Electricity and Magnetism
Electricity and Magnetism tests current, potential difference, resistance, Ohm’s law, series and parallel circuits, electrical power and energy, plus magnetic fields, electromagnets, the motor effect and electromagnetic induction.
Core rules and concepts
- Current I = Q/t is the rate of flow of charge; conventional current is from positive to negative. Ammeters go in series; voltmeters go in parallel.
- Ohm’s law: V = IR for a conductor at constant temperature. Resistance R = V/I.
- Series: same current, voltages add, R_total = R₁ + R₂ + … Parallel: same voltage, currents add, 1/R_total = 1/R₁ + 1/R₂ + …
- Electrical power P = VI = I²R = V²/R. Energy E = Pt.
- Magnetic field lines run from north to south outside a magnet. A current-carrying wire has a circular field; a solenoid behaves like a bar magnet.
- The motor effect: a current-carrying conductor in a magnetic field experiences a force. Electromagnetic induction: a changing magnetic field can induce a voltage in a conductor.
Worked example
A 6.0 Ω resistor and a 3.0 Ω resistor are connected in parallel across a 12 V supply. Find the total resistance, total current and power. For two resistors in parallel: R = (6.0 × 3.0)/(6.0 + 3.0) = 18/9 = 2.0 Ω. Total current I = V/R = 12/2.0 = 6.0 A. Power P = VI = 12 × 6.0 = 72 W. Check: branch currents are 12/6.0 = 2.0 A and 12/3.0 = 4.0 A, which add to 6.0 A. Reasoning: parallel branches have the same voltage but share the current.
Traps that cost marks
- Connecting an ammeter in parallel or a voltmeter in series.
- Thinking total parallel resistance is larger than the smallest resistor; it is smaller.
- Using V = IR for the whole circuit when V is across only one component.
- Confusing conventional current with electron flow.
- Forgetting that transformers and generators depend on changing magnetic fields, not steady ones.