Physical Science / Unit 10: Electricity and Magnetism
10.3 Ohm's Law and Electric Power
Notes
Ohm's law: $V = IR$, so $\displaystyle I = \frac{V}{R}$ and $\displaystyle R = \frac{V}{I}$.
- Example: 12 V across 4 $\Omega$ gives $\displaystyle I = \frac{12}{4} = 3$ A.
- Electric power: $P = IV$ in watts. Example: 2 A at 120 V is $P = 240$ W.
- Energy used = power × time (kilowatt-hours on your electric bill).
- More resistance with the same voltage means less current.
Practice
A circuit has a 9 V battery and a resistance of 3 $\Omega$. What is the current?
$\displaystyle I = \frac{V}{R} = \frac{9}{3} = 3$ A.
A 2 A current flows through a 5 $\Omega$ resistor. What is the voltage across it?
$V = IR = 2 \times 5 = 10$ V.
A device draws 3 A from a 120 V outlet. What is its power?
$P = IV = 3 \times 120 = 360$ W.
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