Electricity and Circuits
Core concepts
Charge is measured in coulombs; current (amperes) is the rate of charge flow; voltage (volts) is potential difference; resistance (ohms) opposes current. Ohm's law states V = IR. In series circuits, current is the same everywhere, voltage divides, and resistances add. In parallel circuits, voltage is the same across branches, current divides, and total resistance is less than the smallest branch. Power is P = VI. Conductors allow charge flow; insulators resist it. Know the difference between conventional current and electron flow, and the function of fuses, switches, and batteries.
Traps to watch
- Adding parallel resistances as if in series.
- Assuming current is "used up" around a series loop.
- Mixing up voltage and current when reading a meter.
- Forgetting that a broken series circuit stops all current, while a broken parallel branch does not.
- Using P = VI without checking units.
Worked example
A 12 V battery drives a 4 Ω resistor. I = V/R = 3 A. Power = VI = 36 W. If two 4 Ω resistors are in series, total R = 8 Ω and I = 1.5 A. In parallel, total R = 2 Ω and I = 6 A.
Rule of thumb
Series: same current, add resistance. Parallel: same voltage, reciprocal resistance. Always check whether the circuit is closed.