Electricity, magnetism, light, sound and waves
This domain covers circuits, magnets, ray diagrams and wave behaviour. Expect Ohm's law calculations, series/parallel comparisons, refraction and the wave equation.
Circuits
Current (A) is the rate of charge flow; voltage (V) is energy per charge; resistance (Ω) opposes current. Ohm's law: V = IR. In series the current is the same everywhere, voltages add and resistances add. In parallel the voltage is the same across each branch and currents add. Power P = VI = I²R; energy in kWh = power (kW) × time (h). Ammeters are connected in series, voltmeters in parallel.
Magnetism
Magnets have north and south poles; like poles repel. A current-carrying wire has a magnetic field; a solenoid with an iron core is an electromagnet; a motor uses the force on a current in a magnetic field.
Light and sound
Light reflects with the angle of incidence equal to the angle of reflection. It refracts towards the normal on entering a denser medium and away from the normal on leaving. Convex lenses converge light and can form real images; concave lenses diverge. White light disperses into a spectrum. Sound needs a medium, travels faster in solids, and is a longitudinal wave; frequency determines pitch and amplitude determines loudness. For all waves v = fλ. The electromagnetic spectrum, long to short: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
Worked example
A 12 V battery is connected across a 3 Ω resistor. I = V/R = 12/3 = 4 A. Power = VI = 12 × 4 = 48 W. Two 3 Ω resistors in series give 6 Ω, so the current falls to 2 A.
Traps
- Swapping the series and parallel rules.
- Drawing refraction bending the wrong way.
- Using v = fλ with frequency in kHz without converting to Hz.