Formula grid
| Quantity | Equation | Units |
|---|---|---|
| Power | $P=IV$ | W, A, V |
| Energy | $E=IVt$ | J, A, V, s |
| Ohm’s law | $V=IR$ | V, A, Ω |
| Charge | $Q=It$ | C, A, s |
| Energy (from charge) | $E=QV$ | J, C, V |
Series
Same $I$. Voltages add. $R_{\text{T}}=R_1+R_2$. One break turns everything off.
Parallel
Same $V$ on each branch. Currents add at a junction. House lights: one can fail, others stay on.
Components
- Resistor / wire: straight I–V through origin; $R$ constant.
- Filament lamp: curve; $R$ rises as it heats.
- Diode: current one way only after a threshold.
- LDR: $R$ down when light up.
- Thermistor: $R$ down when temperature up.
Traps
- Fuse = next rating above calculated $I$. Too large is unsafe.
- Convert mA, kW, minutes first.
- Electrons: negative → positive. Conventional current: the other way.
- Ammeter series, voltmeter parallel.
- Current is not used up in series.
- A lamp is not ohmic.