around the wire at different distances?
Now imagine this wire is
located in a magnetic field
N
x
S
Compare the field lines of the wire and the magnet.
What would happen to the field lines?
Draw the net magnetic field.
Now add the magnetic
field lines together.
N
x
Add an arrow to show the direction of the force.
Describe the magnetic field lines around the wire.
Why have you drawn the arrow in this direction?
S
Now add the magnetic
field lines together.
N
x
Add an arrow to show the direction of the force.
Describe the magnetic field lines around the wire.
Why have you drawn the arrow in this direction?
S
The importance of the motor
effect
This effect is the basis for the conversion of
electrical energy into kinetic energy.
It is used in all electric motors, loud speakers
and any other device where we produce
movement from an electrical power source.
We can easily build simple versions of these
devices should we not be miserable.
List five devices which uses the motor effect.
Describe the energy transfer involved with the motor effect.
Explain why some students prefer not to perform experiments.
What is the energy transfer involved in the generator?
What happens as the voltage increases?
What is the frequency of the sound generated.
How could the loudspeaker be modified to be louder?
Why does the sound not increase with voltage?
Why does the coil begin to smoke?
What is the energy transfer involved?
How can the motor’s speed be increased?
Explain why the top layer of insulation needs to be removed.
Can the motor spin in both directions?
What factors affect the spin speed?
How would you predict the direction of rotation?
What is the energy transfer involved?
How can the brightness of the LED be changed?
Explain why the LED changes colour during generation.
What factors affect the voltage generated?
Why must the coil be around 1 cm in width?
Why are the colour changes not constant?
The motor effect
There have been no electrostatic effects
because we have dealt with conductors
in which there are is an exact balance
of positive and negative charges.
Magnetism can be thought of as the
residual effect that arises when charges
are moving with respect to each other.
How can we think about magnetism?
Why have there been no electrostatic effects to observe?
Explain why permanent magnets can occur if there is no current.
What is the relationship between current and magnetic force?
Calculate the gradient.
Calculate the max and min gradients.
Force on a current-carrying
wire
The force acting on the wire is
proportional to:
The length of the wire, l
The current in the wire, I
What does the force acting on wire depend upon?
Describe the relationship between these factors.
Explain the relationship between these factors.
Force on a current-carrying
wire
This leads us to the definition of
magnetic field strength rather different
from that of electric field strength
and gravitational field strength.
We cannot define the magnetic field
strength in terms of force/single
quantity as it depends on I and l.
Define gravitational field strength.
Deifne electric field strength.
Define magnetic field strength.
Force on a current-carrying
wire
Instead we define magnetic field strength
as B = F/IL
This is similar to that of gravitational and
electric field strength.
However we must remember the two factors
which are on the bottom of the fraction.
What is the definition of magnetic field strength?
What are the fundamental units of B?
Why other factors must be taken into account which is currently
missing?
Force on a current-carrying
wire
Which equation is better?
Why is sine used rather than cosine?
What is the force when the field and the current are parallel?
The Tesla, T
The unit of magnetic field strength
is the tesla, T.
This is equivalent to kg s-2 A-1 in
fundamental units.
We can think of it as 1 N A-1 m-1.
What is the unit of magnetic field strength?
What is this in everyday language?
What is this in fundamental units?
How big is a tesla?
The Tesla turns out to be a very large unit.
The largest magnetic field strengths in the
lab are a few kT and the magnetic field
of the Earth is roughly 10-4 T.
The very largest fields are associated with
some neutron stars of the order of 100
GT.
What is the strength of the Earth’s magnetic field?
What is the largest magnetic field we can generate?
What is the largest magnetic field in nature?
Angle
Force on a
current-carrying
conductor in a
magnetic field, N
Magnetic field
between
Current in
element, A
element and
field,
o
Length of element, m
strength, T
Write down the equation.
Define the terms.
Rearrange to show the different version you may use.
Fields or particles
F = BIL sinθ is written
in terms of the current
in the wire.
The current is the
result of moving charge
carriers.
The equation can be
changed to reflect this
What are the two forms of the magnetic force equation?
What is the purpose of the two forms?
How does this affect the units of magnetic force?
Angle
Force on a
between
charge moving
in a magnetic
Drift speed,
field, N
m s-1
Charge, C
element
and field,
Magnetic field
strength, T
Write down the equation.
Define the terms.
Rearrange to show the different version you may use.
o
5.4 Magnetic effects of current
Write down your best
answer to this question.
Include any key words or
diagrams you think are
necessary.