PHY042 · Electricity & Circuits · Class 12 + College
Kirchhoff voltage law
Objective: To verify Kirchhoff's voltage law: the sum of potential changes around a closed loop is zero.
🧰 Apparatus
- Cell or low-voltage supply
- Two resistors
- Voltmeter
- Ammeter
- Wires
📐 Formulas and SI units
ΣV = 0 around a loop
E = IR₁ + IR₂
💡 Theory
Energy is conserved as charge goes round a loop: the rises in potential (across cells) equal the drops (across resistors). Going round any closed loop, the algebraic sum of the voltages is zero.
🪜 Procedure
- Connect the cell and two resistors in a single loop.
- Measure the EMF of the cell with the circuit open.
- Close the circuit and measure the voltage across each resistor.
- Add the drops and compare with the cell voltage.
- Repeat with different resistors.
📋 Observations Simulated values
✅ Result and conclusion
With 12 V and 2 Ω + 4 Ω, I = 2 A and the drops are 4 V and 8 V: 12 − 4 − 8 = 0.
🔬 In a real lab
A real cell has internal resistance, so the terminal voltage is a little less than the EMF when current flows.
⚠️ Safety
Low-voltage DC only; avoid shorting the cell.
❓ Quick check
1. Kirchhoff's voltage law follows from conservation of…
2. A 9 V cell drives current through two equal resistors. Drop across each?
3. Kirchhoff's current law says the currents at a junction…
🏋️ Practice
EasyE = 6 V, drops 2 V and x. x?
Answer: 4 V
MediumE = 10 V, 3 Ω + 2 Ω. I?
Answer: 2 A
ChallengeWhy is the measured terminal voltage below the EMF?
Answer: Internal resistance of the cell