Worked example: a 12 V car headlamp

Let's put Ohm's Law to work on a real, concrete circuit. A car headlamp circuit uses a 3 Ω resistive bulb. What current flows, and how much power does the lamp dissipate?

1. Given. A 12 V car battery ($V = 12\,\text{V}$) connected across a 3 Ω resistive headlamp ($R = 3\,\Omega$). Find current $I$ and power $P$.
2. Equation. Ohm's Law rearranged for current is $I = V / R$. Power follows from $P = V \cdot I$.
3. Substitution.
   $$I = \frac{12\,\text{V}}{3\,\Omega}$$
4. Result. $I = 4\,\text{A}$, so $P = 12\,\text{V} \cdot 4\,\text{A} = 48\,\text{W}$. A typical car headlamp draws 4 A and burns 48 W — heat mostly, light as a side effect.