Electricity and Magnetism
What this domain tests
Electric current, resistance and resistivity, EMF and internal resistance, DC networks, capacitors, electric fields, magnetic forces, electromagnetic induction and alternating current. This practice separates source EMF from terminal potential difference, applies circuit conventions consistently and explains induced-current directions without asserting a particular target paper structure.
Current, resistance and circuits
- I = nAve links current to drift velocity; V = IR; R = ρL/A, so resistance rises with length and falls with cross-sectional area.
- EMF is the energy converted per unit charge by the source; for a source delivering current, terminal p.d. is V = E − Ir in the stated internal-resistance model.
- Kirchhoff: ΣI = 0 at a junction; around any closed loop ΣEMF = Σp.d.
- A potential divider gives Vout = Vin × R₂/(R₁ + R₂) for ideal (high-impedance) meters.
Fields, capacitors and magnetism
- Uniform field: E = V/d; point charges obey Coulomb's law; field lines run from positive to negative.
- Capacitor: Q = CV, energy = ½CV² = ½QV; discharge follows exponential decay with time constant RC.
- Force on a current: F = BIL sinθ; on a moving charge: F = Bqv sinθ, always perpendicular to v.
- Flux Φ = BA cosθ; induced EMF = −N dΦ/dt; Lenz's law says the induced current opposes the change producing it.
- For sinusoidal AC: Irms = I₀/√2, Vrms = V₀/√2; transformers obey Vp/Vs = Np/Ns for an ideal transformer.
Worked example
A cell of EMF 12 V and internal resistance 0.5 Ω drives a 5.5 Ω resistor. Find the current, terminal p.d. and efficiency.
I = E/(R + r) = 12/6.0 = 2.0 A. Terminal p.d. = E − Ir = 12 − (2.0 × 0.5) = 11 V. Power in load = I²R = 4 × 5.5 = 22 W; internal power = I²r = 2 W. Efficiency = 22/24 = 91.7%.
Traps that cost marks
- Quoting EMF as the terminal p.d. when current flows.
- Saying current is "used up" around a circuit - energy is transferred, charge is not consumed.
- Using peak values where RMS is required, or the reverse.
- Getting Lenz's law backwards and predicting the wrong induced current direction.
- Writing capacitor energy as CV² instead of ½CV².
- Adding parallel resistances directly instead of using reciprocals.