4.2 Electricity — 8463 spec coverage

4.2.1.1

Standard circuit diagram symbols

Circuit diagrams represent components with standard symbols rather than pictures of the apparatus.
Place an ammeter in series with the component whose current is measured and a voltmeter in parallel across that component.
A valid measuring circuit has a source of potential difference, a complete conducting loop and correctly joined component symbols.
A common error is to put the voltmeter in series or the ammeter in parallel, which changes or prevents the intended measurement.

Tier 1 · Easy

3 marks

ORIGINAL

Draw a circuit diagram containing one cell, one open switch and one lamp, all connected in a single loop.

Tier 2 · Standard

4 marks

ORIGINAL

Draw a circuit that can measure both the current through a fixed resistor and the potential difference across it. Include a battery and a switch.

Tier 3 · Hard

4 marks

ORIGINAL

A student draws a resistance-measuring circuit with the ammeter connected across the resistor and the voltmeter inserted in the main loop. State both corrections and describe the corrected circuit.

4.2.1.2

Electrical charge and current

Electric current is the rate of flow of electrical charge, linked by $Q=It$ , where charge is in coulombs, current in amperes and time in seconds.
A source of potential difference and a closed circuit are needed for charge to flow; current has the same value at every point in one closed loop.
For example, a current of $0.50\,\text{A}$ for $20\,\text{s}$ transfers $Q=(0.50)(20)=10\,\text{C}$ .
A common error is to treat current as an amount of charge rather than a rate, or to use minutes directly in $Q=It$ .

Tier 1 · Easy

2 marks

ORIGINAL

A current of $0.35\,\text{A}$ flows for $40\,\text{s}$ . Calculate the charge that flows.

Tier 2 · Standard

3 marks

ORIGINAL

$240\,\text{C}$ of charge passes a point in a circuit in $3.0$ minutes. Calculate the current.

Tier 3 · Hard

4 marks

ORIGINAL

A device carries $0.75\,\text{A}$ for $80\,\text{s}$ and then $0.30\,\text{A}$ for a further $150\,\text{s}$ . Determine the total charge transferred.

4.2.1.3

Current, resistance and potential difference

Potential difference, current and resistance are linked by $V=IR$ ; at a fixed potential difference, a greater resistance gives a smaller current.
Measure current with an ammeter in series and potential difference with a voltmeter in parallel, then calculate $R=V/I$ .
For a $12\,\Omega$ component carrying $0.25\,\text{A}$ , the potential difference is $V=(0.25)(12)=3.0\,\text{V}$ .
A common error is to divide in the wrong order when finding resistance: it is potential difference divided by current, not current divided by potential difference.

Tier 1 · Easy

2 marks

ORIGINAL

A $15\,\Omega$ resistor has a potential difference of $6.0\,\text{V}$ across it. Calculate the current.

Tier 2 · Standard

4 marks

ORIGINAL

A component carries $0.24\,\text{A}$ when the potential difference across it is $3.6\,\text{V}$ . Its resistance remains constant. Calculate its resistance and the current when the potential difference is $9.0\,\text{V}$ .

Tier 3 · Hard

5 marks

ORIGINAL

A $0.80\,\text{m}$ uniform wire at constant temperature has $3.0\,\text{V}$ across it and carries $0.25\,\text{A}$ . The wire is replaced by $0.50\,\text{m}$ of the same wire. Assume resistance is proportional to length. Calculate the new resistance and current at $3.0\,\text{V}$ .

4.2.1.4

Resistors

At constant temperature an ohmic conductor has current directly proportional to potential difference; a diode conducts in one direction and has very high resistance in reverse.
Use an ammeter in series and a voltmeter in parallel, vary the supply, and calculate $R=V/I$ from corresponding readings to investigate a component.
A filament lamp's resistance rises as its filament gets hotter; thermistor resistance falls as temperature rises, and LDR resistance falls as light intensity rises.
A common error is to call every curved current-potential difference graph an experimental mistake; curvature can show that the component is non-linear.

Tier 1 · Easy

1 mark

ORIGINAL

An automatic garden light must switch on when it becomes dark. State how the resistance of its LDR changes as darkness increases.

Tier 2 · Standard

4 marks

ORIGINAL

A filament lamp carries $0.40\,\text{A}$ at $2.0\,\text{V}$ and $0.80\,\text{A}$ at $6.0\,\text{V}$ . Calculate its resistance at each potential difference and explain the change.

Tier 3 · Hard

6 marks

ORIGINAL

Describe an investigation of how the resistance of a thermistor changes with temperature. Include the circuit, measurements, one control and the processing of results.

4.2.2

Series and parallel circuits

In series, current is the same through each component, supply potential difference is shared and resistances add: $R_{\text{total}}=R_1+R_2+\ldots$ .
In parallel, potential difference is the same across each branch and total current is the sum of the branch currents.
Adding a series resistor makes charge flow through more resistance, while adding a parallel branch provides another path and reduces total resistance.
A common error is to add parallel resistances as if they were in series; this specification requires only the qualitative result that the parallel total is below the smallest branch resistance.

Tier 1 · Easy

1 mark

ORIGINAL

Two resistors of $4\,\Omega$ and $7\,\Omega$ are connected in series. Calculate their total resistance.

Tier 2 · Standard

5 marks

ORIGINAL

A $2.0\,\Omega$ resistor and a $4.0\,\Omega$ resistor are connected in series to a $12\,\text{V}$ supply. Calculate the circuit current and the potential difference across each resistor.

Tier 3 · Hard

5 marks

ORIGINAL

Two branches are connected in parallel across a $12\,\text{V}$ supply. One branch contains a $6.0\,\Omega$ resistor. The other branch carries $3.0\,\text{A}$ . Calculate the resistance in the second branch and the current from the supply.

4.2.3.1

Direct and alternating potential difference

A direct potential difference keeps one polarity, so current in the circuit has one direction; cells and batteries supply dc.
An alternating potential difference repeatedly reverses polarity, so the current also repeatedly reverses direction.
The UK mains supply is approximately $230\,\text{V}$ ac at $50\,\text{Hz}$ , meaning $50$ complete cycles each second.
A common error is to say ac merely changes size; its defining feature is that the potential difference reverses direction.

Tier 1 · Easy

2 marks

ORIGINAL

State whether a battery supplies direct or alternating potential difference, and give the defining feature of that supply.

Tier 2 · Standard

3 marks

ORIGINAL

The UK mains supply has a frequency of $50\,\text{Hz}$ . Calculate the number of complete cycles in $0.30\,\text{s}$ and describe what happens to the polarity.

Tier 3 · Hard

5 marks

ORIGINAL

Source A maintains a potential difference of $+6.0\,\text{V}$ . Source B varies between positive and negative values and completes $25$ cycles in $0.50\,\text{s}$ . Identify each supply type, calculate the frequency of B and compare the current directions they produce.

4.2.3.2

Mains electricity

In a three-core cable the live wire is brown, the neutral wire is blue and the earth wire has green-and-yellow stripes.
The live wire carries the alternating potential difference, the neutral completes the circuit, and the earth is a safety wire that carries current only during a fault.
Live-to-earth is about $230\,\text{V}$ , while neutral and earth are at or close to $0\,\text{V}$ .
A common error is to assume an open switch makes the live wire safe; if the switch is in the neutral wire, internal parts can remain connected to the live supply.

Tier 1 · Easy

3 marks

ORIGINAL

State the insulation colour of the live, neutral and earth wires in a UK three-core mains cable.

Tier 2 · Standard

4 marks

ORIGINAL

A fault makes the metal case of a mains appliance touch the live wire. Explain how the earth wire reduces the danger to a user.

Tier 3 · Hard

4 marks

ORIGINAL

A mains lamp is wired so that its switch opens the neutral wire rather than the live wire. The lamp goes out when the switch is opened. Explain why the lamp holder may still be dangerous to touch.

4.2.4.1

Power

Electrical power is the rate of energy transfer and can be calculated using $P=VI$ or $P=I^2R$ .
Choose $P=VI$ when potential difference and current are known, and $P=I^2R$ when current and resistance are known.
A device with $12\,\text{V}$ across it and current $2.0\,\text{A}$ transfers energy at $P=(12)(2.0)=24\,\text{W}$ .
A common error in $P=I^2R$ is to forget to square the current; one watt means one joule transferred per second.

Tier 1 · Easy

2 marks

ORIGINAL

A motor has a potential difference of $12\,\text{V}$ across it and a current of $3.0\,\text{A}$ . Calculate its power.

Tier 2 · Standard

2 marks

ORIGINAL

A heating resistor has resistance $8.0\,\Omega$ and carries $2.5\,\text{A}$ . Calculate the power transferred.

Tier 3 · Hard

5 marks

ORIGINAL

A mains heating element is rated at $1.8\,\text{kW}$ when connected to $230\,\text{V}$ . Calculate its current and resistance at this operating point.

4.2.4.2

Energy transfers in everyday appliances

Electrical appliances transfer energy to other stores, such as the kinetic energy of a motor or the thermal energy of a heating device.
Calculate transferred energy with $E=Pt$ or $E=QV$ , using power in watts and time in seconds for an answer in joules.
A $500\,\text{W}$ appliance used for $30\,\text{s}$ transfers $E=(500)(30)=15\,000\,\text{J}$ .
A common error is to compare appliance power ratings as if they were amounts of energy; power states how quickly energy is transferred.

Tier 1 · Easy

3 marks

ORIGINAL

A $60\,\text{W}$ fan runs for $5.0$ minutes. Calculate the energy transferred.

Tier 2 · Standard

3 marks

ORIGINAL

$9000\,\text{C}$ of charge flows through a $12\,\text{V}$ motor. Calculate the energy transferred and name the main useful energy transfer.

Tier 3 · Hard

5 marks

ORIGINAL

A $1.5\,\text{kW}$ appliance operates for $18$ minutes from a $230\,\text{V}$ supply. Calculate the energy transferred and the charge that flows through the appliance.

4.2.4.3

The National Grid

The National Grid is the system of cables and transformers that links power stations to consumers.
Step-up transformers increase potential difference for transmission, and step-down transformers reduce it to a much lower value for consumers.
For the same transferred power, $P=VI$ shows that increasing potential difference reduces current, so $P_{\text{loss}}=I^2R$ gives smaller heating losses in the cables.
A common error is to claim that a step-up transformer creates energy; it changes potential difference and current while allowing efficient energy transfer.

Tier 1 · Easy

2 marks

ORIGINAL

Name the transformer used before long-distance transmission and the transformer used before electricity enters homes.

Tier 2 · Standard

4 marks

ORIGINAL

A transmission line transfers $6.0\,\text{MW}$ at $300\,\text{kV}$ . Calculate the current in the line and explain why this is preferable to transmitting the same power at a much lower potential difference.

Tier 3 · Hard

6 marks

ORIGINAL

A cable of resistance $0.80\,\Omega$ transfers $2.4\,\text{MW}$ . Compare the power lost in the cable when transmission is at $12\,\text{kV}$ and at $240\,\text{kV}$ .

4.2.5.1

Static charge (physics only)

Rubbing certain insulating materials transfers electrons: the material gaining electrons becomes negative and the material losing them is left equally positive.
Determine the sign from electron movement, remembering that the charged particles transferred between the materials are electrons.
Like charges repel and unlike charges attract, and both effects are non-contact forces; a large build-up of charge can discharge as a spark.
A common error is to say positive charge moves onto an object during rubbing; in this model, electrons move and the positive charge is due to an electron deficit.

Tier 1 · Easy

2 marks

ORIGINAL

During rubbing, material X gains electrons from material Y. State the charge left on each material.

Tier 2 · Standard

3 marks

ORIGINAL

A negatively charged insulating strip repels a second charged strip without touching it. What does this show about the charge on the second strip and the type of force involved?

Tier 3 · Hard

5 marks

ORIGINAL

A student rubs an insulating rod with a cloth and the rod becomes negatively charged. The rod is then brought close to a metal object and a spark occurs. Explain the charging and the spark in terms of electrons.

4.2.5.2

Electric fields (physics only)

A charged object creates an electric field around itself; another charged object placed in the field experiences a force.
Draw the field around an isolated charged sphere with radial lines: arrows point away from a positive sphere and towards a negative sphere.
Closer field lines represent a stronger field, so the field and the force on a given charge are greater nearer the charged object.
A common error is to draw field lines crossing; at any point the force on a positive test charge has only one direction.

Tier 1 · Easy

2 marks

ORIGINAL

Draw the electric field pattern around an isolated positively charged sphere.

Tier 2 · Standard

3 marks

ORIGINAL

A small positive test charge is placed first near a positively charged sphere and then farther away. State the force direction in both positions and compare the force sizes.

Tier 3 · Hard

5 marks

ORIGINAL

A charged metal dome is brought progressively closer to an earthed metal sphere until a spark crosses the gap. Use the electric-field model to explain why there is a force before contact and why a spark becomes more likely as the gap decreases.