ECAT — Engineering College Admission Test (UET)

ECAT Physics — Magnetism, Electromagnetic Induction and AC (Set 1)

Take the ECAT Physics — Magnetism, Electromagnetic Induction and AC (Set 1): 30 multiple-choice questions (40 minutes) with instant scoring and a full…

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. A straight wire 0.2 m long carries a current of 4 A at right angles to a uniform magnetic field of 0.5 T. What force acts on it?
- A. 0.1 N
- B. 0.4 N ✓
- C. 4 N
- D. 1.6 N
Answer: B. 0.4 N
Explanation: F = BIL sin 90° = 0.5 × 4 × 0.2 = 0.4 N.
Q2. A wire 0.5 m long carrying 5 A lies at 30° to a uniform 0.2 T magnetic field. What is the magnetic force on it?
- A. 0.5 N
- B. 2.5 N
- C. 0.25 N ✓
- D. 0.43 N
Answer: C. 0.25 N
Explanation: F = BIL sin θ = 0.2 × 5 × 0.5 × sin 30° = 0.25 N.
Q3. A proton moves at 2 × 10⁶ m/s perpendicular to a uniform 0.5 T magnetic field. What is the magnetic force on it? (e = 1.6 × 10⁻¹⁹ C)
- A. 3.2 × 10⁻¹³ N
- B. 1.6 × 10⁻¹³ N ✓
- C. 6.4 × 10⁻¹³ N
- D. 1.6 × 10⁻¹⁹ N
Answer: B. 1.6 × 10⁻¹³ N
Explanation: F = qvB = 1.6 × 10⁻¹⁹ × 2 × 10⁶ × 0.5 = 1.6 × 10⁻¹³ N.
Q4. The magnetic force on a charged particle moving in a uniform magnetic field does no work on it because:
- A. the force is zero for moving charges
- B. the particle's charge changes during motion
- C. the force is always perpendicular to the particle's velocity ✓
- D. the force is always parallel to the velocity
Answer: C. the force is always perpendicular to the particle's velocity
Explanation: F = q(v × B) is perpendicular to v, so it changes only the direction of motion, not the speed or kinetic energy.
Q5. A charged particle moves in a circle perpendicular to a uniform magnetic field. If the field strength is doubled (speed unchanged), the radius of the circle becomes:
- A. double
- B. four times
- C. half ✓
- D. unchanged
Answer: C. half
Explanation: r = mv/(qB), so r ∝ 1/B.

The other 25 questions

Take the timed test (free, no sign-up) to see your score and the answer and explanation for every question.

Q6. Electrons accelerated from rest through a potential difference V enter a uniform magnetic field at right angles and move in a circle of radius r. If V is made four times larger, the new radius is:
- A. 4r
- B. 2r
- C. 16r
- D. r/2
Q7. A solenoid 0.5 m long is wound with 500 turns and carries a current of 5 A. What is the magnetic field near its centre? (μ₀ = 4π × 10⁻⁷ T m A⁻¹)
- A. 2π × 10⁻⁶ T
- B. π × 10⁻³ T
- C. 2π × 10⁻³ T
- D. 4π × 10⁻³ T
Q8. Ampere's circuital law states that the line integral of B around any closed loop equals:
- A. zero always
- B. ε₀ times the net current enclosed
- C. μ₀ times the net current enclosed by the loop
- D. the magnetic flux through the loop
Q9. A rectangular coil of 100 turns and area 0.01 m² carries 2 A in a uniform 0.5 T field, with its plane parallel to the field. What torque acts on it?
- A. 0.5 N m
- B. 0 N m
- C. 2 N m
- D. 1 N m
Q10. A galvanometer of resistance 50 Ω gives full-scale deflection at 1 mA. What resistance must be connected in series with it to convert it into a voltmeter of range 0–10 V?
- A. 9950 Ω
- B. 10000 Ω
- C. 0.005 Ω
- D. 10050 Ω
Q11. To convert a galvanometer into an ammeter, one should connect:
- A. a low resistance in parallel with it
- B. a high resistance in parallel with it
- C. a high resistance in series with it
- D. a low resistance in series with it
Q12. A uniform 0.4 T magnetic field makes an angle of 60° with the normal to a flat coil of area 0.05 m². What is the magnetic flux through the coil?
- A. 0.01 Wb
- B. 0.02 Wb
- C. 0.0173 Wb
- D. 0.005 Wb
Q13. The magnetic flux through each turn of a 200-turn coil falls from 0.05 Wb to 0.01 Wb in 0.2 s. What is the average induced emf?
- A. 8 V
- B. 0.2 V
- C. 60 V
- D. 40 V
Q14. A metal rod 0.5 m long moves at 4 m/s perpendicular to a uniform 0.2 T magnetic field. What emf is induced between its ends?
- A. 0.4 V
- B. 0.1 V
- C. 1.6 V
- D. 4 V
Q15. Lenz's law is a consequence of the law of conservation of:
- A. momentum
- B. charge
- C. energy
- D. mass
Q16. The current in a coil of self-inductance 0.2 H changes by 5 A in 0.1 s. What is the average self-induced emf?
- A. 2.5 V
- B. 1 V
- C. 0.1 V
- D. 10 V
Q17. How much energy is stored in the magnetic field of a 0.5 H inductor carrying a current of 4 A?
- A. 4 J
- B. 8 J
- C. 2 J
- D. 1 J
Q18. An ideal transformer has 500 turns on its primary and 50 turns on its secondary. If 220 V AC is applied to the primary, what is the secondary voltage?
- A. 2200 V
- B. 22 V
- C. 44 V
- D. 11 V
Q19. An ideal transformer steps 220 V up to 1100 V. If the primary current is 2 A, what is the secondary current?
- A. 4 A
- B. 2 A
- C. 0.4 A
- D. 10 A
Q20. A coil of 100 turns and area 0.02 m² rotates at 100 rad/s in a uniform 0.5 T magnetic field. What is the maximum emf induced?
- A. 1000 V
- B. 10 V
- C. 100 V
- D. 1 V
Q21. An alternating current has a peak value of 10 A. What is its r.m.s. value?
- A. 5 A
- B. 7.07 A
- C. 14.14 A
- D. 10 A
Q22. What is the reactance of a 0.1 H inductor in a 50 Hz AC circuit?
- A. 10π Ω
- B. 1/(10π) Ω
- C. 5 Ω
- D. 100π Ω
Q23. A capacitor of capacitance (100/π) μF is connected to a 50 Hz AC supply. What is its reactance?
- A. 50 Ω
- B. 100 Ω
- C. 10⁴ Ω
- D. 0.01 Ω
Q24. In a series AC circuit, a 6 Ω resistor is connected with an inductor of reactance 8 Ω. What is the impedance?
- A. 10 Ω
- B. 2 Ω
- C. 4.8 Ω
- D. 14 Ω
Q25. At resonance in a series RLC circuit:
- A. the impedance is maximum and the current is minimum
- B. the current lags the voltage by 90°
- C. the impedance is minimum and equal to R, so the current is maximum
- D. the inductive reactance is zero
Q26. When an AC voltage is applied across a pure capacitor, the current in the circuit:
- A. lags the voltage by 90°
- B. leads the voltage by 180°
- C. is in phase with the voltage
- D. leads the voltage by 90°
Q27. A series AC circuit has a resistance of 5 Ω and a net inductive reactance of 12 Ω. What is its power factor?
- A. 5/12
- B. 5/13
- C. 12/13
- D. 13/5
Q28. The core of a transformer is made of thin insulated (laminated) sheets in order to:
- A. reduce energy losses due to eddy currents
- B. reduce hysteresis to zero
- C. increase the magnetic flux leakage
- D. increase the resistance of the copper windings
Q29. A DC motor connected to a 220 V supply draws 10 A. Its armature resistance is 2 Ω. What is the back emf in the motor?
- A. 20 V
- B. 240 V
- C. 200 V
- D. 220 V
Q30. A series circuit contains an inductor of 0.1 H and a capacitor of 10 μF. What is its resonant angular frequency?
- A. 1000 rad/s
- B. 10⁴ rad/s
- C. 159 rad/s
- D. 100 rad/s