Topic 2 of 5

Ohm's law

Ohm's law links the three quantities in every circuit: push (voltage), flow (current) and opposition (resistance).

The formula

V = I × R

Cover the one you want to find. Cover V and you're left with I × R. Cover I and you get V ÷ R. Cover R and you get V ÷ I.

Worked examples

• A 46 Ω element on 230 V: I = V ÷ R = 230 ÷ 46 = 5 A
• 2 A flowing through 12 Ω: V = I × R = 2 × 12 = 24 V
• A lamp draws 0.5 A from 12 V: R = V ÷ I = 12 ÷ 0.5 = 24 Ω

What changes a conductor's resistance

• Length: double the length, double the resistance. That's why long runs have more voltage drop.
• Cross-sectional area: double the area, half the resistance. A 2.5 mm² conductor has less resistance than a 1.5 mm² one of the same length.
• Material: copper conducts better than aluminium, so it has lower resistance for the same size.
• Temperature: copper's resistance rises as it heats up.

Exam tip: if a question gives mA or kΩ, convert to A and Ω first. 230 V across 4.6 kΩ is 230 ÷ 4,600 = 0.05 A, or 50 mA.

Try it: Ohm's law calculator