Electricity, Magnetism and Thermal Physics
Circuits
Current is the rate of flow of charge. In series, current is the same everywhere and voltage splits; in parallel, voltage is the same across branches and current splits. Ohm's law: V = IR. Resistance increases with length and temperature and decreases with thickness. Electrical power: P = VI, and energy = power × time. Series resistances add; parallel resistance is less than the smallest branch.
Magnetism
Magnets have north and south poles; unlike poles attract and like poles repel. A current-carrying wire has a magnetic field, and a coil with an iron core makes an electromagnet. The motor effect occurs when a current-carrying conductor is in a magnetic field. Fleming's left-hand rule predicts the direction of force.
Thermal physics
Specific heat capacity is the energy needed to raise 1 kg of a substance by 1 °C: Q = mcΔT. Latent heat is energy absorbed or released during a change of state without a temperature change. Conduction, convection and radiation transfer thermal energy.
Worked example
A 12 V supply drives a current of 3 A through a resistor. Resistance R = V ÷ I = 12 ÷ 3 = 4 Ω. Power P = VI = 12 × 3 = 36 W. In 10 s the energy transferred is 36 × 10 = 360 J.
Thermal example
Heating 2 kg of water from 10 °C to 60 °C with c = 4200 J/kg°C: Q = mcΔT = 2 × 4200 × 50 = 420 000 J.
Traps that cost marks
In series the current is shared? No: current is the same, voltage splits. Use ΔT, not the final temperature alone. Parallel resistance is not the sum. Convert mA to A and minutes to seconds before calculating energy.