ECAT — Engineering College Admission Test (UET)

ECAT Physics — Magnetism, Electromagnetic Induction and AC (Set 7)

Take the ECAT Physics — Magnetism, Electromagnetic Induction and AC (Set 7): 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.

1. Q1. A straight wire 0.3 m long carries 4 A in a uniform field of 0.5 T, the wire making 30° with the field. The force on it is:

   • A. 1.2 N
   • B. 0.52 N
   • C. 0.3 N ✓
   • D. 0.6 N

   Answer: C. 0.3 N

   Explanation: F = BIL sinθ = 0.5 × 4 × 0.3 × 0.5 = 0.3 N.

2. Q2. An electron moving at 2 × 10⁶ m/s perpendicular to a magnetic field of 0.1 T experiences a force (e = 1.6 × 10⁻¹⁹ C):

   • A. 0 N
   • B. 1.6 × 10⁻¹⁴ N
   • C. 3.2 × 10⁻²⁵ N
   • D. 3.2 × 10⁻¹⁴ N ✓

   Answer: D. 3.2 × 10⁻¹⁴ N

   Explanation: F = evB = (1.6 × 10⁻¹⁹)(2 × 10⁶)(0.1) = 3.2 × 10⁻¹⁴ N.

3. Q3. A proton and a deuteron (same charge, twice the mass) are accelerated from rest through the same potential difference and then enter the same uniform magnetic field perpendicular to it. The ratio of their radii, r_d/r_p, is:

   • A. √2 ✓
   • B. 1
   • C. 2
   • D. 1/√2

   Answer: A. √2

   Explanation: After acceleration mv = √(2mqV), so r = √(2mV/q)/B ∝ √m for equal q and V: r_d/r_p = √2.

4. Q4. Taking μ₀ = 4π × 10⁻⁷ T m/A, the magnetic field at 5 cm from a long straight wire carrying 10 A is:

   • A. 2 × 10⁻⁷ T
   • B. 4π × 10⁻⁵ T
   • C. 4 × 10⁻⁵ T ✓
   • D. 2 × 10⁻⁴ T

   Answer: C. 4 × 10⁻⁵ T

   Explanation: B = μ₀I/2πr = (2 × 10⁻⁷)(10)/0.05 = 4 × 10⁻⁵ T.

5. Q5. A flat circular coil of 50 turns and radius 10 cm carries 2 A. Taking μ₀ = 4π × 10⁻⁷ T m/A, the field at its centre is:

   • A. π × 10⁻⁴ T
   • B. 2π × 10⁻⁴ T ✓
   • C. 4π × 10⁻⁴ T
   • D. 2π × 10⁻³ T

   Answer: B. 2π × 10⁻⁴ T

   Explanation: B = μ₀NI/2r = (4π × 10⁻⁷)(50)(2)/(0.2) = 2π × 10⁻⁴ T.

The other 25 questions

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6. Q6. A long solenoid has 1000 turns per metre and carries 2 A. Taking μ₀ = 4π × 10⁻⁷ T m/A, the field inside its middle is:

   • A. 2π × 10⁻³ T
   • B. 4π × 10⁻⁴ T
   • C. 8π × 10⁻⁴ T
   • D. 8π × 10⁻⁷ T

7. Q7. A rectangular coil of 100 turns and area 0.02 m² carries 0.5 A in a uniform field of 0.4 T, the field lying in the plane of the coil. The torque on the coil is:

   • A. 0.2 N m
   • B. 0.04 N m
   • C. 0.4 N m
   • D. 0 N m

8. Q8. A galvanometer of resistance 100 Ω gives full-scale deflection at 10 mA. To convert it into an ammeter reading up to 1.01 A, the shunt resistance needed is:

   • A. 1 Ω in series
   • B. 1 Ω in parallel
   • C. 0.1 Ω in parallel
   • D. 10 Ω in parallel

9. Q9. The same galvanometer (100 Ω, full scale 10 mA) is to read up to 10 V as a voltmeter. The series resistance needed is:

   • A. 9900 Ω
   • B. 900 Ω
   • C. 100 Ω
   • D. 1000 Ω

10. Q10. A coil of 50 turns has its flux fall uniformly from 0.04 Wb to 0.01 Wb in 0.1 s. The magnitude of the induced emf is:

    • A. 150 V
    • B. 0.3 V
    • C. 15 V
    • D. 1.5 V

11. Q11. A 0.5 m long rod, resistance 0.2 Ω overall in a closed circuit, slides at 4 m/s perpendicular to a 0.2 T field. The external force needed to keep it moving at constant speed is:

    • A. 0.1 N
    • B. 0.2 N
    • C. 0.4 N
    • D. 2 N

12. Q12. The north pole of a bar magnet is pushed towards a closed coil. According to Lenz's law, the induced current:

    • A. flows only after the magnet stops
    • B. creates a north pole facing the magnet to oppose its approach
    • C. is zero because the coil has no battery of its own
    • D. creates a south pole facing the magnet to help its approach

13. Q13. The current in a 0.5 H coil falls uniformly from 4 A to zero in 0.1 s. The average induced emf is:

    • A. 200 V
    • B. 0.2 V
    • C. 2 V
    • D. 20 V

14. Q14. A secondary coil of 200 turns has a flux of 3 × 10⁻⁴ Wb through each turn when the primary current is 2 A. The mutual inductance of the pair is:

    • A. 0.03 H
    • B. 0.3 H
    • C. 1.5 × 10⁻⁴ H
    • D. 0.06 H

15. Q15. A transformer with 90% efficiency takes 5 A from a 220 V supply and delivers its output at 99 V. The output current is:

    • A. 9 A
    • B. 2.5 A
    • C. 10 A
    • D. 11 A

16. Q16. An alternating current with peak value 3 A flows through a 20 Ω resistor. The mean power dissipated is:

    • A. 127 W
    • B. 180 W
    • C. 90 W
    • D. 60 W

17. Q17. An inductor of inductance 25/π mH is connected to a 400 Hz AC source. Its reactance is:

    • A. 40 Ω
    • B. 5 Ω
    • C. 20 Ω
    • D. 10 Ω

18. Q18. In a series RLC circuit R = 30 Ω, X_L = 60 Ω and X_C = 20 Ω. The impedance is:

    • A. 110 Ω
    • B. 70 Ω
    • C. 50 Ω
    • D. 90 Ω

19. Q19. A series LC circuit has L = 0.1 H and C = 10 μF. Its resonant angular frequency is:

    • A. 10 000 rad/s
    • B. 1 rad/s
    • C. 1000 rad/s
    • D. 100 rad/s

20. Q20. In the circuit with R = 30 Ω and Z = 50 Ω, an rms current of 2 A flows. The average power consumed is:

    • A. 80 W
    • B. 120 W
    • C. 100 W
    • D. 200 W

21. Q21. A choke coil is preferred to a resistor for reducing the current in an AC circuit because:

    • A. its large resistance stores energy as heat
    • B. it increases the supply frequency
    • C. it blocks all AC while passing DC
    • D. its very small resistance means little power is wasted

22. Q22. A coil of 200 turns and area 0.05 m² rotates at an angular speed of 100 rad/s in a field of 0.1 T. The peak emf generated is:

    • A. 20 V
    • B. 100 V
    • C. 10 V
    • D. 1000 V

23. Q23. A DC motor of armature resistance 2 Ω is connected to a 220 V supply. At the instant of switching on, before the armature turns, the current drawn is:

    • A. 440 A
    • B. 10 A
    • C. 220 A
    • D. 110 A

24. Q24. In a single-loop DC generator with a split-ring commutator, the output current during one complete revolution:

    • A. reaches its maximum value once and zero once
    • B. reaches its maximum value twice and falls to zero twice
    • C. is constant at its maximum value
    • D. reverses its direction twice

25. Q25. A coil of area 0.05 m² is placed in a uniform field of 0.4 T so that the field makes 60° with the normal to the coil. The magnetic flux is:

    • A. 0.017 Wb
    • B. 0.02 Wb
    • C. 0.01 Wb
    • D. 0.03 Wb

26. Q26. In a velocity selector the electric field is 2000 V/m and the magnetic field 0.05 T, mutually perpendicular. The speed of the particles that pass undeflected is:

    • A. 4 × 10⁴ m/s
    • B. 1 × 10² m/s
    • C. 1 × 10⁵ m/s
    • D. 2.5 × 10⁻⁵ m/s

27. Q27. Taking e/m = 1.76 × 10¹¹ C/kg for an electron, one moving at 1.76 × 10⁷ m/s perpendicular to a 1 mT field follows a circle of radius:

    • A. 0.01 m
    • B. 10 m
    • C. 0.1 m
    • D. 1 m

28. Q28. In a series circuit containing a resistor and an ideal capacitor driven by an AC source, the current:

    • A. leads the source voltage by exactly 90°
    • B. is in phase with the source voltage
    • C. leads the source voltage by an angle between 0° and 90°
    • D. lags the source voltage by an angle between 0° and 90°

29. Q29. The energy stored in the magnetic field of a 2 H inductor carrying 3 A is:

    • A. 4.5 J
    • B. 6 J
    • C. 18 J
    • D. 9 J

30. Q30. A current i = 5 sin(100πt) A (t in seconds) flows in a circuit. Its rms value is:

    • A. 5 A
    • B. 5/2 A
    • C. 5√2 A
    • D. 5/√2 A