JEE Advanced (Assertion-Reason Type): Moving Charges & Magnetism | Chapter-wise Tests for JEE Main & Advanced PDF Download

Directions: These questions consist of two statements, each printed as Assertion and Reason. While answering these questions, you are required to choose any one of the following four responses.
Q.1. Assertion: Cyclotron does not accelerate electron.
Reason: Mass of the electron is very small.
(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.
(c) If assertion is true but reason is false.
(d) If the assertion and reason both are false.
(e) If assertion is false but reason is true.

Correct Answer is option (a)
Cyclotron is suitable for accelerating heavy particles like protons, a-particles etc, and not for electrons because of low mass. Because electrons acquire very high velocities very near to velocity of light and appreciable variation in their mass, occurs.

Q.2. Assertion: Cyclotron is a device which is used to accelerate the positive ion.
Reason: Cyclotron frequency depends upon the velocity.
(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.
(c) If assertion is true but reason is false.
(d) If the assertion and reason both are false.
(e) If assertion is false but reason is true.

Correct Answer is option (c)
Cyclotron is utilised to accelerate the positive ion. And cyclotron frequency is given by . It means cyclotron frequency doesn't depends upon velocity. Therefore, assertion is true and reason false.

Q.3. Assertion: Magnetic field interacts with a moving charge and not with a stationary charge.
Reason: A moving charge produces a magnetic field.
(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.
(c) If assertion is true but reason is false.
(d) If the assertion and reason both are false.
(e) If assertion is false but reason is true.

Correct Answer is option (a)
A moving charge experiences a force in magnetic field. It is because of interaction of two magnetic fields, one which is produced due to the motion of charge and other in which charge is moving.

Q.4. Assertion: If an electron is not deflected while passing through a certain region of space, then only possibility is that there is no magnetic region.
Reason: Force is directly proportional to the magnetic field applied.
(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.
(c) If assertion is true but reason is false.
(d) If the assertion and reason both are false.
(e) If assertion is false but reason is true.

Correct Answer is option (a)
In this case we cannot be sure about the absence of the magnetic field because if the electron moving parallel to the direction of magnetic field, the angle between velocity and applied magnetic field is zero (F = 0). Then also electron passes without deflection. Also F=evBsinθ
⇒ F=  µ B

Q.5. Assertion: Free electron always keep on moving in a conductor even then no magnetic force act on them in magnetic field unless a current is passed through it.
Reason: The average velocity of free electron is zero.
(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.
(c) If assertion is true but reason is false.
(d) If the assertion and reason both are false.
(e) If assertion is false but reason is true.

Correct Answer is option (a)
In the absence of the electric current, the free electrons in a conductor are in a state of random motion, like molecules in a gas. Their average velocity is zero. i.e. they do not have any net velocity in a direction. As a result, there is no net magnetic force on the free electrons in the magnetic field. On passing the current, the free electrons acquire drift velocity in a definite direction, hence magnetic force acts on them, unless the field has no perpendicular component.

Q.6. Assertion: The ion cannot move with a speed beyond a certain limit in a cyclotron.
Reason: As velocity increases time taken by ion increases.
(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.
(c) If assertion is true but reason is false.
(d) If the assertion and reason both are false.
(e) If assertion is false but reason is true.

Correct Answer is option (c)
 Time taken is independent of velocity and radius of path. However, maximum velocity will be given by v_max = qBR / m where R is radius of Dee's.

Q.7. Assertion: The coil is bound over the metallic frame in moving coil galvanometer.
Reason: The metallic frame help in making steady deflection without any oscillation.
(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.
(c) If assertion is true but reason is false.
(d) If the assertion and reason both are false.
(e) If assertion is false but reason is true.

Correct Answer is option (a)
Due to metallic frame the deflection is only due to current in a coil and magnetic field, not due to vibration in the strings. If string start oscillating, presence of metallic frame in the field make these oscillations damped.

Q.8. Assertion: A circular loop carrying current lies in XY plane with its center at origin having a magnetic flux in negative Z-axis.
Reason: Magnetic flux direction is independent of the direction of current in the conductor.
(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.
(c) If assertion is true but reason is false.
(d) If the assertion and reason both are false.
(e) If assertion is false but reason is true.

Correct Answer is option (c)
The direction of magnetic field due to current carrying conductor can be found by applying right hand thumb rule or right hand palm rule. When electric current is passed through a circular conductor, the magnetic field lines near the center of the conductor are almost straight lines. Magnetic flux direction is determined only by the direction of current.

Q.9. Assertion: The energy of charged particle moving in a uniform magnetic field does not change.
Reason: Work done by magnetic field on the charge is zero.
(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.
(c) If assertion is true but reason is false.
(d) If the assertion and reason both are false.
(e) If assertion is false but reason is true.

Correct Answer is option (a)
The force on a charged particle moving in a uniform magnetic field always acts in direction perpendicular to the direction of motion of the charge. As work done by magnetic field on the charge is zero, [W = FS cosq], so the energy of the charged particle does not change.

Q.10. Assertion: If an electron, while coming vertically from outerspace, enter the earth's magnetic field, it is deflected towards west.
Reason: Electron has negative charge.
(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.
(c) If assertion is true but reason is false.
(d) If the assertion and reason both are false.
(e) If assertion is false but reason is true.

Correct Answer is option (b)
We know that the direction of the earth's magnetic field is toward north and the velocity of electron is vertically downward. Applying Fleming's left hand rule, the direction of force is towards west. Therefore, an electron coming from outer space will be deflected toward west.

Q.11. Assertion: A direct current flows through a metallic rod, produced magnetic field only outside the rod.
Reason: There is no flow of charge carriers inside the rod.
(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.
(c) If assertion is true but reason is false.
(d) If the assertion and reason both are false.
(e) If assertion is false but reason is true.

Correct Answer is option (d)
In the case of metallic rod, the charge carriers flow through whole of the cross section. Therefore, the magnetic field exists both inside as well as outside. However magnetic field inside the rod will go on decreasing as we go towards the axis.

Q.12. Assertion: An electron and proton enters a magnetic field with equal velocities, then, the force experienced by the proton will be more than electron.
Reason: The mass of proton is 1837 times more than electron.
(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.
(c) If assertion is true but reason is false.
(d) If the assertion and reason both are false.
(e) If assertion is false but reason is true.

Correct Answer is option (e)
The force experienced by a charge particle in a magnetic field is given by,  which is independent of mass. As q, v and B are same for both the electron and proton, hence both will experience same force.

Q.13. Assertion: Torque on the coil is the maximum, when coil is suspended in a radial magnetic field.
Reason: The torque tends to rotate the coil on its own axis.
(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.
(c) If assertion is true but reason is false.
(d) If the assertion and reason both are false.
(e) If assertion is false but reason is true.

Correct Answer is option (b)
The torque on the coil in a magnetic field is given by t = nIBA cos q For radial field, the coil is set with its plane parallel to the direction of the magnetic field B, then q = 0o and cos q = 1 Þ Torque = nIBA (1) = nIBA (maximum).

Q.14. Assertion: A loosely round helix made of stiff wire is suspended vertically with the lower end just touching a dish of mercury. When a current is passed through the wire, the helical wire executes oscillatory motion with the lower end jumping out of and inside of mercury.
Reason: When electric current is passed through helix, a magnetic field is produced both inside and outside the helix.
(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.
(c) If assertion is true but reason is false.
(d) If the assertion and reason both are false.
(e) If assertion is false but reason is true.

Correct Answer is option (b)
The winding of helix carry currents in the same direction therefore they experience an attractive force pulling the lower end out of mercury. As a result of this, the circuit breaks, current becomes zero and hence the force of attraction vanishes. Therefore helix comes back to its final position, completing the circuit again. In this way, the process is repeated and helix executes oscillatory motion.

Q.15. Assertion: The magnetic field at the ends of a very long current carrying solenoid is half of that at the center.
Reason: If the solenoid is sufficiently long, the field within it is uniform.
(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.
(c) If assertion is true but reason is false.
(d) If the assertion and reason both are false.
(e) If assertion is false but reason is true.

Correct Answer is option (b)
For a solenoid . Also for a long solenoid, magnetic field is uniform within it but this reason is not explaining the assertion.