What you'll learn
This topic covers the fundamental principles of electrical circuits, including current, voltage, resistance and power. You'll study series and parallel circuits, calculate energy transfers, and apply Ohm's law to solve circuit problems. Electricity questions appear on both Paper 1 and Paper 2 of Edexcel GCSE Physics, typically worth 15-20 marks across multiple questions.
Key terms and definitions
Current (I) — the rate of flow of charge 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, measured in ohms (Ω); higher resistance means less current flows for a given voltage.
Power (P) — the rate of energy transfer 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 the mains electricity supply at 50 Hz in the UK.
Charge (Q) — the quantity of electricity measured in coulombs (C); moved through a circuit by current.
Energy transfer (E) — work done by electrical components, measured in joules (J) or kilowatt-hours (kWh).
Core concepts
Current and charge flow
Current is the flow of electrons through a conductor. In metal wires, free electrons move from the negative terminal towards the positive terminal, though conventional current is drawn flowing from positive to negative.
The relationship between charge, current and time:
Q = I × t
Where:
- Q = charge in coulombs (C)
- I = current in amperes (A)
- t = time in seconds (s)
For example, if a current of 2 A flows for 10 seconds, the charge transferred is 2 × 10 = 20 C.
Current is measured using an ammeter connected in series with the component. Ammeters must have very low resistance to avoid affecting the circuit.
Potential difference and energy transfer
Potential difference represents the energy transferred by each coulomb of charge passing through a component. A 6 V battery transfers 6 joules of energy to each coulomb of charge.
The energy equation:
E = Q × V
Where:
- E = energy transferred in joules (J)
- Q = charge in coulombs (C)
- V = potential difference in volts (V)
Combining this with Q = I × t gives:
E = I × V × t
Potential difference is measured using a voltmeter connected in parallel across the component. Voltmeters must have very high resistance to prevent current flowing through them.
Resistance and Ohm's law
Ohm's law states that for an ohmic conductor at constant temperature, current is directly proportional to potential difference:
V = I × R
Where:
- V = potential difference in volts (V)
- I = current in amperes (A)
- R = resistance in ohms (Ω)
Rearranging: R = V/I or I = V/R
Ohmic conductors maintain constant resistance regardless of current. Examples include fixed resistors and metal wires at constant temperature.
Non-ohmic conductors have resistance that varies with current:
- Filament lamps — resistance increases as temperature rises because metal ions vibrate more, obstructing electron flow
- Diodes — allow current in one direction only; very high resistance in reverse direction
- Thermistors (NTC type) — resistance decreases as temperature increases; used in temperature sensors
- Light-dependent resistors (LDRs) — resistance decreases as light intensity increases; used in automatic lighting circuits
Series circuits
In series circuits, components are connected end-to-end in a single loop:
Current rules:
- Current is the same at all points: I₁ = I₂ = I₃
- Only one path for electrons to follow
Voltage rules:
- Total voltage equals sum of individual voltages: V_total = V₁ + V₂ + V₃
- Voltage is shared between components in proportion to their resistance
Resistance rules:
- Total resistance equals sum of individual resistances: R_total = R₁ + R₂ + R₃
- Adding more components increases total resistance
Series circuits have a disadvantage: if one component fails, the entire circuit stops working because the current path is broken.
Parallel circuits
In parallel circuits, components are connected across common points with multiple paths for current:
Current rules:
- Total current equals sum of branch currents: I_total = I₁ + I₂ + I₃
- Current splits at junctions
Voltage rules:
- Voltage is the same across each branch: V₁ = V₂ = V₃ = V_total
- Each component experiences the full battery voltage
Resistance rules:
- Total resistance is less than the smallest individual resistance
- For two resistors: 1/R_total = 1/R₁ + 1/R₂
- Adding more branches decreases total resistance and increases total current
Parallel circuits are used in household wiring because each appliance operates independently at 230 V, and one appliance failing doesn't affect others.
Electrical power
Power is the rate of energy transfer. The power equations are:
P = E/t
P = I × V
P = I² × R
P = V²/R
Where:
- P = power in watts (W)
- E = energy in joules (J)
- t = time in seconds (s)
For mains appliances, power ratings indicate energy transfer rates. A 2000 W kettle transfers 2000 J of energy per second.
Energy and efficiency
The energy transferred by an appliance:
E = P × t
For practical purposes, energy companies charge for electricity in kilowatt-hours (kWh):
Energy (kWh) = Power (kW) × Time (hours)
One kilowatt-hour is the energy used by a 1 kW appliance running for 1 hour, equivalent to 3.6 million joules (3.6 MJ).
Cost calculations:
Total cost = Energy used (kWh) × Cost per kWh
Mains electricity
The UK mains supply provides:
- Alternating current at 50 Hz frequency
- 230 V potential difference
- Delivered through live, neutral and earth wires
Three-pin plug wiring:
- Live wire (brown) — alternates between +325 V and -325 V; carries current to the appliance
- Neutral wire (blue) — remains close to 0 V; completes the circuit
- Earth wire (green and yellow stripes) — safety wire at 0 V; only carries current if a fault occurs
The fuse or circuit breaker protects the circuit by breaking the circuit if current becomes too large, preventing overheating and fires. Fuses contain a thin wire that melts when excessive current flows.
Fuse rating selection: choose the next rating above the normal operating current. For a 1150 W appliance at 230 V, current = P/V = 1150/230 = 5 A, so use a 5 A or 13 A fuse.
Double insulation eliminates the need for an earth wire. Appliances with plastic casings that cannot become live are marked with the double insulation symbol.
Worked examples
Example 1: Charge and current calculation
Question: A battery charger supplies a constant current of 1.5 A for 2 hours. Calculate the total charge transferred. [3 marks]
Solution:
Convert time to seconds: 2 hours = 2 × 60 × 60 = 7200 s [1 mark]
Use Q = I × t [1 mark]
Q = 1.5 × 7200 = 10,800 C (or 10.8 kC) [1 mark]
Example 2: Series circuit analysis
Question: A 12 V battery is connected to two resistors in series: 4 Ω and 8 Ω. Calculate: (a) the total resistance [1 mark] (b) the current in the circuit [2 marks] (c) the potential difference across the 8 Ω resistor [2 marks]
Solution:
(a) R_total = R₁ + R₂ = 4 + 8 = 12 Ω [1 mark]
(b) Rearrange V = I × R to I = V/R [1 mark] I = 12/12 = 1 A [1 mark]
(c) V = I × R [1 mark] V = 1 × 8 = 8 V [1 mark]
Example 3: Energy and cost calculation
Question: A 2.5 kW electric heater is used for 3 hours per day for 30 days. Electricity costs 15p 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 × Price per unit [1 mark]
Cost = 225 × 15 = 3375p = £33.75 [1 mark]
Common mistakes and how to avoid them
• Confusing current and charge — Current is the rate of flow of charge, not the charge itself. Remember current is measured in amperes (A), charge in coulombs (C), and Q = I × t connects them.
• Mixing up series and parallel rules — In series, current is the same everywhere and voltage adds up. In parallel, voltage is the same across branches and current adds up. Draw clear circuit diagrams and label values systematically.
• Incorrect voltmeter and ammeter placement — Ammeters must be placed in series (breaking the circuit) while voltmeters are placed in parallel (across components). Reversed placement will damage the meter or prevent the circuit working.
• Forgetting to convert units — Power is often given in kW but calculations may require W. Time may be in hours but equations need seconds. Always convert: 1 kW = 1000 W, 1 hour = 3600 s, 1 kWh = 3.6 MJ.
• Wrong fuse rating — Students often just pick 13 A for everything. Calculate the normal operating current using I = P/V, then select the next standard fuse rating above this value (typically 3 A, 5 A or 13 A).
• Misunderstanding resistance changes — Filament lamp resistance increases with temperature, causing the I-V graph to curve. Thermistor (NTC) resistance decreases with temperature. Learn the direction of change for each component type.
Exam technique for Electricity
• Command word 'calculate' requires a numerical answer with working shown. Write the equation first, substitute values with units, then calculate. For 3-4 mark calculations, show each step clearly even if using a calculator.
• Circuit diagram questions may ask you to add components or meters. Use correct symbols from the Edexcel specification: rectangle for resistor, circle with A for ammeter, circle with V for voltmeter. Draw wires as straight lines with right-angle corners.
• 'Explain' questions worth 2-3 marks require linked statements showing cause and effect. For example, explaining why adding resistors in series increases total resistance requires stating that electrons encounter more opposition (1 mark) because they must pass through all resistors in turn (1 mark).
• Rearranging equations is essential. Practice making each variable the subject: from V = I × R derive I = V/R and R = V/I. Show rearrangement on the exam paper before substituting numbers, especially for 3+ mark questions.
Quick revision summary
Current (A) is the rate of charge flow; voltage (V) is energy per unit charge; resistance (Ω) opposes current flow. In series circuits, current stays constant and voltages add; resistances add to give total resistance. In parallel circuits, voltage across branches is identical while currents add; total resistance decreases. Use Q = I × t, V = I × R, P = I × V and E = P × t for calculations. UK mains supply is 230 V a.c. at 50 Hz with live (brown), neutral (blue) and earth (green/yellow) wires. Fuses prevent excessive current causing fires.