4. Electrical Power and Energy

Electrical power is the rate at which a device uses (or converts) electrical energy. Power is measured in watts (W).

\[ P = V \times I \]
where \(P\) = power (W), \(V\) = voltage (V), \(I\) = current (A).

Combined with Ohm's Law (\(V = IR\)), two more useful forms appear:
\[ P = I^2 R \qquad P = \frac{V^2}{R} \]

Worked example: A toaster draws 10 A from a 120 V outlet. What is its power?
\[ P = V \times I = 120 \times 10 = 1{,}200 \text{ W} = 1.2 \text{ kW.} \]

Electrical energy is power multiplied by time:
\[ E = P \times t \]
Energy is measured in watt-hours (Wh) or kilowatt-hours (kWh) on electricity bills.

Worked example: A 100 W light bulb runs for 5 hours. How much energy does it use?
\[ E = 100 \text{ W} \times 5 \text{ h} = 500 \text{ Wh} = 0.5 \text{ kWh.} \]

Cost calculation: if electricity costs \$0.12 per kWh, running the bulb costs \(0.5 \times 0.12 = \$0.06\).

Fuses and circuit breakers protect circuits from overloads. When too much current flows, the fuse wire melts (or the breaker trips), breaking the circuit before wires overheat and start fires.