ECAT — Engineering College Admission Test (UET)
Take the ECAT Physics — Magnetism, Electromagnetic Induction and AC (Set 2): 30 multiple-choice questions (40 minutes) with instant scoring and a full…
What this test covers
The first 5 questions of this test, with the correct answer and explanation.
Q1. Using Ampere's law, find the magnetic field 10 cm from a long straight wire carrying 10 A. (μ₀ = 4π × 10⁻⁷ T m A⁻¹)
Answer: A. 2 × 10⁻⁵ T
Explanation: B = μ₀I/2πr = 4π × 10⁻⁷ × 10/(2π × 0.1) = 2 × 10⁻⁵ T.
Q2. A proton (m = 1.67 × 10⁻²⁷ kg, q = 1.6 × 10⁻¹⁹ C) enters a uniform 0.1 T field at right angles with a speed of 10⁶ m/s. What is the radius of its circular path?
Answer: D. 10.4 cm
Explanation: r = mv/qB = 1.67 × 10⁻²⁷ × 10⁶/(1.6 × 10⁻¹⁹ × 0.1) ≈ 0.104 m.
Q3. An electron moves along the +x direction in a uniform magnetic field pointing along +y. In which direction is the magnetic force on it?
Answer: D. −z
Explanation: F = qv × B; i × j = +k, and the electron's negative charge reverses this, giving −z.
Q4. Crossed electric and magnetic fields of 2 × 10⁴ V/m and 0.1 T let charged particles of only one speed pass through undeflected. What is that speed?
Answer: A. 2 × 10⁵ m/s
Explanation: Undeflected when qE = qvB: v = E/B = 2 × 10⁴/0.1 = 2 × 10⁵ m/s.
Q5. A charged particle circles in a uniform magnetic field perpendicular to its velocity. If its speed is doubled, its time period of revolution:
Answer: C. remains the same
Explanation: T = 2πm/qB, which does not depend on the speed (a faster particle moves in a proportionally larger circle).
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Q6. A solenoid is stretched to twice its length while keeping the same number of turns and the same current. The magnetic field inside it:
Q7. A current-carrying rectangular coil is placed in a uniform magnetic field. The torque on it is zero when:
Q8. A galvanometer of resistance 100 Ω gives full-scale deflection at 10 mA. What shunt resistance converts it into an ammeter of range 0–1 A?
Q9. The SI unit of magnetic flux, the weber, is equivalent to:
Q10. A flat loop of area 0.2 m² is held with its plane parallel to a uniform 0.5 T magnetic field. What is the magnetic flux through the loop?
Q11. A 50-turn coil of area 0.1 m² and total resistance 2 Ω lies with its plane perpendicular to a magnetic field that increases steadily from 0.2 T to 0.6 T in 0.5 s. What current is induced in the coil?
Q12. The north pole of a bar magnet is pushed towards one face of a closed coil. The induced current makes that face of the coil behave as:
Q13. A 1 m long rod slides at a steady 5 m/s along two parallel rails in a perpendicular 0.4 T magnetic field. The rails are joined by a 4 Ω resistor (other resistances negligible). What force is needed to keep the rod moving at this speed?
Q14. Two coils have a mutual inductance of 0.5 H. If the current in the primary changes by 4 A in 0.2 s, what average emf is induced in the secondary?
Q15. If the steady current through an inductor is tripled, the energy stored in its magnetic field becomes:
Q16. If the speed of rotation of the coil of an AC generator is doubled (field and coil unchanged), the peak emf:
Q17. An alternating voltage is given by V = 311 sin 314t (V in volts, t in seconds). What is its frequency?
Q18. The domestic AC supply has an r.m.s. voltage of 220 V. What is its peak-to-peak voltage?
Q19. If the frequency of the AC supply connected to a capacitor is doubled, its capacitive reactance:
Q20. A 30 Ω resistor and a capacitor of reactance 40 Ω are connected in series to a 100 V (r.m.s.) AC supply. What r.m.s. current flows?
Q21. A series circuit has R = 10 Ω, X_L = 20 Ω and X_C = 10 Ω. Which statement about the current relative to the applied voltage is correct?
Q22. In a series LC circuit, the capacitance is made four times larger (L unchanged). The resonant frequency becomes:
Q23. An AC circuit draws an r.m.s. current of 5 A from a 220 V (r.m.s.) supply with a power factor of 0.8. What average power does it consume?
Q24. The average power consumed in an AC circuit containing only a pure inductor is:
Q25. A transformer steps 11,000 V down to 220 V. What is the ratio of primary turns to secondary turns?
Q26. A transformer takes 2 A from a 220 V supply and delivers 20 A at 20 V to its load. What is its efficiency?
Q27. A DC motor with armature resistance 4 Ω runs on a 220 V supply. What current does it draw at the instant it is switched on (armature not yet turning)?
Q28. In a DC generator, the split-ring commutator is used to:
Q29. A choke coil is preferred to a resistor for limiting current in an AC circuit because it:
Q30. In an AC circuit containing only a pure inductor, the voltage across the inductor: