P2.24SD0 · Section P2: Electricity

Mains electricity

Key learnings

Power P = I x V; fuse from I = P / V
Power is the rate of energy transfer (energy per second; unit watt, W = J/s), so $P = E/t$ , and for a mains appliance $P = I \times V$ ( $P$ in W, $I$ in A, $V$ in V). To select a fuse, rearrange to $I = P/V$ and choose the smallest standard rating (3 A, 5 A, 13 A) just ABOVE the operating current. A 920 W hairdryer on 230 V draws $920 / 230 = 4.0$ A, so a 5 A fuse is fitted — a rating below the current blows at once; far above, it fails to protect.
Energy E = I x V x t; the electrical units
Energy transferred is $E = I \times V \times t$ , giving joules (J) when $t$ is in SECONDS ( $I$ in A, $V$ in V); since $P = IV$ this is also $E = P \times t$ . Recognise the electrical units: p.d. in volts (V), current in amperes (A), resistance in ohms (Ω), power in watts (W), energy in joules (J). Domestic energy uses the kilowatt-hour: $1\text{ kWh} = 1000\text{ W} \times 3600\text{ s} = 3.6 \times 10^{6}$ J. Always convert time to seconds for an answer in joules.
Mains safety devices
A current has a heating effect: charge through a resistor transfers electrical energy to thermal energy, so too large a current overheats wires (fire risk). Protection: insulation stops shock; double insulation — two layers (or an insulating case) so no earth wire is needed; earthing — the earth wire takes a fault current from a metal case to ground (0 V), blowing the fuse; a fuse melts above its rating; a circuit breaker is a resettable switch, faster than a fuse.

Common mistakes

Drawn from real examiner reports.

Circuit breaker: "faster", not "safer"

A circuit-breaker-versus-fuse comparison must give a specific difference. Credited: it is resettable (a blown fuse must be replaced) and it operates faster / is more sensitive. Vague words — safer, better, more reliable — score nothing; they do not say HOW it differs. Do not swap the two: it is the fuse, not the breaker, that holds a thin wire which melts.

Nov 2024 1P Q3c

Convert mA to A before P = I x V

Currents are often quoted in milliamps, but $P = IV$ and $E = IVt$ need amps: divide the mA value by 1000. So 250 mA = 0.25 A, and at 12 V the power is $0.25 \times 12 = 3.0$ W — not $250 \times 12 = 3000$ W (1000 times too large). Examiners also penalise a final value given to fewer significant figures than the data, especially in a "show that" question.

Jun 2024 1P Q7b(ii)-(iii)

Double insulation needs TWO layers

Answering only that a device is "insulated" or "covered in plastic" is too vague for the second mark. The mark-scheme idea is two independent layers of insulation (or an insulating case) between the user and any live part, so no earth wire is needed. Note that earthing and insulation guard specifically against electric shock — not vague "harm", and not thermal insulation.

Nov 2024 1P Q3a

Power (W) is not energy (J)

Power is the rate of energy transfer (energy per second), measured in watts (W); energy is measured in joules (J). Writing "power is the energy used" scores nothing. A frequent slip is quoting a power in watts as the answer to an energy question — forgetting to multiply by time in $E = IVt$ — which loses the energy mark.

Fuse blows when the current is too HIGH

A fuse melts and breaks the circuit when the current rises ABOVE its rated value. Saying only "when the current increases" is too vague and is not credited (a "surge of current" is accepted). The fuse sits in the LIVE wire, so breaking it disconnects the live supply from the appliance.

Jun 2023 1P Q3b(i)

a.c. reverses direction continuously

Mains is a.c. — the current CONTINUOUSLY reverses (changes) direction (50 times per second in the UK). An a.c. description that omits "continuously changing direction" loses a mark. A cell or battery supplies d.c., which flows in one direction only; never describe mains as d.c.

Jun 2023 1P Q12b(i)

Exam tips

Fuse-selection scaffold
For a fuse question: find the operating current $I = P/V$ (in amps), then choose the smallest standard fuse (3 A, 5 A, 13 A) just ABOVE it — never below. Work in amps and seconds throughout, and state the unit on the final answer.
Substitute first; watch sig figs
Substitute the data BEFORE rearranging, so a method mark is secured even if the algebra slips. In a "show that" question quote the final value to MORE significant figures than the number given, or lose the mark; elsewhere give at least as many significant figures as the data.
Convert to SI units first
Most lost calculation marks here come from units, not physics. Before substituting into $P = IV$ or $E = IVt$ , convert current to amps (mA divided by 1000) and time to seconds (minutes times 60, hours times 3600). Then the answer comes out in watts or joules as required.

Revision notes