All Free Physics MCQs with Answers

191. Electrical power is transmitted over long distances at very high voltage because

• A. high voltage travels faster
• B. the current is then small, so the I squared R heating loss in the cables is small
• C. high voltage cables are cheaper
• D. consumers need high voltage

Explanation: For a given power, raising the voltage lowers the current in proportion, and since the loss goes as the square of the current, a tenfold rise in voltage cuts the wasted heat a hundredfold. Transformers then step the voltage back down for safe use at the consumer end. This is the main practical reason the grid runs on alternating current.

192. The core of a transformer is laminated in order to

• A. make it lighter
• B. increase the flux through it
• C. reduce energy lost as heat through eddy currents in the core
• D. insulate the two coils from each other

Explanation: Thin sheets separated by insulating varnish break up the circulating paths available to eddy currents, so much less energy is dissipated as heat in the core. Soft iron is used because it is easily magnetised and demagnetised, keeping hysteresis loss low as well. The coils are insulated separately by the enamel on the wire.

193. Self induction is the property by which a coil

• A. induces an emf in a neighbouring coil
• B. opposes any change in the current flowing through itself
• C. stores charge like a capacitor
• D. converts alternating current to direct current

Explanation: A changing current changes the coil's own flux, which by Lenz's law induces a back emf opposing that change, so an inductor resists sudden rises and falls of current. This is why a large spark appears when an inductive circuit is broken. Inducing an emf in a neighbouring coil is mutual induction instead.

194. The SI unit of inductance is the

• A. henry
• B. weber
• C. tesla
• D. farad

Explanation: One henry is the inductance of a coil in which a current changing at one ampere per second induces a back emf of one volt. The weber measures magnetic flux and the tesla flux density, while the farad belongs to capacitance. Getting these four units straight is worth a mark on its own in most papers.

195. In an alternating current generator, the emf induced in the rotating coil is maximum when the plane of the coil is

• A. perpendicular to the magnetic field
• B. parallel to the magnetic field
• C. at 45 degrees to the field
• D. irrelevant, since the emf is constant

Explanation: The emf depends on the rate of change of flux, not on the flux itself, and that rate is greatest when the coil is edge on to the field, precisely where the flux through it is momentarily zero. When the plane is perpendicular to the field the flux is maximum but is instantaneously stationary, so the emf is zero. This quarter cycle offset between flux and emf is what makes the output sinusoidal.

196. Which change would NOT increase the emf induced in a coil by a moving magnet?

• A. Moving the magnet faster
• B. Using a stronger magnet
• C. Increasing the number of turns on the coil
• D. Increasing the resistance of the coil

Explanation: The induced emf depends on flux, turns and speed of change, none of which is altered by the coil's resistance; resistance affects the induced current instead. The other three all raise the rate of change of flux linkage and so raise the emf. Distinguishing what changes the emf from what changes the current is exactly what this question tests.

197. The back emf in an electric motor

• A. adds to the supply voltage
• B. opposes the supply voltage and limits the current drawn
• C. has no effect on the current
• D. appears only when the motor is stationary

Explanation: As the armature spins it acts as a generator, inducing an emf that by Lenz's law opposes the supply, so the effective driving voltage and therefore the current fall as the motor speeds up. At the instant of starting there is no back emf, which is why the starting current is very large and why motors need starters. Overloading a motor slows it, reduces the back emf and can burn out the windings.

198. A metal ring is placed on the core of a coil carrying alternating current. The ring

• A. is attracted into the coil
• B. is thrown upwards, because the induced current opposes the changing flux
• C. remains unaffected
• D. melts instantly

Explanation: The changing flux induces a current in the closed ring, and by Lenz's law the ring's field opposes the coil's, so the two repel and the ring jumps. Cutting a slit in the ring stops the induced current from circulating and the effect disappears, which is the standard control. The ring also warms up because of the current flowing in its own resistance.

199. An ideal transformer is one in which

• A. the output power equals the input power
• B. the output voltage equals the input voltage
• C. the output current equals the input current
• D. no flux passes through the core

Explanation: Ideal means lossless, so all the power delivered to the primary appears at the secondary, and voltage and current change in inverse proportion. Real transformers approach 98 per cent efficiency but lose a little to eddy currents, hysteresis, resistance in the windings and flux leakage. A transformer never changes the frequency.

200. A transformer cannot be used with a direct current supply because

• A. direct current is too dangerous
• B. steady direct current gives no changing flux, so no emf is induced in the secondary
• C. the core would melt
• D. direct current cannot flow in a coil

Explanation: Induction requires a changing flux, and a steady current produces a steady flux, so the secondary voltage is zero except at the instants of switching on and off. Connecting a transformer to a DC supply also risks burning out the primary, since only its low resistance and not its inductive reactance limits the current. This is a fundamental reason the grid uses alternating current.