ECAT — Engineering College Admission Test (UET)

ECAT Physics — Magnetism, Electromagnetic Induction and AC (Set 6)

Take the ECAT Physics — Magnetism, Electromagnetic Induction and AC (Set 6): 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. Two long parallel wires 0.30 m apart carry currents of 10 A and 20 A in opposite directions. The magnetic field at the point midway between them is
- A. 4×10⁻⁵ T ✓
- B. 8×10⁻⁵ T
- C. 2×10⁻⁵ T
- D. 1.33×10⁻⁵ T
Answer: A. 4×10⁻⁵ T
Explanation: Between antiparallel currents the two fields point the same way and add: B = (μ₀/2π)(10 + 20)/0.15 = 2×10⁻⁷×30/0.15 = 4×10⁻⁵ T.
Q2. A long solenoid has 500 turns on a length of 0.50 m and carries 3.0 A. Taking μ₀ = 4π×10⁻⁷ T m/A, the magnetic field inside it is (π = 3.14)
- A. 0.00377 T ✓
- B. 0.00188 T
- C. 0.00754 T
- D. 0.0377 T
Answer: A. 0.00377 T
Explanation: B = μ₀(N/L)I = 4π×10⁻⁷×1000×3 = 0.00377 T.
Q3. A rectangular coil of 100 turns and area 0.020 m² carries 2.0 A in a uniform field of 0.50 T. The normal to the coil makes 30° with the field. The torque on the coil is
- A. 2 N m
- B. 1.73 N m
- C. 1 N m ✓
- D. 0.5 N m
Answer: C. 1 N m
Explanation: τ = NIAB sinθ = 100×2×0.02×0.5×sin30° = 1 N m.
Q4. A proton and an alpha particle are each accelerated from rest through the same potential difference and enter a uniform magnetic field perpendicular to their velocities. The ratio of the radius of the alpha particle's path to that of the proton's path is
- A. 1
- B. 2
- C. 0.71
- D. 1.41 ✓
Answer: D. 1.41
Explanation: r = mv/qB = √(2mV/q)/B ∝ √(m/q). For an alpha particle m/q is twice that of a proton, so the ratio is √2 = 1.41.
Q5. A straight wire 0.50 m long carries 4.0 A in a uniform field of 0.30 T, the wire making 30° with the field. The magnetic force on the wire is
- A. 0.6 N
- B. 0.3 N ✓
- C. 0.15 N
- D. 0.52 N
Answer: B. 0.3 N
Explanation: F = BIl sinθ = 0.30×4×0.5×0.5 = 0.3 N.

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Q6. A galvanometer of resistance 100 Ω gives full-scale deflection for 1.0 mA. To convert it into an ammeter reading up to 11 mA, the shunt resistance required is
- A. 100 Ω
- B. 9.1 Ω
- C. 1 Ω
- D. 10 Ω
Q7. A conducting rod 0.50 m long slides at 4.0 m/s perpendicular to a uniform 0.25 T field along rails of negligible resistance joined by a 2.0 Ω resistor. The mechanical power needed to keep the rod moving at constant speed is
- A. 0.5 W
- B. 0.0625 W
- C. 0.25 W
- D. 0.125 W
Q8. A coil of 200 turns and area 0.010 m² has a field through it, perpendicular to its plane, falling uniformly from 0.80 T to 0.20 T in 0.050 s. The induced emf is
- A. 12 V
- B. 24 V
- C. 6 V
- D. 48 V
Q9. A 0.40 H inductor's current falls from 5.0 A to 3.0 A. The energy released from its magnetic field is
- A. 1.6 J
- B. 0.8 J
- C. 3.2 J
- D. 5 J
Q10. The mutual inductance of two coils is 50 mH. The current in the primary changes uniformly from 2 A to 8 A in 30 ms. The emf induced in the secondary is
- A. 10 V
- B. 0.5 V
- C. 1.5 V
- D. 10000 V
Q11. A step-down transformer reduces 2200 V to 220 V and delivers 9.9 kW to its load with an efficiency of 90%. The current in the primary is
- A. 5 A
- B. 4.5 A
- C. 4.05 A
- D. 45 A
Q12. An alternating current is i = 10 sin(100πt) A. Starting from t = 0, the time taken for the current to first reach 5 A is
- A. 5 ms
- B. 1.67 ms
- C. 2.5 ms
- D. 10 ms
Q13. A series circuit with R = 30 Ω, X_L = 60 Ω and X_C = 20 Ω is connected to a 200 V r.m.s. AC supply. The average power dissipated is
- A. 480 W
- B. 960 W
- C. 640 W
- D. 800 W
Q14. A series RLC circuit has R = 10 Ω, L = 0.10 H and C = 10 μF. Its quality factor (Q = ω₀L/R at resonance) is
- A. 31.6
- B. 10
- C. 100
- D. 1
Q15. An AC source drives a pure capacitor. At the instant when the current in the circuit is at its maximum value, the voltage across the capacitor is
- A. zero
- B. equal to the supply's r.m.s. value
- C. at half its maximum value
- D. at its maximum value
Q16. Which change would INCREASE the energy lost to eddy currents in a transformer core?
- A. Making the laminations thinner
- B. Coating each lamination with insulating varnish
- C. Replacing the laminated iron core with a solid iron block
- D. Using iron that is alloyed with silicon
Q17. A 100-turn coil of area 0.050 m² rotates at 50 revolutions per second in a uniform 0.20 T field. Taking π = 3.14, the peak emf generated is
- A. 31.4 V
- B. 628 V
- C. 157 V
- D. 314 V
Q18. A DC motor on a 220 V supply has an armature resistance of 2.0 Ω. At its running speed the back emf is 200 V. The armature current is
- A. 100 A
- B. 20 A
- C. 110 A
- D. 10 A
Q19. A bar magnet is dropped, north pole first, down the axis of a long vertical copper tube. It falls much more slowly than a non-magnetic bar of the same mass. This is because
- A. copper is strongly attracted to the magnet's north pole
- B. induced currents in the tube produce fields that oppose its motion
- C. the magnet gradually loses its magnetism as it falls
- D. air trapped inside the tube produces a large drag
Q20. A toroid has 1000 turns and a mean radius of 0.20 m and carries 2.0 A. Using B = μ₀NI/2πr, the field along its mean circle is
- A. 0.02 T
- B. 0.0126 T
- C. 0.002 T
- D. 4×10⁻⁴ T
Q21. A wire carrying 5.0 A is bent into a semicircular arc of radius 0.10 m. The magnetic field at its centre of curvature is (μ₀ = 4π×10⁻⁷ T m/A)
- A. 3.14×10⁻⁵ T
- B. 6.28×10⁻⁶ T
- C. 1.571×10⁻⁵ T
- D. 7.85×10⁻⁶ T
Q22. A flat loop of area 0.20 m² sits in a uniform 0.50 T field with its normal at 60° to the field. The magnetic flux through the loop is
- A. 0.1 Wb
- B. 0.05 Wb
- C. 0.0866 Wb
- D. 0.087 Wb
Q23. A 0.30 H inductor of negligible resistance is connected in series with a 6.0 Ω resistor and a 12 V battery. At one time constant (t = L/R) after closing the switch the current is (use 1 − e⁻¹ = 0.632)
- A. 1.264 A
- B. 0.632 A
- C. 0.736 A
- D. 2 A
Q24. A proton moves in the +x direction through a uniform magnetic field directed along +z. The magnetic force on the proton points along
- A. −x
- B. −y
- C. +y
- D. +z
Q25. A half-wave rectified sinusoidal current has peak value 10 A (it is zero during alternate half-cycles). Its r.m.s. value is
- A. 7.07 A
- B. 3.18 A
- C. 6.37 A
- D. 5 A
Q26. A series circuit has R = 40 Ω, L = 0.50 H and C = 50 μF, driven at angular frequency 200 rad/s. Its impedance is
- A. 56.57 Ω
- B. 100 Ω
- C. 40 Ω
- D. 200 Ω
Q27. A load draws 2.0 A r.m.s. from a 100 V r.m.s. supply and dissipates 100 W. Its power factor is
- A. 0.25
- B. 0.87
- C. 0.5
- D. 1
Q28. 100 kW of power is sent along a transmission line of total resistance 5.0 Ω at 20 kV. The power lost in the line is
- A. 500 W
- B. 25 W
- C. 625 W
- D. 125 W
Q29. A lamp rated 60 V, 6 A (resistance 10 Ω) is to be run from a 100 V r.m.s., 50 Hz supply through a pure inductor (choke) in series. The reactance of the choke needed is
- A. 10 Ω
- B. 16.7 Ω
- C. 13.3 Ω
- D. 8 Ω
Q30. A closed coil of resistance 10 Ω has the magnetic flux through it reduced from 0.040 Wb to zero. The total charge that flows through the coil is
- A. 4 mC
- B. 40 mC
- C. 400 mC
- D. 0.4 mC