Physics Multiple Choice Questions on “Magnetism and Gauss’s Law”.
1. What is the net magnetic flux through a closed surface?
a) Positive
b) Negative
c) Zero
d) Depends on the nature of the surface
Answer: c
Clarification: Gauss’s law indicates that there are no sources or sinks of the magnetic field inside a closed surface. A “closed surface” is a surface that completely encloses a volume with no holes. Therefore, the net magnetic flux through a closed surface is zero.
2. What is the torque exerted by a bar magnet on itself due to its field?
a) Maximum
b) Zero
c) Minimum
d) Depends on the direction of the magnetic field
Answer: b
Clarification: A bar magnet does not exert a force or torque on itself due to its field. But an element of a current-carrying conductor experiences forces due to another element of the conductor. So, the torque exerted by a bar magnet on itself is zero.
3. When does a magnetic dipole possess maximum potential energy inside a magnetic field?
a) Magnetic moment and magnetic field are antiparallel
b) Magnetic moment and magnetic field are parallel
c) The magnetic moment is zero
d) The magnetic field is zero
Answer: a
Clarification: A magnetic dipole possess maximum potential energy when its magnetic moment and the magnetic field are antiparallel. When the magnetic dipole is aligned along the magnetic field, i.e. when θ = 1800, it is in unstable equilibrium having maximum potential energy.
4. Calculate the surface integral of a magnetic field over a surface.
a) Maximum
b) Minimum
c) Zero
d) Equal to its magnetic flux through that surface
Answer: d
Clarification: The Gauss’s law states that the surface integral of a magnetic field over a closed surface is always zero. But the surface integral of a magnetic field over a surface gives magnetic flux through that surface.
5. Which of the following is not a consequence of Gauss’s law?
a) The magnetic poles always exist as unlike pairs of equal strength
b) If several magnetic lines of force enter a closed surface, then an equal number of lines of force must leave that surface
c) There are abundant sources or sinks of the magnetic field inside a closed surface
d) Isolated magnetic poles do not exist
Answer: c
Clarification: Gauss‘s law indicates that there are no sources or sinks of the magnetic field inside a closed surface. In other words, there are no free magnetic charges. Hence, magnetic monopoles do not exist.
6. Isolated magnetic poles exist.
a) True
b) False
Answer: b
Clarification: Gauss’s law indicates that there are no sources or sinks of the magnetic field inside a closed surface. So there is no point at which the field lines start or there is no point at which the field lines terminate. Hence, isolated magnetic poles do not exist.
7. Which among the following is the source of the magnetic field (magnetism)?
a) Mechanical origin
b) Electrical origin
c) Chemical origin
d) Potential origin
Answer: b
Clarification: Magnetism is of electrical origin. The electrons revolving in an atom behave as tiny current loops and these current loops give rise to magnetism. An electric current produces a magnetic field. This magnetic field can be visualized as a pattern of circular field lines circling a wire.
8. The line of force in a magnetic field represents the direction at each point that a magnetic needle placed at the point takes up. Do they also represent the direction of the force on a moving charge at each point?
a) Not possible
b) Represent circular motion
c) Represent tangential motion
d) Represents translatory motion
Answer: a
Clarification: No. The force on a charge is perpendicular to the direction of the magnetic field at each point.
F = q (v × B).
It is inappropriate to call magnetic field lines as lines of force.