What you'll learn
This revision guide covers all testable content for P3: Electricity in the OCR GCSE Physics specification. You'll master circuit components, current and potential difference, resistance calculations, series and parallel circuits, and the relationship between power and energy transfer in electrical devices.
Key terms and definitions
Current (I) — the rate of flow of charge around a circuit, measured in amperes (A)
Potential difference (V) — the energy transferred per unit charge between two points in a circuit, measured in volts (V)
Resistance (R) — the opposition to current flow in a component or circuit, measured in ohms (Ω)
Charge (Q) — a property of matter measured in coulombs (C); one coulomb is the charge that flows past a point when a current of 1 A flows for 1 second
Power (P) — the rate of energy transfer, measured in watts (W)
Alternating current (a.c.) — current that repeatedly reverses direction, such as UK mains electricity at 230 V and 50 Hz
Direct current (d.c.) — current that flows in one direction only, such as that supplied by batteries and cells
Ohmic conductor — a conductor that obeys Ohm's law, where current is directly proportional to potential difference at constant temperature
Core concepts
Circuit basics and charge flow
Electric current flows when charged particles (usually electrons in metals) move through a conductor. In a complete circuit, current flows from the positive terminal of a power source, through components, and back to the negative terminal.
The relationship between charge, current and time is:
Q = I × t
Where:
- Q = charge (coulombs, C)
- I = current (amperes, A)
- t = time (seconds, s)
Current is measured using an ammeter connected in series with a component. Potential difference is measured using a voltmeter connected in parallel across a component.
Key circuit components:
- Cell/battery — provides the potential difference to drive current
- Resistor — restricts current flow
- Variable resistor — allows resistance to be adjusted
- Diode — allows current to flow in one direction only
- Light-dependent resistor (LDR) — resistance decreases as light intensity increases
- Thermistor (NTC type) — resistance decreases as temperature increases
- Lamp — resistance increases with temperature as the filament heats up
Resistance and Ohm's law
For ohmic conductors at constant temperature, current is directly proportional to potential difference. This relationship is expressed as:
V = I × R
Where:
- V = potential difference (volts, V)
- I = current (amperes, A)
- R = resistance (ohms, Ω)
This equation can be rearranged to find any of the three quantities:
- I = V ÷ R
- R = V ÷ I
I-V characteristics:
Different components show different relationships between current and potential difference:
- Ohmic conductor (resistor) — straight line through the origin; constant resistance
- Filament lamp — curved line; resistance increases with current as temperature rises
- Diode — current flows only when potential difference exceeds threshold (approximately 0.6 V for silicon); very high resistance in reverse direction
These characteristics are determined experimentally by varying voltage, measuring current, and plotting graphs.
Series and parallel circuits
Series circuits:
- Current is the same at all points: I₁ = I₂ = I₃
- Total potential difference is shared between components: V_total = V₁ + V₂ + V₃
- Total resistance is the sum of individual resistances: R_total = R₁ + R₂ + R₃
Parallel circuits:
- Potential difference across each branch is the same as the supply
- Total current is the sum of currents in each branch: I_total = I₁ + I₂ + I₃
- Total resistance is less than the smallest individual resistance
- For two resistors in parallel: 1/R_total = 1/R₁ + 1/R₂
Adding resistors in series increases total resistance. Adding resistors in parallel decreases total resistance because there are more paths for current to flow.
Energy and power in circuits
When charge flows through a component, energy is transferred. The energy transferred depends on the charge and potential difference:
E = Q × V
Where:
- E = energy transferred (joules, J)
- Q = charge (coulombs, C)
- V = potential difference (volts, V)
Power is the rate of energy transfer. In electrical circuits:
P = I × V
Where:
- P = power (watts, W)
- I = current (amperes, A)
- V = potential difference (volts, V)
Alternative power equations (derived by combining P = IV with V = IR):
- P = I² × R — useful when current and resistance are known
- P = V² ÷ R — useful when voltage and resistance are known
These equations are essential for calculating energy transfer in household appliances and understanding electricity costs.
Mains electricity
UK mains electricity is supplied as alternating current (a.c.) at 230 V and a frequency of 50 Hz. This means the current changes direction 50 times per second.
Three-core cable:
- Live wire (brown) — alternates between positive and negative voltage relative to earth
- Neutral wire (blue) — completes the circuit; at or near 0 V
- Earth wire (green and yellow stripes) — safety wire connected to the metal case of appliances; at 0 V
The potential difference between live and neutral is 230 V. The live wire is dangerous even when a switch is off because it carries the supply voltage.
Electrical safety:
- Fuse — thin wire that melts and breaks the circuit if current becomes too large, preventing overheating and fire
- Circuit breaker — electromagnetic switch that trips when current exceeds a safe value; can be reset
- Earth wire — if a fault causes the live wire to touch a metal case, current flows through the earth wire to the ground, causing the fuse to blow or circuit breaker to trip
The correct fuse rating should be slightly higher than the normal operating current of the appliance.
Power and energy calculations for appliances
The energy transferred by an appliance can be calculated using:
E = P × t
Where:
- E = energy (joules, J)
- P = power (watts, W)
- t = time (seconds, s)
For practical purposes, electricity companies measure energy in kilowatt-hours (kWh):
Energy (kWh) = Power (kW) × Time (hours)
One kilowatt-hour is the energy transferred by a 1 kW appliance in 1 hour.
Cost of electricity = Energy (kWh) × Cost per kWh
Typical appliances and their power ratings:
- Kettle: 2–3 kW
- Microwave: 0.8–1.2 kW
- LED bulb: 5–15 W
- Television: 100–400 W
Worked examples
Example 1: Calculating charge flow
Question: A current of 2.5 A flows through a lamp for 4 minutes. Calculate the charge that flows through the lamp. [3 marks]
Solution:
- Convert time to seconds: 4 × 60 = 240 s [1 mark]
- Use Q = I × t [1 mark]
- Q = 2.5 × 240 = 600 C [1 mark]
Example 2: Series circuit calculation
Question: Two resistors of 15 Ω and 25 Ω are connected in series to a 12 V battery. Calculate: (a) the total resistance [1 mark] (b) the current in the circuit [2 marks] (c) the potential difference across the 15 Ω resistor [2 marks]
Solution: (a) R_total = R₁ + R₂ = 15 + 25 = 40 Ω [1 mark]
(b) Rearrange V = IR to I = V/R [1 mark] I = 12 ÷ 40 = 0.3 A [1 mark]
(c) V = I × R [1 mark] V = 0.3 × 15 = 4.5 V [1 mark]
Example 3: Electricity cost calculation
Question: A 2.5 kW electric heater is used for 3 hours each day for 30 days. Electricity costs 28p per kWh. Calculate the total cost of running the heater. [4 marks]
Solution:
- Total time = 3 × 30 = 90 hours [1 mark]
- Energy = Power × Time = 2.5 × 90 = 225 kWh [1 mark]
- Cost = Energy × Cost per unit [1 mark]
- Cost = 225 × 28 = 6300p = £63.00 [1 mark]
Common mistakes and how to avoid them
Confusing current and charge — Current is the rate of flow of charge. In Q = It, time must be in seconds, not minutes or hours. Always convert time units before calculating.
Mixing up series and parallel rules — In series, current is the same everywhere but voltage is shared. In parallel, voltage is the same across each branch but current is shared. Draw clear circuit diagrams and label known values.
Using the wrong power equation — Choose the power formula based on what information is given. If you know I and V, use P = IV. If you know I and R, use P = I²R. If you know V and R, use P = V²/R.
Incorrect unit conversion — When calculating electricity costs, power must be in kW and time in hours. A 500 W appliance is 0.5 kW. One hour is 60 minutes, not 100 minutes.
Forgetting resistances in parallel decrease total resistance — The total resistance of parallel resistors is always less than the smallest individual resistor. This is because additional paths reduce overall resistance.
Confusing live and neutral wires — The live wire (brown) carries 230 V and is dangerous. The neutral wire (blue) completes the circuit and is near 0 V. The earth wire (green/yellow) is a safety feature.
Exam technique for "P3: Electricity"
"Calculate" questions require working — Show the formula, substitution with units, and final answer. Even if your final answer is wrong, you can gain method marks. For a 3-mark calculation, typically: 1 mark for correct formula, 1 mark for substitution, 1 mark for answer with unit.
Draw circuit diagrams accurately — Use correct symbols with ruler-drawn straight lines. Ammeters go in series; voltmeters go in parallel. Show battery polarity and label component values clearly.
"Explain" and "describe" command words — "Describe" requires stating what happens. "Explain" requires stating what happens and why, often using cause-and-effect language. For example: "The fuse melts because the large current heats the thin wire."
Check your units match the formula — If energy is in joules, power must be in watts and time in seconds. If energy is in kWh, power must be in kW and time in hours. Circle the units in the question to remind yourself.
Quick revision summary
Electricity flows when charge moves through complete circuits. Current (I) is measured in amperes; potential difference (V) in volts; resistance (R) in ohms. Use V = IR, Q = It, E = QV, and P = IV to solve circuit problems. In series circuits, current is constant and voltage is shared; in parallel, voltage is constant and current is shared. UK mains electricity is 230 V a.c. at 50 Hz. Calculate energy costs using Energy (kWh) = Power (kW) × Time (h), then multiply by cost per unit.