Electricity and Magnetism
Domain 4: Electricity and Magnetism
This domain tests electrostatic force and field, potential, capacitance, direct-current circuits, magnetic forces, and electromagnetic induction. Expect quantitative circuit items and direction questions answered with the right-hand rule and Lenz's law.
Core rules
- Coulomb's law: F = k|q1q2|/r². Electric field E = F/q; field lines point away from positive charge and toward negative charge.
- Potential difference is work per charge: V = W/q. Along an equipotential, no work is done; field lines are perpendicular to equipotentials.
- Ohm's law: V = IR, valid for ohmic conductors at constant temperature. Power: P = IV = I²R = V²/R.
- Series: same current, voltages add, Req = R1 + R2 + …. Parallel: same voltage, currents add, 1/Req = 1/R1 + 1/R2 + …; equivalent resistance is less than the smallest branch.
- Kirchhoff's laws: junction rule (charge conservation) and loop rule (energy conservation).
- Magnetic force on a moving charge: F = qvB sin θ, direction by right-hand rule. Induced emf depends on changing magnetic flux; Lenz's law says the induced current opposes the change.
Worked example
A 12 V ideal battery connects a 4 Ω resistor in series with a parallel pair of 6 Ω and 3 Ω. The parallel equivalent is (6×3)/(6+3) = 2 Ω, so total resistance is 6 Ω and total current is I = 12/6 = 2 A. The parallel-pair voltage is IR = 2 × 2 = 4 V. Branch currents are 4/6 = 0.67 A and 4/3 = 1.33 A; they add to 2 A.
Traps that cost marks
- Thinking current is used up around a series circuit; charge is conserved.
- Assigning the same voltage to series resistors.
- Forgetting that an ammeter goes in series and a voltmeter in parallel.
- Reversing the right-hand rule for negative charges.
- Treating induced current as opposing flux itself rather than the change in flux.