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yr11 electricity

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Electricity
• The movement of charged particles
• Electrons – Negatively charged
particles found in atoms
• Ions – atoms which have gained or
lost electrons to get a full outer shell
and become stable
• Flow of electrons around a circuit is
called CURRENT (I) measured in amps
(A)
• Voltage (V) – Increase or decrease in
the amount of electrical energy
carried by the current.
Atoms to Ions
• Na
2,8,1
+ Cl
2,8,7
Electricity occurs in 2 different
forms:
Static
• Is stationary
– Brush your hair
– Wool socks in tumble
drier
Current
• Electrons are forced around a
complete circuit by an
electrical force field
– turn on light
– Ipod
Static Electricity
• Occurs when materials
which are usually insulators
become electrically
charged.
• This is because electrons
have been transferred
(rubbing adds or removes
electrons)
• Like charges repel, opposite
charges attract
Use this information to draw a diagram explaining how the
paper sticks to the balloon (assume your hair removes
electrons)
Conductors and Insulators
• Conductors allow the electrons to transfer through
them easily. Metals are good conductors.
• Insulators do not allow the electrons to move easily
through them. Plastic, wool, wood, rubber etc. are
good insulators
Static electricity in conductors
• Electrons can move freely
through a conductor.
• The charge can therefore be
distributed uniformly over the
surface of the conductor.
Static electricity in an insulator
• Electrons are not free to
move over the surface of an
insulator.
• A temporary polarisation of
the individual atoms can
occur.
• The charge is not uniformly
distributed over the surface
of the object.
• Polarisation is strongest
where the surface is close
to the charged object.
• Which is the conductor,
sphere A or B?
Charge distribution and shape
• Electric fields occur at 90
degrees to the surface of
the object.
• Electrons A and B repel
maximally in a flat
surface.
• Electrons C and D can
accumulate closer
because the repulsive
effect of surrounding
electrons is not as great.
Charge
accumulates
at pointed
surfaces
Excellence Questions
Consider these when answering:
• Define insulator and conductor
• State the charge on electron
• Rubbing adds/ removes electrons
• Like charges repel, opposites attract
• State direction of electron movement
• Distribution of charge in conductor and insulator
• Electrostatic FORCE is greatest in close proximity
Electroscopes
• Electroscopes are
devices that detect
static electricity.
• Explain what is
happening.
• Van De Graff generator
shows the effects of
static electricity.
Lightning
• Caused by static
• water/ ice particles rub past each other due to
convection currents. (hotter is less dense therefore
rises)
• Electrons are transferred creating charged clouds
• When the charge is very large electrons jump by
ionising the air, they take the shortest path to ground
Grounding / Earthing
• Grounding is a physical connection between an
object and the earth. This will stop an object
becoming charged.
• Explain why she gets a shock from touching the car.
Assume the car is negatively charged.
•
•
•
•
How has the car become charged?
Why is the charge evenly distributed on the car?
Why does the charge build up on the car?
Why does the area under the car become positive?
•
•
•
•
•
•
•
How has the car become charged?
Rubbing air gains electrons
Why is the charge evenly distributed on the car?
Metal is a good conductor
Why does the charge build up on the car?
Rubber tyres are a good insulator
Why does the area under the car become
positive?
• Like charges repel, electrons move away leaving
+ve
• Person (+) touches the car (-) and electrons jump
to her = grounded
fire
CURRENT ELECTRICITY
To have an electric current flow we need 2 things…
Power supply
Complete Closed circuit (use wires to join parts)
We should also have a power user, or else we have a…
short circuit, which means that the electricity is getting back to the
power source still with its energy.
This will result in the power source getting very hot, and there may
be an electrical fire.
What is wrong with
this diagram?
The source provides energy to the
electrons, which move away from
the –ve end and towards the +ve
end.
As they pass power users their
energy is used up, and they return
to the power source with none.
ELECTRIC COMPONENTS
Power sources…
Power users:
Single cell
Bulb
Battery (of 6 cells)
Resistor
Variable resistor
Power pack (variable)
Current Controllers..
Also useful…
Connecting Wires
Diode
Switch
Wires joined
LED
Current Controllers
•
•
•
•
Diodes let electrons flow in one direction
Switches allow electrons to flow or not
Fuses protect……………..
Resistor/ Rheostat control the movement of
electrons
As resistance is
increased
current flow…
decreases
CIRCUIT TYPES
The simplest type of circuit involves electricity moving down one
route. (electrons don’t choose!).
This is called a Series circuit.
Draw the path the electrons travel.
The other main type of circuit has two
or more branches.
This is called a Parallel circuit.
Draw on the electron flow.
What sort of circuit is this?
A parallel… but, more
importantly…
A short circuit.
Explain what you would observe if a
bulb breaks in each of the circuits?
A
M
E
• The lights in the series
circuit will go out but
the other light in the
parallel circuit will still
be lit
as there is an
alternative pathway
for current to flow
which means there is
still a complete circuit.
Current (I)
Current is the amount of charge past a point
over time (flow of electrons around a circuit)
I=q/t
I= current, q = charge, t = time
• To measure current we need to “count electrons” using an
ammeter
• 6.25 x1018 electrons = 1 coulomb
• 1 coulomb per second = 1 Amp
• Connect in series where you need to measure the current Red
terminals to Red and Black to Black.
• DO THE TOUCH TEST! If the needle goes backwards or off the
scale – Recheck
Current (I)
Current is the amount of charge
past a point over time
• In series circuits the current is the same
everywhere
• In parallel circuits the current is split
between the branches and adds up to the
total drawn from the supply
Voltage (V)
Change in energy per charge between
two points (energy gained or lost as it passes
through a component)
V = ΔE/q V = voltage (V), E = energy (J), q = charge (c)
• Connect a voltmeter in parallel around the component
Red terminals to Red and Black to Black.
• DO THE TOUCH TEST! If the needle goes backwards or
off the scale – Recheck
Voltage (V)
Change in energy per charge between
two points
• In series circuits the voltage is shared between
components
• Therefore voltage loss = voltage gain
• In parallel circuits the voltage is the same across
all branches
SUMMARY
In Series
Current
Always the same
In Parallel
Split between branches
The branches add to the
total
Voltage
Voltage from source =
voltage used
Voltage is shared
between power users
Voltage is the same in all
branches
Resistance (R)
• The amount that a component slows the
current
• Insulators have high resistance, whereas conductors
have low resistance.
•As the electrons are slowed by a resistor, energy is
lost in the form of heat.
•This means that current, resistance and voltage must
be linked.
•This is Ohms law V=I x R
•The unit of resistance is the ohm, symbol 
V
I × R
ELECTRICITY PROBLEMS
1. What will the voltage be in V1 and V2?
2. If the current in the lower branch is 3A,
what is the resistance in the bulb?
3. If the current at point Y is 4A, what is the
total current in the circuit?
4. What is the current through the resistor
below?
Y
Resistance question
12V
5Ω
I=V/R
I=12/5
I=2.4A
• Find the current
• If an extra 5Ω is inserted in series what is the
voltage and current through each?
• If the 5Ω is added in parallel what happens to
the voltage and current in each?
POWER
In an electrical circuit energy may be used up by components like
lamps and resistors.
The rate at which this occurs is called
Power (P).
It is measured in Watts (W).
One watt means that 1 joule of electrical energy is being used up per
second.
1000 W
Power is often given in kW (kilowatts) =
The power in a circuit depends on…
Current (I) and Voltage (V).
P
I × V
Power
• Which bulb is brightest? Why?
12v
12v
0.4A
0.5V
1
2
0.4A
3
1.4A
4
Describe the effect on battery life and bulb brightness when you
replace a 100W bulb with a 60W one.
Resistance in Series Circuits
• If we add resistances (or any component with
resistance ) the overall resistance of a circuit will
increase, this implies that the current will decrease.
• If R1 = 2 Ohm, R2 = 3 Ohm and R3 = 7 Ohm and 24v
are supplied find:
– The total resistance
– Current in the circuit
– Voltage across each resistor
What happens to P1 if R2
increases?
12 Ohm
I=V/R
I= 24/12
I = 2Amp
V=IR
V =2 x 2
V1=4volts
V2 = 6v, V3 = 14v
What effect does adding more bulbs have?
Series:
• The current in a series circuit is always the same.
• The more energy users (bulbs), the higher the
resistance and the lower the current. So the bulbs
appear dimmer.
Parallel:
• The current in the branches of a parallel circuit add to
equal the current from the power supply.
• More branches, means less resistance so…..
•
As bulbs are added they draw more current with the
same voltage and so appear the same brightness.
Effect of heat on Resistance
run
=
V
I
= gradient = R
rise
run
Current
If the resistor gets hot, the particles start to
vibrate, and the electrons find it harder to get
through, so the current drops more than
expected.
If you change the
voltage across a
resistor the current will
change. In this case
the resistance is
constant.
Voltage
Voltage
rise
Some components like LED, thermistors,
diodes make use of this resistance change. Eg
LDR is on when the resistance is low (gradient
is not steep)
Why do bulbs blow when they are cold?
Current
DIODES
Diodes only allow current to flow in one direction.
And are drawn like this in a circuit diagram:
- end
+ end
Electron flow
-
If you set up this circuit
what would you notice?
A
+
Flow allowed
Lamp A goes, B does not.
Explain what is going on.
Diode allows current to flow
in only one direction.
B
Flow Blocked
Practise question
• Mr D. wants to wire some lights on a trailer. He has 2
lamps available 12v 6W and 12V 12W
• He connects in parallel first with a 12V battery
• Draw the circuit with a switch that controls both
lamps.
• Find the current and resistance in each lamp.
• He then connects in series to see if anything changes.
He measures the current to be 0.33A.
• In series which is brighter and why?
Test Yourself
• A series and parallel circuit are built containing a 4
ohm and 8 ohm bulb and a 12v battery.
• Draw the circuits.
• Explain using the terms resistance, current and
power why the 4 ohm bulb in parallel will be brighter.
• Calculate the power of each bulb.
• The bulbs in parallel have less resistance and draw
more current. Voltage in parallel is the same as the
source which is also higher than it will be in the
series circuit. As P=IV the bulb in parallel will have
more power and be brighter.
ENERGY
Work done by a component
The total energy used in a circuit has to do with the power and…
the amount of time that it is running for.
E
Energy is measured in Joules (J), Power (W), time (s)
Questions:
P × t
1. What is the power in bulb S?
2. What is the voltage across bulb T if the power is 60W?
3. Both circuits are turned
on for 30 seconds.
How much energy is
used?
T
S
Power question
• A vacuum cleaner has
2200W of power. It is
plugged in at 240V. If it
runs for 2 mins find the
current and energy it
uses.
•
•
•
•
•
•
•
I= P/V
I= 2200/240
I= 9.2A
Time = 120s
E= Pxt
E= 2200 x 120
E= 264000J or 264KJ
Overview
Term
Letter
Definition
Measured
with
Units
Symbol
voltmeter
Volts
V
Voltage
V
Energy gained or
lost by electrons
passing through
a component
Current
I
Movement of
ammeter
charged particles
(electrons)
Amp
A
Power
P
Energy used per
second
P=V/I
Watts
W
Energy
E
Work done by a
component
E=P/t
Joules
J
Resistance
R
The amount that
a component
slows current
R=V/I
Ohms

PROBLEMS
1. Fill in the gaps in the table.
a
b
c
d
e
g
i
k
j
f
h
l
Magnetism
• Objects that contain
Iron are magnetic.
• A magnetic field is a
region where a
magnetic object
experiences a force.
• The magnetic field
moves from North to
South pole.
• The closer the lines the
stronger the force.
Compass
• A compass is a
magnetic object.
• They line up with the
magnetic field lines.
• The Earth has an Iron
core which allows it to
form a magnetic field.
• Where is the north?
• What we call the North
Pole is actually the
magnetic South pole!
Effect of Magnets
• Magnetic fields can interact.
• Like poles experience a
repulsive force so they repel.
• Opposite poles experience
an attractive force so they
attract.
Question
• Explain how and why
the magnetic fields
interact to enable this
• In your answer consider
the strength of the
magnetic force
• First nail is attracted, N
at top because N and S
attract (opposite poles
attract)
• Therefore S at position
B. This means the pin
becomes a temporary
magnet.
• The force is weaker as
you get further away.
• Repeat for pin 2
Magnetic Field around a current
carrying wire
• When a current passes
through a wire it
creates a magnetic field
around it.
• It is circular in shape
• It is stronger closer to
the wire.
• X represents current
going into the page.
• . represents current
coming towards us.
Magnetic Field Direction
• Grasp the wire with
your right hand so your
thumb points in the
direction of the
conventional current.
• Your fingers are now
pointing in the direction
of the magnetic field.
• Right Hand Grip Rule
• What is the direction of
the magnetic field?
Solenoid
• A solenoid is a coil of wire.
• This concentrates the
magnetic field so it
becomes stronger.
• Grip the solenoid so your
fingers curl in the
direction of the current,
your thumb points North.
• Use the right hand grip
rule to find the North pole
on the iron bar in this
solenoid.
N
Electromagnets
• Placing an iron core in
the solenoid creates an
electromagnet.
• Electromagnets can be
turned on or off.
• The strength can be
increased by:
– Increasing the number
of turns in the wire
– Increasing the thickness
of the iron core
– Increasing the current
Explain how the bell works
• When the switch is closed the
circuit is completed and current
flows through the electromagnetic
coil.
• The iron striker is attracted
to the electromagnet and strikes
the bell.
• This means the circuit is broken
so the coil loses its magnetism.
The spring returns the striker to its
original position which makes a
new contact and the cycle repeats
itself.
• The bell will continue to ring as
long as the switch is held closed.
Magnetic Field Strength
• B=kI/d
•
•
•
•
B= strength of magnetic field (Tesla)
K = 2.0 x 10-7 TmA-1
I= current
D = distance (metres)
• Calculate the strength of
the magnetic field created
by a 12v, 30 Ω wire at
distance of 1cm and 3cm.
• 8 x 10-6 T and 2.7 x 10-6T
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