Free Magnetic Flux Density MCQs with Answers

1. The SI unit of magnetic flux density is the

• 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. The tesla is a large unit: the Earth's field is only about 50 microtesla, while an MRI scanner reaches 1.5 T or more. The henry is the unit of inductance and the gauss is the old CGS unit, with 1 T equal to 10,000 gauss.

2. The force on a straight conductor of length L carrying current I in a magnetic field B is given by

• 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 current parallel to B feels no force at all. The direction is given by Fleming's left hand rule, at right angles to both the current and the field. This force is what turns an electric motor.

3. Fleming's left hand rule gives the direction of

• 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. The right hand rule is the generator rule, giving the direction of an induced current. Remembering that left is for motors and right is for generators avoids most of the confusion here.

4. The magnetic field around a long straight current carrying wire consists of

• 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 field direction. The strength falls off as one over the distance from the wire, not as one over the distance squared. Inside a long solenoid, by contrast, the field is nearly uniform and parallel to the axis.

5. Two parallel wires carrying currents in the same direction

• 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, outward when they are antiparallel. So like currents attract, which is the opposite of the behaviour of like charges. This force is what defines the ampere.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.