Free Electromagnetism MCQs with Answers

• A. weber
• B. tesla
• C. henry
• D. gauss

Explanation: One tesla is one weber per square metre, so the tesla measures flux density while the weber measures the total flux.

• A. F equals BIL sin theta
• B. F equals BIL cos theta
• C. F equals BI over L
• D. F equals B over IL

Explanation: The sine factor means the force is maximum when the conductor is perpendicular to the field and zero when it lies along the field, since a…

• A. the induced current in a generator
• B. the force on a current carrying conductor in a magnetic field
• C. the magnetic field around a wire
• D. the induced emf

Explanation: With the first finger along the field and the second along the current, the thumb gives the motion or force, which is the motor rule.

• A. B times A times cos theta, where theta is the angle between B and the normal to the surface
• B. B times A times sin theta
• C. B divided by A
• D. B times A only, whatever the orientation

Explanation: Flux counts the field lines threading the surface, so it is greatest when the field is perpendicular to the surface, meaning parallel to…

• A. maximum magnitude
• B. zero
• C. half the maximum
• D. qvB

Explanation: The magnetic force is qvB sin theta, and sin 0 is zero, so a charge travelling along the field lines feels nothing at all.

• A. a straight line
• B. a parabola
• C. a circular path
• D. a spiral of increasing radius

Explanation: The magnetic force is always perpendicular to the velocity, so it changes direction without changing speed, which is exactly the condition…

• A. very small
• B. always perpendicular to the velocity
• C. always opposite to the velocity
• D. not a real force

Explanation: Work is the dot product of force and displacement, and a force at right angles to the motion contributes nothing, so the kinetic energy…

• A. straight lines parallel to the wire
• B. concentric circles centred on the wire
• C. lines radiating outwards from the wire
• D. no field at all

Explanation: The right hand grip rule gives the sense of the circles: point the thumb along the conventional current and the curled fingers show the…

• A. attract each other
• B. repel each other
• C. exert no force on each other
• D. twist about one another

Explanation: Each wire sits in the field of the other, and applying the left hand rule shows the forces to be inward when the currents are parallel…

• A. r equals qB over mv
• B. r equals mv over qB
• C. r equals mvB over q
• D. r equals qvB

Explanation: Equating the magnetic force qvB to the required centripetal force mv squared over r and rearranging gives r equal to mv over qB.

• A. the coil experiences a constant torque whatever its angular position
• B. the coil turns faster
• C. the current is increased
• D. the coil never moves

Explanation: With curved pole pieces and a soft iron core the field is always parallel to the plane of the coil, so the torque stays proportional to…

• A. high resistance is connected in series
• B. low resistance called a shunt is connected in parallel
• C. high resistance is connected in parallel
• D. capacitor is connected in series

Explanation: The shunt carries most of the current so that only a small known fraction passes through the delicate coil, and it also keeps the overall…

• A. parallel to the field
• B. perpendicular to the field
• C. at 45 degrees to the field
• D. irrelevant to the flux

Explanation: Flux depends on the cosine of the angle between the field and the normal to the coil, so it is maximum when the normal points along the…

• A. 1 weber
• B. 0.005 weber
• C. 100 weber
• D. 0.5 weber

Explanation: The flux through one turn is BA, that is 0.5 multiplied by 0.01, giving 0.005 Wb, and the flux linkage multiplies this by the 200 turns to…

• A. zero
• B. strongest at the ends
• C. nearly uniform and parallel to the axis
• D. circular around the axis

Explanation: The overlapping fields of the individual turns reinforce along the axis and largely cancel between adjacent turns, giving a field much…

• A. is cheaper
• B. loses its magnetism quickly when the current is switched off
• C. retains its magnetism permanently
• D. conducts electricity better

Explanation: An electromagnet must be able to be switched off, which requires a core of low retentivity, and soft iron is easily magnetised and just as…

• A. obey an inverse square law
• B. act only over a few millimetres
• C. are always attractive
• D. require a material medium

Explanation: Both fall off as one over the square of the separation and both can be attractive or repulsive depending on the sign or polarity involved.

• A. heavier ions move faster
• B. heavier ions follow a path of larger radius in the magnetic field
• C. lighter ions are stopped by the field
• D. heavier ions lose their charge

Explanation: The radius mv over qB is proportional to mass when charge, speed and field are fixed, so heavier ions curve less sharply and strike the…

• A. a net force that accelerates it across the field
• B. a torque that tends to align its plane perpendicular to the field
• C. neither force nor torque
• D. a force along the field lines

Explanation: The forces on opposite sides of the coil are equal and opposite, so they cancel and there is no net translational force, but because they…

• A. 5 tesla
• B. 50 microtesla
• C. 50 millitesla
• D. zero at the equator

Explanation: At around 50 microtesla the geomagnetic field is weak enough that a compass needle needs to be delicately mounted to respond to it, and…