Electricity, Magnetism, and Waves
Circuits
Electric current is the flow of charge, measured in amperes (A); potential difference (voltage) is energy per unit charge, in volts (V); resistance opposes current, in ohms (Ω). Ohm's law: V = IR. In a series circuit, current is the same everywhere and voltage is shared; in parallel, voltage is the same across branches and current splits. Ammeters are connected in series, voltmeters in parallel. Traps include mixing up series and parallel rules and forgetting that resistance increases with more resistors in series but decreases in parallel.
Magnetism
Magnets have north and south poles; like poles repel, unlike attract. Magnetic field lines run from north to south outside the magnet. Electromagnets are made by passing current through a coil; increasing current, turns, or adding an iron core strengthens them. Motors use the force on a current-carrying wire in a magnetic field. Transformers change voltage using electromagnetic induction.
Waves
Waves transfer energy without transferring matter. Transverse waves (light, water) vibrate perpendicular to travel; longitudinal waves (sound) vibrate parallel. Key quantities: amplitude (height), wavelength (λ), frequency (f in Hz), and wave speed v = f × λ. Sound needs a medium; light does not. Light reflects with angle of incidence equal to angle of reflection; refraction bends light as speed changes between media. The electromagnetic spectrum runs from radio to gamma rays, all travelling at the speed of light in a vacuum.
Worked example
A wave has frequency 50 Hz and wavelength 4 m. Speed = f × λ = 50 × 4 = 200 m/s. If the frequency doubles to 100 Hz, speed stays the same in the same medium, so wavelength halves to 2 m.