Electricity and Magnetism
This section covers Electricity and Magnetism. The exam tests charge, current, potential difference, resistance, circuits, electrical power, magnetic forces and electromagnetic induction.
Core rules
- Current is charge per unit time, I = ΔQ/Δt. Conventional current is opposite to electron drift in metals.
- Ohm's law, V = IR, applies to ohmic conductors at constant temperature. Resistance is R = V/I.
- In series, current is the same and resistances add. In parallel, potential difference is the same and 1/R_total = 1/R₁ + 1/R₂ + ...
- Electrical power is P = VI = I²R = V²/R. Energy transferred is E = Pt.
- Emf is the energy supplied per unit charge; terminal potential difference is less than emf when current flows because of internal resistance.
- A current-carrying wire in a magnetic field can experience F = BIL sinθ. Changing magnetic flux can induce an emf.
Worked example
A 6.0 Ω resistor and a 3.0 Ω resistor are in parallel, and this combination is in series with a 4.0 Ω resistor across a 12 V supply. Parallel resistance: 1/R = 1/6.0 + 1/3.0 = 1/2.0, so R = 2.0 Ω. Total resistance = 2.0 + 4.0 = 6.0 Ω. Current I = V/R = 12/6.0 = 2.0 A. The parallel section has voltage 4.0 V, so the 6.0 Ω resistor carries 4.0/6.0 = 0.67 A.
Traps
Do not add parallel resistances directly; use reciprocals. Ammeters go in series and voltmeters in parallel. Do not confuse emf with terminal potential difference. Power formulas are equivalent only for a particular component; choose the form using known quantities. Magnetic force is maximum when current is perpendicular to the field.