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.
- 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,
- 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
- 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.