All Free Physics MCQs with Answers

181. In a step up transformer, if the voltage is doubled then for an ideal transformer the current in the secondary is

• A. doubled
• B. halved
• C. unchanged
• D. quadrupled

Explanation: Power is conserved in an ideal transformer, so if the voltage doubles the current must halve to keep the product the same. This is why high voltage transmission lines carry surprisingly small currents. Any real transformer loses a few per cent, so the output power is slightly less than the input.

182. Ultraviolet radiation is more damaging to skin than visible light because ultraviolet photons

• A. travel faster
• B. have a longer wavelength
• C. carry more energy, enough to break chemical bonds in DNA
• D. are more numerous in sunlight

Explanation: Photon energy is proportional to frequency, so the shorter wavelength ultraviolet carries enough energy per photon to damage the bonds in biological molecules, which is why it causes sunburn and raises skin cancer risk. Visible photons simply cannot deliver that much energy however intense the beam. All electromagnetic waves travel at the same speed, so speed is not the difference.

183. Faraday's law states that the induced emf in a circuit is proportional to

• A. the magnetic flux through it
• B. the rate of change of magnetic flux linkage
• C. the current in it
• D. the resistance of the circuit

Explanation: It is the rate of change that matters, not the flux itself, so a coil sitting in a strong but steady field has no emf induced in it at all. Moving the magnet faster, using more turns or using a stronger field all increase the rate of change and therefore the emf. This is the single most important idea in the chapter.

184. A bar magnet is held stationary inside a coil. The induced emf in the coil is

• A. maximum
• B. zero
• C. half the maximum
• D. reversed in direction

Explanation: With the magnet at rest the flux linkage is constant, so its rate of change is zero and no emf is induced however strong the magnet. An emf appears only while the magnet is moving in or out, or while the field is changing. This is why a generator must keep rotating to keep producing electricity.

185. Lenz's law states that the direction of an induced current is such that it

• A. assists the change producing it
• B. opposes the change producing it
• C. is always clockwise
• D. is independent of the change

Explanation: The induced current sets up a field that opposes the flux change, so pushing a north pole towards a coil induces a current making the near face a north pole, which resists the approach. This opposition is why work must be done to move the magnet, and that work is the source of the electrical energy produced. Lenz's law is therefore conservation of energy applied to induction.

186. The minus sign in the expression for Faraday's law expresses

• A. that the emf is always negative
• B. Lenz's law, that the induced effect opposes the change causing it
• C. that flux always decreases
• D. an error of convention with no physical meaning

Explanation: The sign encodes the direction of the induced emf rather than its magnitude, stating that it acts to oppose the change in flux. Without it, an induced current could reinforce the change producing it and energy would be created from nothing. The magnitude of the emf is given by the rest of the expression.

187. A magnet dropped down a long vertical copper tube falls unusually slowly because

• A. copper is magnetic and attracts it
• B. air resistance inside the tube is very large
• C. induced eddy currents in the tube produce a field opposing the magnet's motion
• D. the magnet loses its magnetism as it falls

Explanation: The changing flux induces circulating eddy currents in the tube wall, and by Lenz's law their magnetic field opposes the motion, retarding the fall. Copper is not itself magnetic, so the effect disappears entirely if a plastic tube is used, which is the standard control for this demonstration. The same principle gives eddy current braking in trains.

188. A transformer works on the principle of

• A. self induction only
• B. mutual induction between two coils linked by a common changing flux
• C. electrostatic attraction
• D. the motor effect

Explanation: Alternating current in the primary produces a continuously changing flux, which the laminated iron core carries to the secondary, where it induces an alternating emf. Since the process depends entirely on a changing flux, a transformer cannot work on direct current. Self induction is the related effect a single coil has on itself.

189. A step down transformer has

• A. more turns on the secondary than the primary
• B. fewer turns on the secondary than the primary, giving a lower output voltage
• C. the same number of turns on both
• D. no core

Explanation: Voltage divides in the ratio of the turns, so fewer secondary turns give a lower voltage, and for an ideal transformer the current rises in the same proportion so that power is conserved. This is what a phone charger does, dropping 220 V to a few volts. A step up transformer reverses the turns ratio.

190. A transformer with 1000 primary turns and 100 secondary turns is supplied at 220 V. The secondary voltage is

• A. 2200 V
• B. 22 V
• C. 220 V
• D. 2.2 V

Explanation: The turns ratio is 10 to 1 down, so the voltage falls by the same factor, from 220 V to 22 V. If the transformer is ideal the secondary current is ten times the primary current, keeping the power the same on both sides. Multiplying instead of dividing gives the 2200 V distractor.