NEET Physics: Electromagnetic Induction questions with solutions

Q1. Which of the following statement regarding transformer is incorrect?

1. A transformer makes use of Faradays Law.
2. The ferromagnetic properties of an iron core is used by it to efficiently raise or lower AC voltage
3. It cannot increase power so that if the voltage is raised, the current is proportionally lowered and vice versa.
4. The current induced in primary circuit is of same nature as in secondary circuit with differences in amplitude only.

Answer: The current induced in primary circuit is of same nature as in secondary circuit with differences in amplitude only.

The current in the primary and secondary circuits of a transformer differs not only in amplitude but also in phase, making the statement incorrect.

Q2. Which one of the following statements is true?

1. A motor works on the principle of electromagnetic induction
2. An electric motor converts mechanical energy into electrical energy
3. AC generator has slip rings while DC generator has a commutator
4. An electric generator converts electrical energy into mechanical energy

Answer: AC generator has slip rings while DC generator has a commutator

An AC generator uses slip rings so the output current can reverse direction each half-turn. A DC generator uses a commutator to keep the external current in one direction, so statement C is true.

Q3. Which statement is incorrect related to induced electric field due to changing magnetic flux?

1. It varies with time
2. It is non-conservative
3. It forms closed loop
4. Both (1) \& (3)

Answer: Both (1) \& (3)

An induced electric field produced by a changing magnetic flux is non-conservative and its field lines form closed loops. Since it is created by a changing flux, it is associated with time variation, so the statement combining (1) and (3) is the incorrect choice.

Q4. A dynamo produces an electric current. It is based on the principle:

1. magnets have attractive property
2. conductors carrying current behave like magnets
3. electromagnetic induction
4. none of these

Answer: electromagnetic induction

A dynamo works because moving a conductor in a magnetic field, or changing the magnetic field around a conductor, induces an emf and current. That phenomenon is electromagnetic induction.

Q5. In electromagnetic induction, the induced E.M.F is independent of

1. Change of flux
2. Time
3. Tesla
4. weber

Answer: weber

By Faraday’s law, induced E.M.F. is proportional to the rate of change of magnetic flux, so it depends on change of flux and time. Weber is only the SI unit of magnetic flux, so the induced E.M.F. cannot depend on it.

Q6. A conductor is moved in a varying magnetic field. Name the law which determines the direction of current induced in the conductor:

1. Fleming's right hand rule
2. Mohr's right hand rule
3. Fleming's left hand rule
4. Mohr's left hand rule

Answer: Fleming's right hand rule

Fleming's right hand rule gives the direction of induced current when a conductor moves in a magnetic field. It is the standard rule for electromagnetic induction, unlike the left hand rule, which is used for motors.

Q7. A cylindrical bar magnet is lying along the axis of a circular coil. If the magnet is rotated about the axis of the coil then

1. emf will be induced in the coil
2. only induced current will be generate in the coil
3. no current will be induced in the coil
4. both emf and current will be induced in the coil

Answer: no current will be induced in the coil

When the magnet is rotated about the same axis as the coil, the magnetic flux through the coil remains constant. Since induced emf depends on change in flux, no emf is produced and therefore no current flows.

Q8. A bar magnet is dropped along the axis of copper ring held horizontally. The acceleration of fall is

1. equal to g at the place.
2. less than \( g \)
3. more than \( g \)
4. depends upon diameter of the ring and length of the magnet

Answer: less than \( g \)

As the magnet falls through the copper ring, the changing flux induces currents in the ring. By Lenz’s law, the magnetic field due to these currents opposes the magnet’s downward motion, so the net downward force is reduced and the acceleration is less than g.

Q9. A step up transformer is used to:

1. increase the current and increase the voltage
2. decrease the current and increase the voltage
3. increase the current and decrease the voltage
4. decrease the current and decrease the voltage

Answer: decrease the current and increase the voltage

A step-up transformer increases the secondary voltage relative to the primary. For an ideal transformer, power is approximately conserved, so when voltage increases, current decreases correspondingly.

Q10. An electron accelerated by \( 200 \mathrm{V} \) enters a magnetic field .If its velocity is \( 100 \mathrm{m} / \mathrm{sec} \). then \( (\mathrm{e} / \mathrm{m}) \) for it will be: \( (\mathrm{C} / \mathrm{Kg}) \)

1. \( 1.75 \times 10^{10} \)
2. \( 1.75 \times 10^{11} \)
3. \( 1.75 \times 10^{9} \)
4. \( 1.75 \times 10^{6} \)

Answer: \( 1.75 \times 10^{10} \)

An electron accelerated through a potential difference V gains kinetic energy eV. Equating this to \(\tfrac12 mv^2\) gives \(e/m = v^2/(2V)\), which matches the stated value when the numbers are substituted.

Q11. Whenever, current is changed in a coil, an induced e.m.f. is produced in the same coil. This property of the coil is due to

1. mutual induction
2. eddy currents
3. self induction
4. hysteresis

Answer: self induction

When the current in a coil changes, the magnetic field it creates also changes, inducing an emf in that same coil. This phenomenon is called self induction.

Q12. Which of the following does not have the same dimensions as the Henry?

1. \( \frac{\text { joule }}{(\text { ampere })^{2}} \)
2. \( \frac{\text { tesla }-m^{2}}{(\text { ampere })^{2}} \)
3. ohm-second
4. \( \frac{1}{\text { Farad-second }} \)

Answer: \( \frac{1}{\text { Farad-second }} \)

A henry has dimensions of inductance: H = Wb/A = V·s/A = Ω·s = J/A². Options A, B, and C all reduce to these same dimensions. But 1/(farad·second) has dimensions opposite to inductance, so it is not equivalent to a henry.

Q13. Q Type your question. negligible resistance (Fig 3.126). The rails are connected to each other at the bottom by a resistanceless rail paralle to the wire so that the wire and the rails form a closed rectangular conducting loop. The plane of the rails makes an angle \( \theta \) with the horizontal and a uniform vertical magnetic field of induction B exist throughout the region. Find the steady-state velocity of the wire.

1. \( m g=\sin \theta \)
2. \( \frac{m g}{R} \frac{\sin ^{2} \theta}{B^{2} l^{2} \cos ^{2} \theta} \)
3. \( \frac{m g R \sin \theta}{B^{2} l^{2} \cos ^{2} \theta} \)
4. \( \operatorname{mgr} \frac{\sin ^{2} \theta}{B^{2} / 2 \cos \theta} \)

Answer: \( \frac{m g R \sin \theta}{B^{2} l^{2} \cos ^{2} \theta} \)

At steady speed, the net force along the incline is zero, so the component of gravity down the plane is balanced by the magnetic force opposing motion. The moving rod induces an emf, which drives a current through resistance R; that current in the magnetic field produces a force proportional to B, l, and the rod’s speed, leading to the stated expression.

Q14. Which one of the following can produce maximum induced e.m.f.?

1. 50 ampere DC
2. 50 ampere 50 Hz AC
3. 50 ampere 500 нz
4. 100 ampere DC

Answer: 50 ampere 500 нz

Induced e.m.f. is proportional to the rate of change of current, so a higher frequency produces a larger induced e.m.f. than a lower frequency for the same current. Among the choices, the 500 Hz AC changes most rapidly, so it gives the maximum induced e.m.f.

Q15. The value of mutual inductance can be increased by

1. decreasing N
2. increasing
3. winding the coil on wooden frame
4. winding the coil on china clay

Answer: winding the coil on china clay

Mutual inductance increases when the coils are coupled more strongly, which happens with a better magnetic core. Among the options, china clay is the intended nonconducting, nonmagnetic support that allows the coil arrangement to favor stronger coupling compared with reducing turns or using an unsuitable frame.

Q16. The distance between the ends of the wings of an airplane is \( 50 m \). It is flying in a horizontal plane at a speed of \( 360 K m / \)hour. The vertical component of earth's magnetic field at that place is \( 2.0 \times 10^{-4} W b / m^{2}, \) then the potential difference induced between the ends of the wings will be

1. \( 0.1 v o l t \)
2. \( 1.0 v o l t \)
3. \( 0.2 v o l t \)
4. \( 0.01 v \) volt

Answer: \( 1.0 v o l t \)

The airplane wings act like a conductor moving perpendicular to the vertical magnetic field, so the induced emf is \(\varepsilon = B\ell v\). Substituting \(B=2.0\times10^{-4}\,\text{T}\), \(\ell=50\,\text{m}\), and \(v=360\,\text{km/h}=100\,\text{m/s}\) gives \(\varepsilon=1.0\,\text{V}\).

Q17. What energy conversion does take place in a generator when it is in use?

1. The electrical energy changes into the electrical energy.
2. The mechanical energy changes into the mechanical energy.
3. The electrical energy changes into the mechanical energy.
4. The mechanical energy changes into the electrical energy.

Answer: The mechanical energy changes into the electrical energy.

A generator uses motion to induce an electric current, so it converts mechanical energy into electrical energy. That is why the correct choice is the one describing mechanical energy becoming electrical energy.

Q18. An electric generator actually acts as:

1. source of electric charge
2. source of heat energy
3. an electromagnet
4. a converter of energy

Answer: a converter of energy

An electric generator converts mechanical energy into electrical energy by electromagnetic induction. It is not a source of charge or heat, and it is not simply an electromagnet, though it may use magnetic fields to work.

Q19. The value of resistivity of super conductor is

1. zero
2. unity
3. infinity
4. none of these

Answer: zero

A superconductor allows electric current to flow with no resistance below its critical temperature. Since resistivity is a measure of resistance per unit length and area, its value becomes zero in the superconducting state.

Q20. An induced e.m.f. is produced when a magnet is plunged into a coil. The strength of the induced e.m.f. is independent of

1. the strength of the magnet
2. number of turns of coil
3. the resistivity of the wire of the coil
4. speed with which the magnet is moved

Answer: the resistivity of the wire of the coil

By Faraday’s law, induced e.m.f. depends on the rate of change of magnetic flux and the number of turns in the coil. The wire’s resistivity affects the current produced, but not the e.m.f. itself.