All Free Physics MCQs with Answers

211. 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.

212. The radius of the circular path of a charged particle in a magnetic field is given by

• 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 heavier or faster particle therefore curves less, while a stronger field curves it more tightly. Because the radius depends on mass for a given charge and speed, this expression is the basis of the mass spectrometer.

213. 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.

214. 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.

215. 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.

216. 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.

217. 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.

218. 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.

219. 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.

220. 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.