Electricity and Magnetism
Electricity and Magnetism
Electricity and Magnetism is Section D. It tests charge, current, potential difference, resistance, circuit rules, electrical energy, magnetism, the motor effect and electromagnetic induction.
Core rules and concepts
- Current and charge. Current is rate of flow of charge: I = Q/t. Conventional current is from positive to negative. Potential difference is energy per unit charge: V = W/Q.
- Ohm's law and resistance. R = V/I for an ohmic conductor at constant temperature. A steeper current–voltage graph means lower resistance.
- Series circuits. Current is the same everywhere; voltages add; R_total = R1 + R2 + ...
- Parallel circuits. Voltage is the same across each branch; currents add; 1/R_total = 1/R1 + 1/R2 + ...
- Power and energy. P = VI = I²R = V²/R; energy E = Pt. The kilowatt-hour is a practical energy unit.
- Magnetism. 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. Fleming's left-hand rule gives the force on a conductor in a magnetic field; electromagnetic induction depends on changing magnetic flux.
Worked example
A 6 Ω resistor and a 3 Ω resistor are connected in parallel across a 12 V supply. Find the equivalent resistance and the total current. Reasoning: for parallel resistors, 1/R = 1/6 + 1/3 = 1/6 + 2/6 = 3/6 = 1/2, so R = 2 Ω. The supply voltage is across each branch, so I = 12/2 = 6 A. Check: branch currents are 12/6 = 2 A and 12/3 = 4 A, which add to 6 A. The equivalent resistance is smaller than either resistor, as expected for a parallel combination.
Traps that cost marks
- Adding parallel resistances directly instead of using reciprocals.
- Assuming total voltage divides equally in a series circuit when resistances differ.
- Connecting an ammeter in parallel or a voltmeter in series.
- Mixing mA and A, or kW and W, without converting.
- Saying a magnetic field line points from south to north outside a magnet.