Electricity and Magnetism
Electricity and Magnetism tests circuit analysis, electrical power, electrostatic forces, magnetic fields, and electromagnetic induction. HKDSE questions often require you to combine Ohm’s law with series and parallel rules, interpret a graph of p.d. against current, or apply Fleming’s rules and Lenz’s law to a changing magnetic flux.
Core rules and concepts
- Current is rate of flow of charge, I = Q/t. Potential difference is energy transferred per unit charge.
- Ohm’s law, V = IR, applies to ohmic conductors at constant temperature. Filament lamps and diodes are non-ohmic.
- In series, current is the same and p.d.s add; in parallel, p.d. is the same and currents add. Equivalent resistance: series R = R₁ + R₂; parallel 1/R = 1/R₁ + 1/R₂.
- Electrical power is P = VI = I²R = V²/R. Energy transferred is Pt.
- A real cell has emf ε and internal resistance r: terminal p.d. = ε − Ir.
- A current-carrying conductor in a magnetic field may experience a force; a changing magnetic flux induces an emf, and Lenz’s law gives the direction of the induced current.
Worked example
A battery of emf 12 V and internal resistance 0.50 Ω is connected to a 5.5 Ω resistor. Find the current, terminal p.d., and power dissipated in the external resistor.
Total resistance = 5.5 + 0.50 = 6.0 Ω. Current I = ε/R_total = 12/6.0 = 2.0 A. Terminal p.d. = ε − Ir = 12 − (2.0)(0.50) = 11 V. This equals IR = 2.0 × 5.5 = 11 V. External power = I²R = (2.0)² × 5.5 = 22 W. The lost power inside the cell is I²r = 2.0 W, so total power from the cell is 24 W, matching εI.
Traps that cost marks
- Using emf as terminal p.d. when internal resistance is present.
- Adding parallel resistances directly instead of using reciprocal values.
- Applying V = IR to a diode or lamp as if resistance were constant.
- Reversing the direction in Fleming’s left-hand rule or Lenz’s law.
- Forgetting to convert mA to A or cm to m before calculating power.