Free Electromagnetism MCQs with Answers

11. A moving coil galvanometer uses a radial magnetic field so that

• 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 the current at every angle and the scale is linear. Without it the torque would fall off as the cosine of the deflection and the scale would be cramped at the ends. The restoring couple comes from a hair spring.

12. To convert a galvanometer into an ammeter, a

• 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 resistance very low as an ammeter requires. Converting to a voltmeter is the opposite operation: a large resistance in series limits the current and gives the high resistance a voltmeter needs. Both conversions preserve the meter's own full scale deflection current.

13. The magnetic flux through a coil is maximum when the plane of the coil is

• 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 field, which means the plane of the coil is at right angles to it. When the plane contains the field lines, none of them thread the coil and the flux is zero. Rotating a coil between these extremes is what generates alternating current.

14. A coil of 200 turns and area 0.01 m squared lies perpendicular to a field of 0.5 T. The flux linkage is

• 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 give 1 Wb. Forgetting the number of turns leaves the 0.005 distractor. Flux linkage rather than flux is what appears in Faraday's law for a coil.

15. Inside a long current carrying solenoid the magnetic field is

• 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 like that of a bar magnet but uniform inside. Its strength is proportional to the current and to the number of turns per unit length, and inserting a soft iron core multiplies it many times. The field weakens and spreads near the ends.

16. Soft iron rather than steel is used for the core of an electromagnet because soft iron

• 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 easily demagnetised. Steel retains magnetism and is therefore used for permanent magnets instead. This difference in hysteresis behaviour is the whole basis of choosing between them.

17. The force between the poles of two magnets and the force between two charges are alike in that both

• 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. The essential difference is that an isolated magnetic pole has never been found, whereas an isolated electric charge is ordinary. Cutting a magnet in half simply produces two smaller magnets, each with both poles.

18. In a mass spectrometer, ions of the same charge and speed but different masses are separated because

• 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 detector further along. Measuring where each beam lands therefore measures the mass. This is how isotopes were discovered and how their relative abundances are determined.

19. A current carrying coil placed in a uniform magnetic field experiences

• 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 act along different lines they form a couple that rotates the coil. The rotation continues until the plane of the coil is perpendicular to the field, at which point the torque is zero. A motor uses a commutator to reverse the current at that instant so rotation continues.

20. The Earth's magnetic field is approximately

• 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 strong enough to deflect charged particles from the Sun. The field is horizontal near the magnetic equator and dips steeply near the poles. A laboratory electromagnet is thousands of times stronger.