ECAT — Engineering College Admission Test (UET)

ECAT Physics — Electrostatics and Current Electricity (Set 2)

Take the ECAT Physics — Electrostatics and Current Electricity (Set 2): 30 multiple-choice questions (40 minutes) with instant scoring and a full answer…

What this test covers

30 ECAT Physics

Sample questions with answers

The first 5 questions of this test, with the correct answer and explanation.

Q1. Two point charges exert a force F on each other in air. When the same arrangement is placed in a medium of relative permittivity 4, the force becomes:
- A. 4F
- B. F/2
- C. F/16
- D. F/4 ✓
Answer: D. F/4
Explanation: F = kq₁q₂/(εᵣr²), so the force is divided by εᵣ = 4.
Q2. Point charges +4q and +q are 30 cm apart. At what point on the line joining them is the electric field zero?
- A. 10 cm from the +4q charge
- B. 15 cm from either charge
- C. 20 cm from the +q charge
- D. 10 cm from the +q charge ✓
Answer: D. 10 cm from the +q charge
Explanation: k(4q)/(30 − x)² = kq/x² gives 2x = 30 − x, so x = 10 cm from +q (20 cm from +4q).
Q3. For two large parallel plates carrying equal and opposite uniform charge densities ±σ, the electric field:
- A. is zero between the plates
- B. is uniform between the plates (σ/ε₀) and practically zero outside ✓
- C. decreases as 1/r² between the plates
- D. is strongest near the positive plate and weakest near the negative plate
Answer: B. is uniform between the plates (σ/ε₀) and practically zero outside
Explanation: Between the plates the fields of both sheets add to σ/ε₀, uniform; outside they cancel.
Q4. A uniform electric field of 200 N/C passes through a flat surface of area 0.5 m² at an angle of 60° to the normal of the surface. What is the electric flux through the surface?
- A. 86.6 N m²/C
- B. 400 N m²/C
- C. 100 N m²/C
- D. 50 N m²/C ✓
Answer: D. 50 N m²/C
Explanation: Φ = EA cos θ = 200 × 0.5 × cos 60° = 50 N m² C⁻¹.
Q5. Charges of +3 μC and −1 μC are placed 0.2 m apart. What is the electric potential at the midpoint between them? (k = 9 × 10⁹ N m² C⁻²)
- A. 2.7 × 10⁵ V
- B. 1.8 × 10⁵ V ✓
- C. 3.6 × 10⁵ V
- D. 9 × 10⁴ V
Answer: B. 1.8 × 10⁵ V
Explanation: Potentials add as scalars: V = k(3 − 1) × 10⁻⁶/0.1 = 9 × 10⁹ × 2 × 10⁻⁵ = 1.8 × 10⁵ V.

The other 25 questions

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Q6. What is the electric potential energy of two point charges of +1 μC and +2 μC held 0.1 m apart? (k = 9 × 10⁹ N m² C⁻²)
- A. 0.018 J
- B. 1.8 J
- C. 1.8 × 10⁵ J
- D. 0.18 J
Q7. An electron and a proton are placed in the same uniform electric field. Which statement is correct?
- A. They have equal accelerations in the same direction
- B. They experience forces of equal magnitude, but the electron has a much larger acceleration
- C. The proton experiences the larger force
- D. Both experience forces in the same direction
Q8. An α-particle (charge +2e) starting from rest is accelerated through a potential difference of 100 V. What kinetic energy does it gain?
- A. 200 eV
- B. 100 eV
- C. 400 eV
- D. 50 eV
Q9. The plate area of a parallel-plate capacitor is halved and the separation between its plates is also halved. Its capacitance:
- A. is halved
- B. remains unchanged
- C. becomes four times
- D. doubles
Q10. A 4 μF and a 6 μF capacitor, joined side by side in parallel, are charged from a 5 V cell. How much charge flows from the cell?
- A. 25 μC
- B. 2.4 μC
- C. 12 μC
- D. 50 μC
Q11. A 3 μF and a 6 μF capacitor are connected in series across a 9 V supply. What is the potential difference across the 3 μF capacitor?
- A. 9 V
- B. 3 V
- C. 4.5 V
- D. 6 V
Q12. A parallel-plate capacitor is charged and then disconnected from the battery. If the plate separation is now doubled, the energy stored:
- A. is halved
- B. becomes four times
- C. doubles
- D. stays the same
Q13. What is the time constant of a circuit with a 2 MΩ resistor in series with a 5 μF capacitor?
- A. 2.5 s
- B. 0.4 s
- C. 10 ms
- D. 10 s
Q14. A steady current of 2 A flows in a wire for 5 minutes. How much charge passes any cross-section of the wire?
- A. 150 C
- B. 10 C
- C. 2.5 C
- D. 600 C
Q15. For a metallic conductor obeying Ohm's law at constant temperature, the graph of current against potential difference is:
- A. a parabola through the origin
- B. a curve that flattens at high voltage
- C. a straight line through the origin
- D. a horizontal straight line
Q16. Two 6 Ω resistors are connected in parallel and the combination is connected in series with a third 6 Ω resistor. What is the total resistance?
- A. 4 Ω
- B. 18 Ω
- C. 12 Ω
- D. 9 Ω
Q17. A carbon resistor has bands coloured brown, black, red and gold. What is its resistance?
- A. 100 Ω ± 10%
- B. 10 kΩ ± 5%
- C. 102 Ω ± 5%
- D. 1000 Ω ± 5%
Q18. Two wires A and B are made of the same metal. A is twice as long as B but has half its diameter. What is the ratio of their resistances R_A : R_B?
- A. 1 : 8
- B. 2 : 1
- C. 8 : 1
- D. 4 : 1
Q19. A copper coil has a resistance of 50 Ω at 25 °C. Taking α = 0.004 K⁻¹ and R = R₂₅(1 + αΔT), at what temperature will its resistance be 60 Ω?
- A. 30 °C
- B. 100 °C
- C. 50 °C
- D. 75 °C
Q20. A cell of emf 6 V gives a terminal potential difference of 5 V when connected to a 10 Ω resistor. What is its internal resistance?
- A. 1 Ω
- B. 1.2 Ω
- C. 2 Ω
- D. 0.5 Ω
Q21. A source of emf 10 V can deliver at most 25 W to an external load, however the load is chosen. What is its internal resistance?
- A. 2.5 Ω
- B. 0.4 Ω
- C. 1 Ω
- D. 4 Ω
Q22. A 60 W, 220 V bulb and a 100 W, 220 V bulb are connected in series across a 220 V supply. Which statement is correct?
- A. Both glow equally
- B. The 100 W bulb glows brighter
- C. Both glow with their rated brightness
- D. The 60 W bulb glows brighter
Q23. A heater of fixed resistance is connected to a supply whose voltage is halved. Its power becomes:
- A. one half
- B. double
- C. one quarter
- D. unchanged
Q24. Kirchhoff's first (current) rule is a consequence of the conservation of:
- A. charge
- B. mass
- C. energy
- D. momentum
Q25. In a metre bridge, an unknown resistance X is in the left gap and a 30 Ω resistor in the right gap. The balance point is 40 cm from the left end. What is X?
- A. 12 Ω
- B. 45 Ω
- C. 20 Ω
- D. 30 Ω
Q26. A cell balances at 80 cm on a potentiometer wire on open circuit. When a 6 Ω resistor is connected across the cell, the balance length becomes 60 cm. What is the internal resistance of the cell?
- A. 2 Ω
- B. 1.5 Ω
- C. 4.5 Ω
- D. 8 Ω
Q27. Which statement about electric field and potential is correct?
- A. The electric field points in the direction in which the potential decreases most rapidly
- B. The electric field is zero wherever the potential is zero
- C. The electric field points towards higher potential
- D. The potential is constant wherever the field is non-zero
Q28. Which of these CANNOT be the charge on an isolated body? (e = 1.6 × 10⁻¹⁹ C)
- A. 1.6 × 10⁻¹⁸ C
- B. 2.4 × 10⁻¹⁹ C
- C. 3.2 × 10⁻¹⁹ C
- D. 4.8 × 10⁻¹⁹ C
Q29. Two electric field lines never cross each other because:
- A. field lines are always parallel
- B. the field is zero where two lines would meet
- C. field lines carry opposite charges
- D. the field at any point has only one direction
Q30. The resistance of an NTC thermistor, as its temperature rises:
- A. increases
- B. stays constant
- C. first increases then becomes zero
- D. decreases