Electricity and Electromagnetism
Electricity and Electromagnetism tests Ohm's law, resistor networks, electric power and the magnetic effect of current. Expect numericals on series and parallel circuits plus qualitative questions on electromagnets and induction.
Current electricity
- Electric current is rate of flow of charge: I = Q/t, measured in ampere. Potential difference is work per unit charge, measured in volt.
- Ohm's law: V = IR at constant temperature. Resistance R = rho l/A; longer and thinner wires have more resistance.
- In series, current is the same and resistances add. In parallel, voltage is the same and 1/R = 1/R1 + 1/R2 + ...
- Power P = VI = I^2R = V^2/R. Electrical energy in kWh = power in kW x time in hours.
Magnetic effects
- A current-carrying conductor produces a magnetic field. The field direction follows the right-hand thumb rule; a solenoid behaves like a bar magnet.
- Electromagnetic induction: a changing magnetic flux through a coil induces an emf. Fleming's rules give directions for motors and generators.
Worked example
A 6 ohm and a 3 ohm resistor are connected in parallel across 6 V. Find total current and total power. Parallel resistance: 1/R = 1/6 + 1/3 = 1/2, so R = 2 ohm. Total current I = V/R = 6/2 = 3 A. Power P = VI = 6 x 3 = 18 W. Branch currents are 1 A and 2 A, confirming the total.
Traps
- Adding parallel resistances directly instead of using reciprocals.
- Placing an ammeter in parallel or a voltmeter in series.
- Using V^2/R with the wrong voltage in a series circuit.
- Confusing motor and generator directions of energy conversion.