What you'll learn
This guide covers everything you need to know about electricity and circuits for your Edexcel GCSE Physics examination. You'll master the fundamental concepts of current, voltage and resistance, learn to analyse series and parallel circuits, and apply these principles through calculations and practical contexts. These topics form a substantial portion of Paper 1 and are frequently tested through calculation, explanation and investigation questions.
Key terms and definitions
Current (I) — the rate of flow of electrical charge through a conductor, measured in amperes (A). One ampere equals one coulomb of charge passing a point per second.
Potential difference (V) — the energy transferred per unit charge between two points in a circuit, measured in volts (V). Also called voltage.
Resistance (R) — the opposition to current flow in a component or conductor, measured in ohms (Ω). Greater resistance means less current for a given voltage.
Ohmic conductor — a component that obeys Ohm's law, maintaining constant resistance as current changes. The I-V graph is a straight line through the origin.
Power (P) — the rate of energy transfer in an electrical component, measured in watts (W). One watt equals one joule per second.
Direct current (d.c.) — current that flows in one direction only, supplied by cells and batteries.
Alternating current (a.c.) — current that repeatedly reverses direction, supplied by mains electricity at 50 Hz in the UK.
Charge (Q) — a property of matter measured in coulombs (C). Moving charges constitute an electric current.
Core concepts
Current, voltage and resistance fundamentals
Electric current flows when charge carriers (usually electrons) move through a conductor. In metal conductors, free electrons drift from negative to positive, though conventional current is defined as flowing from positive to negative.
The relationship between current, voltage and resistance is given by Ohm's law:
V = I × R
Where:
- V = potential difference in volts (V)
- I = current in amperes (A)
- R = resistance in ohms (Ω)
This equation applies to all components, but only ohmic conductors maintain constant resistance regardless of current. Non-ohmic components like filament lamps and diodes have resistance that varies with current or voltage.
Current is related to charge flow by:
Q = I × t
Where:
- Q = charge in coulombs (C)
- I = current in amperes (A)
- t = time in seconds (s)
This tells us that one ampere represents one coulomb of charge passing a point every second.
I-V characteristics of components
Different components produce distinctive current-voltage graphs:
Resistor (at constant temperature)
- Straight line through the origin
- Constant resistance (ohmic behaviour)
- Doubling voltage doubles current
Filament lamp
- Curved line through the origin
- Resistance increases as temperature rises
- Current increases less than proportionally with voltage
- Atoms vibrate more at higher temperatures, impeding electron flow
Diode
- Negligible current in reverse bias (negative voltage)
- Rapid current increase above threshold voltage (~0.6V for silicon) in forward bias
- Very low resistance when conducting
- Used for rectification (converting a.c. to d.c.)
Light-dependent resistor (LDR)
- Resistance decreases as light intensity increases
- Used in automatic lighting circuits and light sensors
- Not typically shown as an I-V graph but as resistance vs light intensity
Thermistor (NTC type)
- Resistance decreases as temperature increases
- Used in temperature sensors and thermostats
- Common in fire alarms and electronic thermometers
Series and parallel circuits
Understanding how current, voltage and resistance behave in different circuit configurations is essential.
Series circuits:
- Current is the same at all points: I₁ = I₂ = I₃
- Voltage is shared between components: V_total = V₁ + V₂ + V₃
- Total resistance equals sum of individual resistances: R_total = R₁ + R₂ + R₃
- If one component fails, the whole circuit breaks
Parallel circuits:
- Voltage is the same across each branch: V₁ = V₂ = V₃
- Current splits between branches: I_total = I₁ + I₂ + I₃
- Total resistance is less than the smallest individual resistance
- For two resistors: 1/R_total = 1/R₁ + 1/R₂
- Components work independently; if one fails, others continue
In practical contexts, household lighting uses parallel circuits so each light can be switched independently and all receive 230V mains voltage.
Electrical power and energy
Power represents the rate of energy transfer in electrical components. Two key equations apply:
P = I × V
P = I² × R
Where:
- P = power in watts (W)
- I = current in amperes (A)
- V = potential difference in volts (V)
- R = resistance in ohms (Ω)
The second equation derives from combining P = IV with V = IR and is particularly useful when resistance is known.
Energy transferred depends on power and time:
E = P × t
E = Q × V
Where:
- E = energy in joules (J)
- P = power in watts (W)
- t = time in seconds (s)
- Q = charge in coulombs (C)
- V = potential difference in volts (V)
For practical calculations involving electricity costs, energy is often measured in kilowatt-hours (kWh). One kWh equals 3.6 million joules.
Cost = power (kW) × time (hours) × price per kWh
A typical Caribbean or UK household might pay 15-35 pence per kWh depending on location and tariff.
Mains electricity and the National Grid
UK and Caribbean territories using British standards operate mains electricity at:
- Voltage: 230V a.c.
- Frequency: 50 Hz
The three-pin plug contains three wires with specific functions:
Live wire (brown) — carries alternating voltage at 230V; poses electrocution risk
Neutral wire (blue) — completes the circuit; normally at 0V but still potentially dangerous
Earth wire (green/yellow stripes) — safety wire connected to metal case; carries current to earth if fault occurs, triggering the fuse or circuit breaker
Fuses and circuit breakers protect circuits by breaking the connection if current exceeds a safe threshold. This prevents overheating, fire and electric shock.
Fuse ratings (3A, 5A, 13A) should be slightly higher than the normal operating current of an appliance. To calculate required fuse rating:
I = P/V
Where P is the appliance power and V is mains voltage (230V).
The National Grid transmits electrical energy from power stations to consumers. Electricity is transmitted at very high voltages (up to 400,000V) to reduce energy losses.
Power loss in transmission cables is given by P = I²R. For a given power transmitted (P = IV), increasing voltage allows lower current. Since power loss depends on I², halving current reduces power loss to one quarter. Step-up transformers increase voltage at power stations; step-down transformers reduce it for safe domestic use.
Practical circuits and applications
Sensing circuits combine LDRs or thermistors with fixed resistors in potential divider arrangements. As the variable resistor changes resistance, the voltage across it changes, triggering outputs like alarms or switching circuits.
For example, a fire alarm uses a thermistor in series with a fixed resistor connected across a power supply. At normal temperatures, most voltage drops across the thermistor (high resistance). When temperature rises, thermistor resistance falls, so more voltage appears across the fixed resistor, activating an alarm circuit.
Light-dependent circuits work similarly with LDRs, useful in street lighting that switches on automatically at dusk.
Worked examples
Example 1: Ohm's law calculation
Question: A 6Ω resistor is connected to a 12V battery. Calculate the current flowing through the resistor. (2 marks)
Solution: Use V = I × R, rearranging for I:
I = V/R = 12/6 = 2 A ✓✓
Mark scheme: 1 mark for correct rearrangement or substitution; 1 mark for correct answer with unit.
Example 2: Series circuit analysis
Question: Three resistors of 2Ω, 3Ω and 5Ω are connected in series to a 15V power supply.
(a) Calculate the total resistance. (1 mark) (b) Calculate the current in the circuit. (2 marks) (c) Calculate the potential difference across the 5Ω resistor. (2 marks)
Solution:
(a) R_total = 2 + 3 + 5 = 10Ω ✓
(b) I = V/R = 15/10 = 1.5 A ✓✓
(c) V = I × R = 1.5 × 5 = 7.5 V ✓✓
Mark scheme: (a) 1 mark for correct addition. (b) 1 mark for correct substitution, 1 mark for answer with unit. (c) 1 mark for correct method, 1 mark for answer with unit.
Example 3: Power and energy
Question: A kettle rated at 2.2 kW operates on 230V mains supply.
(a) Calculate the current drawn by the kettle. (2 marks) (b) Calculate the energy transferred in 5 minutes of operation. (3 marks) (c) If electricity costs 25p per kWh, calculate the cost of boiling the kettle for 5 minutes. (2 marks)
Solution:
(a) I = P/V = 2200/230 = 9.57 A ✓✓ (Accept 9.6 A)
(b) Convert time: 5 minutes = 300 seconds ✓ E = P × t = 2200 × 300 = 660,000 J (or 660 kJ) ✓✓
(c) Time in hours = 5/60 = 0.0833 hours Cost = 2.2 × 0.0833 × 25 = 4.58p ✓✓ (Accept 4.6p or 5p)
Mark scheme: (a) 1 mark for rearrangement, 1 mark for answer. (b) 1 mark for time conversion, 1 mark for substitution, 1 mark for answer. (c) 1 mark for method, 1 mark for answer.
Common mistakes and how to avoid them
Confusing current and voltage — Remember current is the flow of charge (like water flowing), while voltage is the energy per unit charge (like water pressure). They are different quantities with different units.
Adding resistances incorrectly in parallel — Don't just add R₁ + R₂. Use 1/R_total = 1/R₁ + 1/R₂, then find the reciprocal. Total resistance in parallel is always less than the smallest individual resistance.
Wrong fuse selection — Calculate I = P/V first, then choose the next fuse rating above this value. A fuse too low will blow unnecessarily; too high won't provide protection.
Unit conversion errors — Always convert minutes to seconds (×60), kW to W (×1000), or hours to seconds (×3600) before calculation. Write down conversions explicitly to avoid mistakes.
Misidentifying wire colours — Learn the modern UK standard: brown = live, blue = neutral, green/yellow = earth. Older wiring used different colours but questions use current standards.
Assuming all components are ohmic — Only some resistors at constant temperature obey Ohm's law. Filament lamps, diodes, LDRs and thermistors are non-ohmic.
Exam technique for "Electricity and Circuits"
"Calculate" questions require working — Always show the formula, substitution with numbers and units, then the answer. You can gain method marks even with arithmetic errors. A naked answer gets zero marks if wrong.
Draw circuit diagrams carefully — Use standard symbols from the exam specification. Make connections clear, especially in series vs parallel arrangements. A badly drawn circuit loses marks in drawing tasks and may lose understanding marks in calculations.
Explain using cause and effect — Questions asking "explain why" need logical chains: "When temperature increases, thermistor resistance decreases, so current increases (using V = IR with constant V)." Don't just state facts without connecting them.
Practical questions test understanding, not memory — You may face unfamiliar contexts (tropical cooling systems, solar panels, etc.) but the physics is identical. Extract information from diagrams and apply standard principles. Read voltage/current values carefully from meters or graphs.
Quick revision summary
Current is charge flow (Q = It); voltage is energy per charge; resistance opposes current (V = IR). In series circuits, current is constant and voltages add; in parallel, voltage is constant and currents add. Power P = IV = I²R; energy E = Pt = QV. Ohmic conductors have constant resistance; filament lamps and diodes don't. UK mains operates at 230V, 50Hz a.c. The National Grid transmits at high voltage to minimize power losses (P = I²R). Always show working in calculations, use correct units, and apply physics systematically to unfamiliar contexts.