Current Electricity — Previous Year Questions (Class 12 Physics)
Current Electricity (Class 12) extends Ohm's law to EMF, internal resistance, Kirchhoff's laws, and complex circuits. JEE and CBSE focus on circuit analysi
TL;DR: Current Electricity (Class 12) extends Ohm's law to EMF, internal resistance, Kirchhoff's laws, and complex circuits. JEE and CBSE focus on circuit an…
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Current Electricity (Class 12) extends Ohm's law to EMF, internal resistance, Kirchhoff's laws, and complex circuits. JEE and CBSE focus on circuit analysi
Current Electricity — Previous Year Questions with Solutions
Q (2023, 2 marks): A cell of EMF 6 V and internal resistance 2 Ω is connected to an external resistance of 4 Ω. Find the current and terminal voltage.
Answer: EMF ε = 6 V, internal resistance r = 2 Ω, external resistance R = 4 Ω
Using ε = I(R + r):
6 = I(4 + 2)
6 = 6I
I = 1 A
Terminal voltage V = ε - Ir = 6 - 1 × 2 = 4 V
Or V = I × R = 1 × 4 = 4 V
Final Answer: Current = 1 A, Terminal voltage = 4 V
Q (2022, 2 marks): Find the equivalent resistance between points A and B in a circuit with a 1 Ω resistor in series with a parallel combination of 2 Ω and 3 Ω resistors.
Answer: Series: 1 Ω
Parallel combination of 2 Ω and 3 Ω:
1/R_p = 1/2 + 1/3 = (3 + 2)/6 = 5/6
R_p = 6/5 = 1.2 Ω
Total equivalent resistance:
R_eq = 1 + 1.2 = 2.2 Ω
Final Answer: R_eq = 2.2 Ω or 11/5 Ω
Q (2021, 3 marks): Apply Kirchhoff's voltage law to the circuit: EMF source 10 V, internal resistance 1 Ω, external resistance 9 Ω. Verify the current using Ohm's law.
Answer: ε = 10 V, r = 1 Ω, R = 9 Ω
Kirchhoff's Voltage Law (KVL): Around any closed loop, sum of EMFs = sum of potential drops.
ε - Ir - IR = 0
10 - I(1) - I(9) = 0
10 = 10I
I = 1 A
Verification using Ohm's law:
I = ε/(R + r) = 10/(9 + 1) = 10/10 = 1 A ✓
Final Answer: Current = 1 A; KVL and Ohm's law both give the same result.
Q (2023, 3 marks): Two cells of EMF 1.5 V each with internal resistances 0.5 Ω each are connected in parallel. Find the equivalent EMF and internal resistance.
Answer: When identical cells are connected in parallel:
Equivalent EMF = ε (same as individual EMF) = 1.5 V
Equivalent internal resistance: 1/r_eq = 1/r1 + 1/r2 = 1/0.5 + 1/0.5 = 4
r_eq = 0.25 Ω
Additional note: Parallel connection is preferred when internal resistance of the source is significant. Total current capacity increases but voltage remains the same.
Final Answer: Equivalent EMF = 1.5 V, Equivalent internal resistance = 0.25 Ω
Q (2022, 3 marks): A wire of resistance 10 Ω is bent into a circular loop. Find the equivalent resistance between two diametrically opposite points.
Answer: Original wire resistance = 10 Ω
When bent into a circle, the resistance is distributed uniformly around the circumference.
Between two diametrically opposite points (like A and B at ends of a diameter):
The wire forms two semi-circular paths in parallel.
Each semi-circle has resistance = 10/2 = 5 Ω (half of the original)
Equivalent resistance: 1/R_eq = 1/5 + 1/5 = 2/5
R_eq = 2.5 Ω
Final Answer: Equivalent resistance = 2.5 Ω
Q (2021, 3 marks): State Kirchhoff's current law (KCL) and explain its basis.
Answer: Kirchhoff's Current Law (KCL): At any junction in an electrical circuit, the sum of currents entering the junction equals the sum of currents leaving the junction.
Mathematically: Σ I_in = Σ I_out
Basis (Conservation of Charge):
Charge is conserved. In steady state, charge cannot accumulate at a junction. Any charge flowing into the junction must flow out.
If more charge entered than left, charge would accumulate (impossible in steady state).
Therefore, total current in = total current out.
Example: If 5A enters a junction and splits into two branches, currents in branches must be 3A and 2A (total 5A out).
Final Answer: At a junction, Σ I_in = Σ I_out. This is based on conservation of charge — charge cannot accumulate at any point in steady state.
Frequently Asked Questions
What is the difference between EMF and terminal voltage?
EMF (ε) is the total energy per unit charge provided by the cell, always constant for a given cell. Terminal voltage (V) is the potential difference across the cell terminals when current flows, given by V = ε - Ir. Terminal voltage is less than EMF due to internal resistance.
When is a cell most efficient?
A cell is most efficient when current drawn is small (internal loss Ir is minimal). Maximum power is delivered to the external circuit when external resistance equals internal resistance (R = r).
More Class 12 Physics PYQs
- Ray Optics
- Electrostatics
- Moving Charges and Magnetism
- Electromagnetic Induction
- Alternating Current
- Atoms and Nuclei
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