Current, potential difference and resistance
Current is the flow of charge: Q = It. Potential difference is work done per coulomb. Ohm’s law: V = IR, for an ohmic conductor at constant temperature. Resistance increases with temperature in a filament lamp. A diode only allows current in one direction. An LDR’s resistance falls as light intensity rises; a thermistor’s resistance falls as temperature rises.
Series and parallel
- Series: current the same; PD shared; resistances add.
- Parallel: PD the same; current splits; total resistance is less than the smallest branch.
Mains electricity
UK mains is about 230 V a.c. at 50 Hz. The live wire carries the alternating PD, the neutral completes the circuit, and the earth is a safety wire. The fuse and earth protect the user if an appliance becomes live. Power P = VI = I²R.
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Taught with support from Joseph Danso, Science Lecturer in London. Book a session.