Physics / Unit 9: Electricity and Circuits
9.4 Resistance and Ohm's Law
Notes
Resistance $R$ opposes current. It is measured in ohms ($\Omega$).
- Ohm's law: $V = IR$. For a fixed resistance, current is proportional to voltage.
- A wire's resistance is larger when it is longer, thinner or hotter, and depends on the material.
- Example: a 12 V battery across a $4 \text{ } \Omega$ resistor gives $\displaystyle I = \frac{12}{4} = 3 \text{ A}$.
Practice
A $6 \text{ } \Omega$ resistor carries 2 A. What voltage is across it?
$V = IR = 2 \times 6 = 12 \text{ V}$.
A toaster draws 10 A from a 120 V outlet. What is its resistance?
$\displaystyle R = \frac{V}{I} = \frac{120}{10} = 12 \text{ } \Omega$.
A copper wire is replaced by a copper wire twice as long with the same thickness. Its resistance:
Resistance is proportional to length.
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