Physics
Electric circuits — Ohm's law & Kirchhoff — quick study summary
GCSE PhysicsA-Level PhysicsAP Physics 1 & 2IB Physics
An electric circuit moves charge round a closed loop, driven by a voltage source. Ohm's law: V = IR. Power P = IV = I²R = V²/R. In series, currents are equal but voltages add: R_total = R₁ + R₂ + … In parallel, voltages are equal but currents add: 1/R_total = 1/R₁ + 1/R₂ + … Kirchhoff's current law (sum at any node = 0) and voltage law (sum around any loop = 0) generalise this to any network.
Key points
- V = IR (Ohm's law); P = IV = I²R = V²/R
- Series: same I, voltages add, R_total = sum
- Parallel: same V, currents add, 1/R_total = sum of 1/R_i
- Kirchhoff's current law: ΣI in = ΣI out at any node
- Kirchhoff's voltage law: ΣV around any closed loop = 0
Practice quiz
Click each question to reveal the answer.
1. A 12 V battery drives a current of 2 A through a resistor. What's the resistance?
- 2 Ω
- 6 Ω
- 14 Ω
- 24 Ω
Answer: 6 Ω
Ohm's law V = IR → R = V/I = 12 ÷ 2 = 6 Ω.
2. Two 4 Ω resistors are connected in parallel. What's the total resistance?
Answer: 2 Ω
1/R = 1/4 + 1/4 = 1/2 → R = 2 Ω. Parallel resistance is always less than the smallest individual resistor.
3. A light bulb has resistance 6 Ω and runs at 3 A. How much power does it dissipate?
Answer: 54 W
P = I²R = 9 × 6 = 54 W.
Last reviewed: May 2026