Free Magnetic Flux MCQs with Answers

1. Magnetic flux through a surface is defined as

• 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 its normal, and zero when the field lies in the plane of the surface. Its unit is the weber. Every question on induction depends on tracking how this quantity changes with time.

2. 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.

3. 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.