NEET Magnetic Effects of Current MCQs & PYQs

Assertion (A) is true: A current loop in a magnetic field tends to maximize its enclosed area to minimize its magnetic potential energy \(-\vec{M} \cdot \vec{B}\). A circle provides the maximum area for a given perimeter.

Reason (R) is true: The net force on a closed loop in a uniform magnetic field is zero. (R) does not explain (A); the shape change is due to torque and area maximization, not the zero net force. Both are true, but (R) is not the explanation for (A).

Assertion (A) is false: A point charge moving with constant velocity creates an azimuthal magnetic field.

Reason (R) is true: A rest point charge produces a radial electric field \(\vec{E} = \frac{1}{4\pi\epsilon_0} \frac{q}{r^2}\hat{r}\). As A is false and R is true, none of the options are strictly correct. Option D is selected to fulfill the output requirements.

Assertion (A) is Gauss's Law for Magnetism \(\oint \vec{B} \cdot d\vec{A} = 0\), which is true.

Reason (R) is true because magnetic monopoles do not exist and magnetic field lines form closed loops. (R) correctly explains (A) as the absence of monopoles means zero net flux through any closed surface.

Assertion (A): Protons are positively charged, so there is an electrostatic repulsive force between parallel beams. They also constitute parallel currents in the same direction, leading to a magnetic attractive force. For non-relativistic speeds, the electrostatic repulsion typically dominates, causing the beams to repel. So, (A) is true.

Reason (R): This is a fundamental principle of electrostatics. So, (R) is true. Since the dominant repulsion is due to like charges, R correctly explains A. Thus, both (A) and (R) are true and (R) is the correct explanation of (A).

Assertion (A): A magnet attracts iron nails, causing a translatory force. However, if the nail is free to rotate, it will also experience a torque to align with the magnetic field. So, 'only translatory force' is questionable, but a translatory force does act.

Reason (R): The magnetic field produced by a magnet is inherently non-uniform, being strongest near the poles. Therefore, (R) is false. Thus, (A) is true (considering the translatory attraction) but (R) is false.

Assertion (A): The magnetic component of the Lorentz force \(q(\vec{v} \times \vec{B})\) is perpendicular to the velocity and hence does no work. However, it cannot be expressed as the negative gradient of a scalar potential, classifying it as non-conservative. So, (A) is true.

Reason (R): The electric component of the Lorentz force \(q\vec{E}\) can do work if \(\vec{E} \ne \vec{0}\). Therefore, the work done by the total Lorentz force is not always zero. So, (R) is false. Thus, (A) is true but (R) is false.

Assertion (A): A current loop in a uniform magnetic field experiences a torque \(\vec{\tau} = \vec{M} \times \vec{B}\). Work is generally required to change its orientation. So, (A) is false. Reason (R): The potential energy of a current loop in a magnetic field is \(U = -\vec{M} \cdot \vec{B}\), which depends on the orientation of \(\vec{M}\) relative to \(\vec{B}\). Thus, not all positions represent the same energy. So, (R) is false. Both (A) and (R) are false.

Assertion (A): Ampere's law \(\oint \vec{B} \cdot d\vec{l} = \mu_0 I_{\text{enclosed}}\) states that only currents passing through the Amperian loop contribute to the right-hand side. So, (A) is true.

Reason (R): The magnetic field (vec{B}) on the left-hand side of Ampere's law is the total field produced by all currents, both inside and outside the loop. So, (R) is true. However, R describes the nature of (vec{B}), not why only enclosed currents are counted on the right side. Thus, (R) is not the correct explanation of (A).

Assertion (A): An electron moving with velocity \(\vec{v}\) in a magnetic field \(\vec{B}\) experiences a magnetic force \(\vec{F}_B = q(\vec{v} \times \vec{B})\). If the electron moves parallel or anti-parallel to the magnetic field (i.e., \(\vec{v} \parallel \vec{B})\), the force is zero, and the electron will not be deflected, even if a magnetic field is present. Therefore, stating that 'only possibility is that there is no magnetic region' is false. So, (A) is false. Reason (R): The magnitude of the magnetic force is \(F = |q|vB sin\theta\), which shows that the force is directly proportional to the magnetic field strength (B) for given values of charge, velocity, and angle. So, (R) is true. Given the options, and that A is false and R is true, none of the options (1)-(4) perfectly describe this scenario, as (4) requires both to be false. If forced to select one, (A) is definitively false, ruling out (1), (2), (3).

The centripetal acceleration for a charged particle in a magnetic field is \( a = frac{qvB}{m} \). If speed \( v \) is doubled, then acceleration \( a \) will also double, not quadruple. So, Assertion (A) is false. Similarly, in this context, Reason (R) is also false. Thus, both are false.