Ohm’s law links three quantities: V = IR, where V is the potential difference in volts, I is the current in amperes and R is the resistance in ohms. Identify which two you know, and solve for the third.
This lesson belongs to the SPM Physics electricity section. After it, go on to calculating electrical energy and power and circuits with more than one component.
How do I rearrange V = IR safely?
Write the three forms once, and check them with units.
| Find | Formula | Units check |
|---|---|---|
| Voltage | V = I × R | A × Ω = V |
| Current | I = V ÷ R | V ÷ Ω = A |
| Resistance | R = V ÷ I | V ÷ A = Ω |
A formula triangle helps some students. A units check helps everyone, because the wrong form produces units that make no sense.
Worked example: two calculations
A resistor of 15 Ω carries a current of 0.40 A. The potential difference is V = IR = 0.40 × 15 = 6.0 V.
A second circuit has 12 V across a 48 Ω resistor, and the question asks for the current in milliamperes.
- I = V ÷ R = 12 ÷ 48 = 0.25 A.
- Convert to milliamperes: 0.25 × 1 000 = 250 mA.
Check the pair: 250 mA = 0.25 A, and 0.25 × 48 = 12 V. The numbers agree.
The mistakes that cost marks
Two slips are common. The first is to substitute milliamperes directly. The second is to invert the formula.
| Step | Wrong | Right |
|---|---|---|
| Current given as 150 mA, V = 4.5 V | Use 150 directly | Convert: 150 mA = 0.15 A |
| Calculate R | 4.5 ÷ 150 = 0.03 Ω | 4.5 ÷ 0.15 = 30 Ω |
| Formula form | R = I ÷ V | R = V ÷ I |
If an ordinary lamp comes out at 0.03 Ω, stop and recheck. A lamp’s resistance is normally tens or hundreds of ohms.
Check yourself
A lamp has a potential difference of 3.0 V across it and a current of 120 mA. Find its resistance.
Answer
Convert the current: 120 mA = 0.12 A.
R = V ÷ I = 3.0 ÷ 0.12 = 25 Ω.
Check: 0.12 × 25 = 3.0 V, which matches the given value.
What to study next
Continue with calculating electrical energy and power, which uses the same quantities. Then work through the electricity practice set. The simple circuit calculation tutor gives more practice with V, I and R.
For a teacher to check your working on questions like these, see online one-to-one Physics tuition.