Electrical work, power, heating and circuit safety
An electric circuit transfers energy. Current tells how quickly charge moves, while voltage tells the energy transferred per unit charge.
1. Work and power
Potential difference is energy transferred per charge: U = W/q. If current I carries charge q = It through a component, the electrical energy transferred is W = UIt. Power is the transfer rate: P = W/t = UI. For a resistor that obeys U = IR, equivalent forms are P = I²R = U²/R. These forms describe the same transfer but are useful with different known quantities. Energy is measured in joules; power in watts.
2. Heating and energy use
Collisions of charge carriers with the material transfer energy to the lattice. For a constant resistive current, Joule heating is Q = I²Rt. Doubling current at fixed R and t makes four times as much heat. Electrical energy billing often uses the kilowatt-hour: 1 kWh = 3.6 × 10⁶ J. A device rated 2.0 kW transfers 2.0 kJ each second when operating at its rated conditions.