Electric current
The rate of flow of charge
Potential difference
work done (energy transferred) per unit charge
Resistance
a measure of how difficult it is for charge carriers to pass through a component
Potential difference equation
Work done (W)/ Charge (Q)
Resistance equation
Potential difference(V) / Current (I)
Energy change equation
number of kg x joules per kg difference
Graph of energy coulomb vs distance
The closer one (like) charge is to another...
More work was done to get it there, because it had to be ushed further against an increasingly bigger force and so the more EPE
Electrical potential energy
for ONE particular object in one place
Electrical potential
tells you the energy of ANY object at that place, as long as you know its charge
Electrical potential definition
Electrical potential energy per coulomb
The nearer a charge is to another...
the greater the electrical potential it has
Energy transferred (J) equation
Number of Coulombs x Potential energyE = QV
Pushing charges of the same sign...
gives it energy, we know this because when you let it go it rushes away and gains kinetic energy. That energy must have already been there as potential energy.
The nearer the two charges...
the bigger the forces pushing them apart
The faster one chaeged particle rushes away from the other...
the more energy it has
What is charge?
A basic property of matter
What are the two forms of charge?
Positive and negative
In most electric circuits what do electrons do?
Move and transfer energy
What objects can be charged up?
Insulators
What objects conduct charge (away to Earth)
conductors
What is charge measured in
Coulombs (C), amps per second
Charge of one electron
-1.6x10^-19C
What is current?
The flow of charge
What is current measured in
Amps (A), coulombs per second
Is current a fundamental unit?
Yes, the base electrical quantity is current all other electricl quantities derive from it
Current equation
change in charge/ change in time ( I = ∆ Q/ ∆ t )
When one ampere flows
one coulomb of charge passes a given point in a circuit each second
Ohm's law
for an ohmic conductor, current is directly proportional to the potential difference across it, given that physical conditions are kept constant
Potential difference (voltage) equation interms of ohm's law
V = IR
Current voltage graph for semiconductor diodes
semiconductor diode forward bias
is the direction in which it will allow current to flow easily past the threshold voltage (the smallest voltage needed for current to flow, usually 0.7V)
semiconductor diode reverse bias
In the direction of the reverse bias, the resistance off the diode is extremely high meaning that only a very small current can flow.
semiconductor diode forward bias resistance
The current gets very big quickly as resistance decreases
What happens at ~ -50V for semiconductor diode reverse bias
the diode breaks down so the current increases
Current voltage graph & voltage current graph for filament lamp
As current increases for the filament lamp
This component heats up as current increasesSo the resistance of this component increases as current increases
filament lamp at low currents vs higher (ohm's law)
At low currents the metal wire will not heat up significantly therefore for very low current, Ohm’s law is obeyed, but as current increases (in either direction) the graph begins to curve due to the increasing resistance (due to the wire heating up)
Negative current direction vs positive direction for IV & VI graph for filament lamp
Negative current direction is the exact same thing in the positive direction but reversed and so the graph is symmetrical
The higher the resistance of the component
the hotter and brighter it will get
The current passing through the filament of the light bulb is the same as the current passing through the leads.So why does the filament glow and heat up while the leads do not?
The filament has much higher resistance than the leads.It is made of a different metal (tungsten rather than copper), it is longer, it is thinner and it works at a higher temperature.
Light dependent resistor (LDR)
Resistance decreases as light intensity increases
Thermistor
Resistance decreases as temperature increases
Light emitting diode (LED) note for diagram
for diodes the direction of the triangle is the direction of current
Diagram for Diode, LED, Thermistor and LDR
Resistance equation (in terms of resistivity)
resistivity x length/cross-sectional area (R=ρ *L/A)
Resistivity equation
resistance x cross-sectional area/length (ρ = R * A/L)
Resistivity definition
is the nature of the material to resist current (measured in ohm m ), it’s a way to compare different materials to each other
Resistance of a component in a circuit meaning
is a measure of the difficulty of making current pass through the component
What does resistance do?
Resistance restricts the flow of charge so it makes the current smaller
What is resistance caused by?
the repeated ‘collisions’ between the charge carriers in the materials with each other and with the fixed positive ions in the materials
When a metal wire is placed in an electric circuit, the potential difference from the battery...
makes the electrons flow through the wire (current). As they do so, they collide with the metal ions and this slows down the flow.
What does resistance indicate?
the potential difference needed to make 1 amp of current flow.
If a component has a high resistance (pd)
a large potential difference is needed for a given current
Effect of temperature on the resistivity of metals
Increasing the temperature of a metal causes the ions in the wire to vibrate faster and more as they gain KE. This increases the number of collisions the charge carriers ( in metals conduction electrons)and so are likely to have which slows them down resulting in a lower current Higher temperature -> lower current -> higher resistivity
Effect of temperature on the resisitivity of semi-conductors
Increasing the temperature in a semi-conductor causes an enormous release of new charge carriers. This reduces resistivity dramatically.Higher temperature -> higher charge carrier density -> higher current -> lower resistivity
Factors impacting resisitance
Length, cross sectional area, type of wire
How does length impact resistance?
the longer the wire the larger the resistance as electrons collide with more ions as they pass through
How does the cross sectional area impact resistance?
the larger the cross sectional area the smaller the resistance, as there are more routes for charge carriers/ electrons to travel and less dense metal
How does the type of wire impact resistace?
conductivity or resistivity of wire varies from wire to wire
Resistors in series equation
Rtotal = R1+ R2 + R3...
Resistors in parallel equation
1/RT = 1/R1 + 1/R2 + 1/R3 ...
energy equation
Current x Potential difference x time
Electrical power equation
Current x Potential difference = Current^2xResistance = Potential difference^2/Resistance
P = IV = I^2R = V^2/ R
current in a series circuit
The current is the same everywhere in the circuit
potential difference in a series circuit
The battery potential difference is shared across all elements in the circuit, therefore the total sum of the voltages across all elements is equal to the supply potential differenceThe total voltage across the cells is equal to the sum of the individual voltages of the cells : VT = V1 + V2 + V3 …
current in a parallel circuit
The sum of the currents in each parallel set of branches is equal to the total current
Potential difference in a parallel circuit
The potential difference across each branch is the sameThe total voltage is equal to the voltage of one cell. This is because the current is split equally between branches, therefore the overall potential difference is the same as if the total current was flowing through a single cell: VT = V1 = V2 = V3…
In DC circuits what is conserved?
charge and energy are always conserved
Kirchoff's first law
the total current flowing into a junction is equal to the current flowing out of that junction. This shows that no charge is lost at anypoint in the circuit, conservation of charge principle
Kirchoff's second law
around any closed loop in a circuit the (vector) sum of all voltages (potential differences) in a series circuit is equal to 0. This shows that no energy is lost at any point in a circuit, Conservation of energy principle.
What is a potential divider?
is a circuit with several resistors in series connected across a fixed voltage source, used to produce a required fraction of the source potential difference, which remains constant
Potential divider in a circuit properties
The potential difference of the source is divided between the components in the circuitThe ratio of the voltages is equal to the ratio of the resistors
Why would you use a potential divider?
We use a potential divider circuit when we want to use only part of the voltage provided by the batteryThe potential divider used to supply constant or variable potential difference from a power supply
What can variable resistors be used for?
Variable resistors can be used to trigger certain events.
How does an LDR work?
in the circuit a LDR (resistivity decreases as light intensity increases, when it gets darker the resistance increases) is used, as the light intensity falls the resistance across R1 will increase so the circuit current decreases and resistance across R2 decreases, so the potential difference decreases
If you want this effect reversed switch the position of the LDR and resistor and so the potential difference out would increase as light intensity decreased and the circuit could go on to cause a light bulb to be switch on because a threshold voltage has been met.
Real life uses of LDR
fridge
What does a temperature sensor consist of?
A temperature sensor consists of a thermistor and a variable resistor with potential difference source
What does a potentiometer do and how does it work?
To supply a variable output pd, the input is connected to a length of uniform resistance bare wire. A sliding contact on the wire can then be moved along the wire giving a variable output pd between the contact and the one end of the wire.
potentiometer diagram
Potentiometer used as a dimmer switch diagram
Do batteries have an internal resistance?
yes, batteries have an internal resistance which is caused by electrons colliding with atoms inside the battery
consequences of batteries having an internal resistance
some energy is lost before electrons even leave the battery, It is represented as a small resistor inside the battery
what is electromotive force?
is the energy transferred by a cell per coulomb of charge that passes through it (the amount of energy put in the circuit per coulomb of charge)
what devices give out emf
Any device putting energy into a circuit is providing an electromotive force (emf)
what do devices taking in emf have?
any device taking it out emf has a potential difference across it
can emf be recorded in a broken circuit?
yes
when the cell is supplying no current... (emf)
terminal potential difference = emf
When the cell supplies current
the terminal potential difference decreases, the larger the current you draw the less terminal potential difference
Electromotive force equations
ε = E/ Q = V(terminal potential) + v(lost volts) = I(R+r)
The potential difference available to the circuit equations
V = ε - Ir
in a V against I graph what is r (internal resistance) and ε
V against I graph r = -gradient, and the ε is the y-intercept
terminal potential difference equation
V = IR
lost volts equation
v=Ir
Power supplied by the cell equation
Power supplied by the cell = εI = I2(R+r)This shows that the power supplied by the cell = power delivered to the load + power wasted in the cell (due to internal resistance)
power (external resistor)
Power = ε2R/R+r
total resistance equation
Internal resistance + load resistance = Total resistance (r + R = RT)
What is terminal potential difference (V)?
The potential difference across the resistor R is known as the terminal potential (V) difference
What is lost volts?
the potential difference across the resistor is known as lost volts (v) as this value is equal to the energy wasted by the cell per coulomb of charge
Power vs resistance graph
When is maximum power delivered to the load?
the maximum power is delivered to the load when the load resistance is equal to the internal resistance
adding cells in series (EPE)
When two cells are connected in series there is an increase in EPE, therefore there is a greater voltage and the the voltage of the cells add up
Adding cells in parallel (EPE)
When two cells are connected in parallel the EPE remains the same as a single cell but twice as much electric charge moves so the cell will operate twice as longThe voltage is the same as a single cell
positives of having a cell in series
Cells connected in series would provide a higher potential difference therefore could drive more components
positives of having a cell in parallel
The same number of cells in parallel would deliver the same amount of electrical power as in series P=IV
In a parallel circuit there is more than one path for the current to flow along. When cells are connected in parallel the total value of the cells is the same as each individual cell. For two 1.5 V cells in parallel, the voltage stays at 1.5 V, but the life of the battery is doubled.
negatives of having a cell in series
a higher potential difference is dangerous as electric shocks can be more severe
negatives of having cells in parallel
can cause overheating
coulomb definition
One coulomb (C) is defined as the amount of charge that passes in 1 second if the current is 1 ampere (A)
semiconductor characteristics and uses
Semiconductors are nowhere near as good at conducting electricity as metals. This is because there are far, far
fewer charge carriers available. However, if energy is supplied to the semiconductor, more charge carriers can
be released. This means that they make excellent sensors for detecting changes in their environment.
You need to know about the two semiconductor components on the next page — thermistors and diodes.
what is an ntc?
‘Negative Temperature Coefficient’.
This means that the resistance decreases
as the temperature goes up.
Describe the shape of a current-voltage characteristic graph for a filament lamp.
the graph is curved strating steep and then levellin off as voltage and current increases
Explain, with reference to charge carriers, how an NTC thermistor connected in a circuit
can be used as a temperature sensor.
As the temperature increases more charge carriers are release.
More charge carriers available allows more current to flow in the crcuit.
So the current in the circuit can be used to monitor the temperature
How can you lower the resistance of many materials?
cooling them down
What happens when a material is cooled down to below the transition temperature?
Ther resistivity disappears entirely and they become a superconductor
Without any resistance none of the electrical resistance is turned into heat, so none of it's wasted. That means you can start a current flowing in a circuit using a magnetic field, then once the magnet is taken away the current would carry on flowing forever
Uses of supreconductors
Power cables that transmit electricity without any loss of power.
Really strong electromagnets that don’t need a constant power source
(for use in medical applications and Maglev trains).
Electronic circuits that work really fast, because there’s no resistance to slow them down.
When does the brightness of a lamp increase
Current through it increases so power through it imcreases
Explain, in terms of charge carriers, why the resistance of the thermistor decreases as the temperature rises
Extra charge carriers released as the temperature rises
Increased thermal agitation of atoms resists flow of charge carriers
First effect overwhelms the second
Kirchoff's second law
In a given closed loop, the sum of the potential differences is equal to the sum of the EMFs: ΣP Di = ΣEMFi
explain how changes in temperature cause a change in the resistance of a thermistor. For [2] an ntc thermistor. will the resistance increase or decrease as temperature rises?
thermal energy can excite electrons allowing them to cross the bandgap
so at higher temperatures we expect more electrons in the conduction band and so a lower resistance
The mechanism behind a light dependent resistor's variable resistance
light landing on the material excites electrons from valence to conduction band and so more electrons are free to carry charge and the effective resistance is lowered