Ohm's Law
Voltage across a resistor equals current times resistance.
Formula
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Example
A 9 V battery across a 3 Ω resistor drives 3 A of current.
How it works
Ohm's law says that for many ordinary conductors, the voltage needed to push charge through the material is proportional to the current that flows. The proportionality constant is the resistance, which measures how strongly the material opposes current. So a larger resistance needs more voltage for the same current, while a larger current through the same resistor requires a larger voltage.
When to use it
Use this when analyzing simple electric circuits with resistors, such as finding the current from a battery and resistor, or determining the resistor value needed for a target current. It is especially useful in basic circuit problems where the component behaves ohmically.
Worked example, step by step
Suppose a 12 V source is connected to a 4 Ω resistor. Use Ohm's law: V = IR Rearrange to find current: I = V / R Substitute the values: I = 12 / 4 I = 3 A So the current in the circuit is 3 amperes.
Common mistakes to avoid
- Mixing up voltage, current, and resistance when rearranging the formula
- Using ohm's law for components that do not behave like simple resistors
- Forgetting to keep units consistent, especially volts, amperes, and ohms
Did You Know?
Ohm's law is the single most-used relationship in all of electronics.
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