# 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

• 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

• 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

• 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