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Chapter 2: Electricity
2.1-Current and Circuit Symbols
Current is the flow of Electrical Charge
Electric current is a flow of electrical charge.
Electrical charge will only flow round a complete circuit if there is a potential difference.
The unit of current is the ampere,A.
In a single, closed loop the current has the same value everywhere in the circuit.
Potential difference is the driving force that pushes the charge round.
Its unit is the volt,V
Resistance is anything that slows the flow down. Unit:ohm
The current flowing through a component depends on the potential difference across it and the resistance of the component
Total Charge through a circuit depends on Current and Time
The size of the current is the rate of flow of charge.
When current flows past a point in a circuit for a length of time then the charge that has passed is given by this formula
Formula: Q=It
More charge passes around the circuit when a larger current flows
Learn these Circuit Diagram Symbols
You need to be able to understand circuit diagrams and draw them using the correct symbols.
Make sure all the wires in your circuit are straight lines and that the circuit is closed.
2.2-Resistance and V=IR
There’s a formula linking Potential Difference and Current
Potential Difference=Current x Resistance
You can investigate the factors affecting resistance:
The resistance of a circuit can depend on a number of factors, like whether components are in series or parallel, or the length of wire used in the circuit.
You can investigate the effect of wire length using the circuit:
The Ammeter: Measures the current flowing through the test wire.
The ammeter must always be placed in series with whatever you’re investigating
The Voltmeter: Measures the potential difference across the test wire.
The voltmeter must always be placed in parallel around whatever you’re investigating, NOT around the other bit of the circuit.
There can be parallel or series circuits
Measuring the length of wire per resistance:
Attach a crocodile clip to the wire level with 0cm on the ruler
Attach the second crocodile clip to the wire.
Write down the length of the wire between the clips
Close the switch, then record the current through the wire and the pd across it
Open the switch then move the second crocodile clip.
Close the switch again, then record the new length, current and pd
Repeat this for a number of different lengths of the test wire
Use your measurements of current and pd to calculate the resistance for each length of wire,
using R=V/I
Plot a graph of resistance against wire length and draw a line of best fit
Your graph should be a straight line through the origin, meaning resistance is directly proportional to length
the longer the wire, the greater the resistance
If your graph doesn’t go through the origin, it could be because the first clip isn’t attached exactly at 0cm, so all of your length readings are a bit out
2.3-Resistance and I-V Characteristics
Ohmic conductors have a constant resistance
The resistance of ohmic conductors doesn’t change with the current.
At a constant temperature, the current flowing through an ohmic conductor.
At a constant temperature. the current flowing through on ohmic conductor is directly proportional to the potential difference across it.
The resistance of some resistors and components DOES change
When an electrical charge flows through a filament lamp, it transfers some energy to the thermal energy store of the filament which is designed to heat up.
Resistance increases with temperature, so as the current increases, the filament lamp heats up more and the resistance increases.
For diodes, the resistance depends on the direction of the current.
They will happily let the current flow in one direction, but have a very high resistance if it is reversed
Three very important I-V Characteristics
The term I-V Characteristics refers to a graph which shows how the current flowing through a component changes as the potential difference across it is increased.
Linear components have an I-V characteristic that’s a straight line.
Non-linear components have a curved I-V characteristic.
You can do the experiment by:
Set up the test circuit shown in the diagram:
Begin to vary that variable resistor.
This alters the current flowing though the circuit and the potential difference across the component
Take several pair of readings from the ammeter and voltmeter to see how the potential difference across the component varies as the current changes.
Repeat each reading twice more to get an average pd at each current
Swap over the wires connected to the cell, so the direction of the current is reversed
Plot a graph of current against voltage for the component
The I-V characteristics you get for an ohmic conductor, filament lamp and diode should look like this:
The calculate the resistance you can do: R=V/I
2.4-Circuit Devices
LDR is short for Light Dependence Resistor
An LDR is a resistor that is dependent on the intensity of light.
In bright light, the resistance falls
In darkness, the resistance is highest
They have lots of applications
including automatic night lights, outdoor lighting and burglar detectors
The resistance of a Thermistor depends on Temperature
A thermistor is a temperature dependent resistor
In hot conditions, the resistance drops
In cool conditions, the resistance goes up
Thermistors make useful temperature detectors, temperature sensors and electronic thermostats
You can use LDRs and Thermistors in Sensing Circuits
Sensing circuits can be used to turn on or increase the power to components depending on the condition that they are in
The circuit on the right is a sensing circuit used to control a fan in a room
The fixed resistor and the fan will always have the same potential difference across them
The pd of the power supply is shared out between the thermistor and the loop made up of the fixed resistor and the fan according to their resistances
the bigger a component’s resistance, the more of the pd it takes
As the room gets hotter, the resistance of the thermistor decreases and it takes a smaller share of the pd from the power supply.
So the pd across the fixed resistor and the fan rises, making the fan go faster
You can also connect the components across the variable resistor instead.
For examples, if you connect a bulb in parallel to an LDR, the pd across both the LDR and the bulb will be high when it’s dark and the LDR’s resistance is high.
The greater the pd across a component, the more energy it gets.
So a bulb connected across an LDR would get brighter as the room got darker
2.5-Series Circuits
Series Circuits-All or Nothing
In series circuits, the different components are connected in a line, end to end, between the +ve and -ve of the power supply
except for voltmeters, which are always connected in parallel, but they don’t count as part of the circuit
If you remove or disconnect one component, the circuit is broken and they all stop.
This is generally not very handy, and in practice very few things are connected in series
You can use the following rules to design series circuits to measure quantities and test components
Potential Difference is Shared
In series circuits the total pd of the supply is shared between the various components.
So the potential difference round a series circuit always add up to equal the source pd:
V(total) = V1 + V2 +…
Current is the same everywhere
In series circuits the same current flows through all components: R(total) = R1 + R2
The size of the current is determined by the total pd of the cells and the total resistance of the circuit
I = V / R
Resistance Adds Up
In series circuits the total resistance of two components is just the sum of their resistance.
This is because by adding a resistor in series, the two resistors have to share the total pd.
The potential difference across each resistor is lower, so the current through each resistor is also lower.
In a series circuit, the current is the same everywhere so the total current in the circuit is reduced when a resistor is added.
This means the total resistance of the circuit increases.
The bigger a component’s resistance, the bigger its share of the total potential difference
Cell Potential Difference Adds Up
There is a bigger pd when more cells are in series, if they’re all connected the same way.
For example when two cells with a potential difference of 1.5V are connected in series they supply 3V between them.
2.6-Parallel Circuits
Parallel Circuits-Independence and Isolation
In Parallel Circuits, each components is separately connected to the +ve and -ve of the supply, except ammeters, which are always connected in series.
If you remove or disconnect one of them, it will hardly effect the others at all
This obviously how most things must be connected, for example in cars and in household electrics.
You have to be able to switch everything on and off separately
Everyday circuits often include a mixture of series and parallel parts
Potential Difference is the Same Across all Components
In parallel circuits all components get the full source pd, so the potential difference is the same across all components
This means that identical bulbs connected in parallel will all be at the same brightness
Current is Shared between Branches
In parallel circuits the total current flowing around the circuit is equal to the total of all the currents through the separate components
In a parallel circuit, there are junctions where the current either splits or rejoins.
The total current going into a junction has to equal the total current leaving
If two identical components are connected in parallel then the same current will flow through each component
Adding a Resistor in Parallel Reduces the Total Resistance
If you have two resistors in parallel, their total resistance is less than the resistance of the smallest of the two resistors
This can be tough to get your head around, but think about it like this:
In parallel, both resistors have the same potential difference across them as the source
This means the ‘pushing force’ making the current flow is the same as the source pd for each resistor that you add
But by adding another loop, the current has more than one direction to go in
This increases the total current that can flow around the circuit.
Using V=IR, an increase in current means a decrease in the total resistance of the circuits
2.7-Investigating Resistance
You can Investigate adding Resistors in series
First, you’ll need to find at least four identical resistors
Then build the circuit shown on the right using one of the resistors.
Make a note of the potential difference of the battery
Measure the current through the circuit using the ammeter.
Use this to calculate the resistance of the circuit using R=V/I
Add another resistor, in series with the first
Again, measure the current through the circuit and use this and the potential difference of the battery to calculate the overall resistance of the circuit
Repeat steps 4 and 5 until you’ve added all of your resistors
Plot a graph of the number of resistors against the total resistance of the circuit
Or in Parallel
Using the same equipment as before, build the same initial circuit
Measure the total current through the circuit and calculate the resistance of the circuit using R=V/I
Next, add another resistor, in parallel with the first
Measure the total current through the circuit and use this and the potential difference of the battery to calculate the overall resistance of the circuit
Repeat steps 3 and 4 until you’ve added all of your resistors
Plot a graph of the number of resistors in the circuit against the total resistance
Your results should match the Resistance Rules
You should find that adding resistors in series increases the total resistance of the circuit
The more resistors you add, the larger the resistance of the whole circuit
When you add resistors in parallel, the total current through the circuit increases-so the total resistance of the circuit has decreased
The more resistors you add, the smaller the overall resistance becomes
These results agree with what you’ve learnt about resistance in series and parallel circuits.
2.8-Electricity in the Home
Mains supply is ac, Battery supply is dc
There are two types of electricity supplies- alternating current(ac) and direct current(dc)
In ac supplies the current is constantly changing direction. Alternating currents are produced by alternating voltages in which the positive and negative ends keep alternating
The UK mains supply is an ac supply at around 50Hz
By contrast, cells and batteries supply direct current
Direct current is a current that is always flowing in the same direction. It’s created by a direct voltage.
Most cables have Three Separate Wires
Most electrical appliances are connected to the mains supply by three-core cables.
This means that they have three wires inside them, each with a core of copper and a coloured plastic coating.
The colour of the insulation on each cable shows its purpose
The colours are always the same for every appliance. This is so that it is easy to tell the different wires apart.
You need to know the colour of each wire, what each of them is for and what their pd is:
LIVE WIRE=brown. The live wire provides the alternating potential difference(about 230V) from the mains supply
NEUTRAL WIRE=blue. The neutral wire completes the circuit-when the appliance is operating normally, current flows through the live and neutral wires, at 0V
EARTH WIRE=green and yellow. It is for protecting the wiring, and for safety-it stops the appliance casing from becoming live.
It doesn’t usually carry a current-only when there’s a fault. It’s also at 0V
The Live Wire can give you an Electric Shock
You body is at 0V.
This means that if you touch the live wire-a large potential difference is produced across your body and a current flows through you.
This causes a large electric shock which could injure or even kill you
Even if a plug socket or a light switch is turned off there is still a danger of an electric shock.
A current isn’t flowing but there’s still a pd in the live wire.
If you made contact with the live wire, your body would provide a link between the supply and the earth, so a current would flow through your body
Any connection between live and earth can be dangerous.
If the link creates a low resistance path to earth, a huge current will flow, which could result in a fire.
2.9-Power of Electrical Appliances
Energy is Transferred from Cells to other Sources
You know that a moving circuit transfers energy.
This is because the charge does work against the resistance of the circuit
Electrical Appliances are designed to transfer energy to components in the circuit when a current flows
Of course, no appliance transfers all energy completely usefully.
The higher the current, the more energy is transferred to the thermal energy stores of the components.
You can calculate the efficiency of any electrical appliance
Energy transferred depends on the Power
The total energy transferred by an appliance depends on how long the appliance is on for and its power
The power of an appliance is the energy that is transfers per second.
So the more energy it transfers on a given time, the higher its power
The amount of energy transferred by electrical work is given by:
Energy Transferred(J)= power(W) x time(s)
Appliances are often given a power rating-they’re labelled with the maximum safety power that they can operate at.
You can usually take this to be their maximum operating power
The power rating tells you the maximum amount of energy transferred between stores per second when the appliance is in use
This helps customers choose between models-the lower the power rating, the less electricity an appliance uses in a given time and so the cheaper it is to run
But, a higher power doesn’t necessarily mean that it transfers more energy usefully.
An appliance may be more powerful than another, but less efficient, meaning that it might still only transfer the same amount of energy to useful stores.
2.10-More on Power
Potential Difference is Energy Transferred per Charge Passed
When an electrical charge goes through a change in potential difference, then energy is transferred
Energy is supplied to the charge at the power source to ‘raise’ it through a potential
The charge gives up this energy when it ‘falls’ through any potential drop in components elsewhere in the circuit.
Formula=E = QV
Energy Transferred= Charge flow x potential difference
That means that a battery with a bigger pd will supply more energy to the circuit for every coulomb of charge which flows around it,
because the charge is raised up ‘higher’ at the start
Power also depends on Current and Potential Difference
As well as energy transferred in a given time, the power of an appliance can be found with
Power=Potential Difference x Current P=VI
You can also find the power if you don’t know the potential difference
P=I(2)R
2.11-The National Grid
Electricity is distributed via the Nation Grid
The national grid is a giant system of cables and transformers that covers the UK and connects power stations to consumers
The national grid transfers electrical power from power stations anywhere on the grid to anywhere else on the grid where it’s needed
Electricity production has to meet demand
Throughout the day, electricity usage changes.
Power stations have to produce enough electricity for everyone to have it when they need it
They can predict the most electricity will be used through.
Demand increases when people get up in the morning and when it starts to get dark.
Popular events on TV also cause peak in demand
Power stations often run at well below their maximum power output, so there’s spare capacity to cope with high demand, even if there’s an unexpected shut down of other stations
Lots of smaller power stations that can start up quickly are also kept in standby just incase
The national grid uses a high pd and a low current
To transmit the huge amount of power needed, you need either a high potential difference or a high current
The problem with a high current is that you lose loads of energy as the wires heat up and is transferred into thermal energy of the surroundings
It’s much cheaper to boost the pd really high, 400,000V, and keep the current as low as possible
For a given power, increasing the pd decreases the current, which decreases the energy lost by heating the wires and the surroundings.
This makes the national grid an efficient way of transferring energy
Potential difference is changed by a transformer
To get the voltage up for efficient transmission we use transformers
Transformers all have two coils, a primary coil and a secondary coil joined with an iron coil
Potential difference is increased using a step-up transformer.
They have more turns on the secondary coil than the primary coil.
As the pd is increased by the transformer, the current is decreased
The pd then reduced again at the local consumer end using a step-down transformer.
They have more turns on the primary coil than the secondary
The power of a primary coil is given by power=pd x current.
Transformers are nearly 100% efficient, so the power in primary coil = power in secondary coil.
This means that:
P.d. across secondary coil x current in secondary coil = p.d across primary coil x current in primary coil
2.12-Static Electricity
Build-up of static is caused by friction
When certain insulating materials are rubbed together, negatively charged electrons will be scraped off one and dumped on the other
This will leave the materials electrically charged, with a positive static charge on one and an equal negative static charge on the other
Which way the electrons are transferred depends on the two materials involved
The classic examples are polythene and acetate rods being rubbed with a cloth duster
Only electrons move- never positive charges
But +ve and -ve electrostatic charges are only ever produced by the movement of electrons.
The positive charges definitely do not move.
A positive static charge is always caused by electrons moving away elsewhere.
The material that loses the electrons loses some negative charge, and is left with an equal positive charge.
Too much static causes sparks
As electric charge builds on an object, the potential difference between the object and the earth increases
If the potential difference gets large enough, electrons can jump across the gap between the charged object and the earth
this is the spark
They can also jump to any earthed conductor that is nearby-which is why you can get static shocks getting out of a car.
A charge builds up on the car’s metal frame, and when you touch the car, the charge travels through you to earth
This usually happens when the gap is fairly small
Like charges repel, opposite charges attract
When two electrically charged objects are brought close together they exert a force on one another
Two things with opposite electric charges are attracted to each other, while two things with the same electric charge will repel each other
These forces get weaker the further apart the two things are
These forces will cause the objects to move if they are able to do so.
This is known as electrostatic attraction/repulsion and is a non-contact force
One way to see this force is to suspend a rod with a known charge from a piece of string.
Placing an object with the same charge nearby will repel with rod-the rod will move away from the object.
An oppositely charged object will cause the rod to move towards the object
2.13-Electric Fields
Electric charges create an electric field
An electric field is created around any electrically charged object
The closer to the object you get, the stronger the field is
You can show an electric field around an object using field lines.
For example, you can draw the field lines for an isolated, charged sphere:
Electric field lines go from positive to negative
They’re always at a right angle to the surface
The closer together the line, the stronger the field is
Charged objects in a electric field feel a force
When a charged object is placed in the electric field of another object, it feels a force
This force causes the attraction or repulsion
The force is caused by the electric fields of each charged object interacting with each other
The force on an object is linked to the strength of the electric field it is in
As you increase the distance between the charged objects, the strength of the field decreases and the force between them gets smaller
Sparking can be explained by electric fields
Sparks are caused when there is a high enough potential difference between a charged object and the earth
A high potential difference causes a strong electric field between the charged object and the earthed object
The strong electric field causes electrons in the air particles to be removed
Air is normally an insulator, but when it is ionised it is much more conductive, so a current can flow through it.
This is the spark