Electricity
Circuits, current, voltage, resistance, power, and domestic electricity.
In this topic
Circuit Basics
Electric current is the flow of charge (electrons) around a circuit. It requires a complete circuit and a source of potential difference (voltage).
Key quantities: - Current (I): rate of flow of charge, measured in amperes (A) - Potential difference (V): the 'push' that drives current, measured in volts (V) - Resistance (R): opposition to current flow, measured in ohms (Ω)
Ohm's Law: V = IR
Charge: Q = It (charge = current × time)
Key Points
- Current is the same at all points in a series circuit
- Voltage is shared between components in series
- In parallel: current splits, voltage is the same across each branch
- Resistance increases with temperature for most conductors
Example Questions
2A 6V battery is connected to a 3Ω resistor. Calculate the current.
V = IR → I = V/R = 6/3 = 2 A
[2 marks]
2A current of 0.5 A flows for 2 minutes. Calculate the charge transferred.
Q = It = 0.5 × 120 = 60 C (remember to convert minutes to seconds)
[2 marks]
Series and Parallel Circuits
Series circuits: components connected in a single loop. - Current is the SAME everywhere - Voltage is SHARED between components (adds up to supply voltage) - Total resistance = R₁ + R₂ + R₃...
Parallel circuits: components connected in separate branches. - Voltage is the SAME across each branch - Current is SHARED between branches (adds up to total) - Total resistance is LESS than the smallest individual resistance - 1/R_total = 1/R₁ + 1/R₂ + 1/R₃...
Key Points
- Adding resistors in series increases total resistance
- Adding resistors in parallel decreases total resistance
- Ammeters are connected in series
- Voltmeters are connected in parallel
Example Questions
2Two resistors (4Ω and 6Ω) are connected in series to a 20V battery. Calculate the current.
Total R = 4 + 6 = 10Ω. I = V/R = 20/10 = 2 A
[2 marks]
2Two resistors (6Ω and 3Ω) are connected in parallel. Calculate the total resistance.
1/R = 1/6 + 1/3 = 1/6 + 2/6 = 3/6 = 1/2. R = 2Ω
[2 marks]
Electrical Power and Energy
Power is the rate of energy transfer.
P = IV (power = current × voltage) P = I²R (power = current² × resistance) P = V²/R
Energy transferred: E = Pt (energy = power × time) E = QV (energy = charge × voltage) E = IVt
Domestic electricity: - UK mains supply: 230V, 50Hz AC - Most electronics use DC (direct current) - Live wire: brown, carries current at high voltage - Neutral wire: blue, completes the circuit - Earth wire: green/yellow, safety wire
Key Points
- Power is measured in watts (W) — 1W = 1 J/s
- Energy is often measured in kilowatt-hours (kWh) for domestic use
- Cost = energy (kWh) × price per kWh
- 1 kWh = power (kW) × time (hours)
Example Questions
2A 2000 W kettle is used for 3 minutes. Calculate the energy transferred in kJ.
E = Pt = 2000 × 180 = 360,000 J = 360 kJ
[2 marks]
3A 100 W light bulb is left on for 8 hours. Electricity costs 15p per kWh. Calculate the cost.
Energy = 0.1 kW × 8 h = 0.8 kWh. Cost = 0.8 × 15 = 12p
[3 marks]