Electricity and Magnetism
Circuits and current
Current is the rate of flow of charge (I = Q/t) in amperes; potential difference is energy per unit charge (V = W/Q) in volts. In series, current is the same everywhere and voltages add; in parallel, voltage is the same across branches and currents add. Know the symbols for cells, resistors, lamps, ammeters (series) and voltmeters (parallel). A frequent trap is placing an ammeter in parallel or a voltmeter in series.
Ohm's law and power
For an ohmic conductor at constant temperature, V = IR. Use P = VI = I²R = V²/R and energy E = Pt. A filament lamp is non-ohmic: its resistance rises as it heats. Calculate combined resistances: series add, parallel use 1/R = 1/R₁ + 1/R₂. Always sketch the circuit and mark known values before substituting.
Magnetism and electromagnetism
Know magnetic field patterns around bar magnets and current-carrying wires, and the motor effect: a current in a magnetic field experiences a force, whose direction is given by Fleming's left-hand rule. Electromagnetic induction produces a current when a conductor cuts magnetic field lines (Faraday). Transformers change alternating voltage: Vp/Vs = Np/Ns, and are efficient at high voltage because power loss in cables is I²R.
Worked example
A 12 V supply drives two 6 Ω resistors in parallel. Combined resistance: 1/R = 1/6 + 1/6 = 1/3, so R = 3 Ω. Total current I = V/R = 12/3 = 4 A. Power P = VI = 12 × 4 = 48 W. Each branch carries 2 A, confirming the parallel current rule.