All Free Physics MCQs with Answers

271. A full wave bridge rectifier uses

• A. one diode
• B. two diodes
• C. four diodes
• D. no diodes

Explanation: Four diodes arranged in a bridge route both halves of the cycle through the load in the same direction, so the output has twice the pulse frequency of a half wave circuit and is far easier to smooth. Its advantage over the two diode centre tap arrangement is that it needs no centre tapped transformer. Two of the four diodes conduct during each half cycle.

272. A capacitor connected across the output of a rectifier acts as

• A. an amplifier
• B. a smoothing element, charging at the peaks and discharging between them to reduce ripple
• C. a switch
• D. an oscillator

Explanation: The capacitor stores charge while the rectified voltage rises and releases it while the voltage falls, filling in the troughs so the output approaches a steady direct voltage. A larger capacitance or a larger load resistance gives less ripple, since the time constant is longer. Full wave rectification smooths better than half wave because the gaps to be filled are shorter.

273. A Zener diode is normally operated

• A. in forward bias as a rectifier
• B. in reverse bias at its breakdown voltage, as a voltage regulator
• C. with no bias at all
• D. only at very high frequency

Explanation: Beyond the breakdown voltage a Zener conducts heavily while the voltage across it stays almost constant, which makes it a simple and effective voltage reference. Unlike an ordinary diode this breakdown is non destructive provided a series resistor limits the current. In forward bias it behaves like any other silicon diode.

274. A light emitting diode produces light when

• A. it is reverse biased and breaks down
• B. it is forward biased and electrons recombine with holes, releasing energy as photons
• C. it is heated
• D. current passes through a filament inside it

Explanation: Each recombination across the junction releases an energy equal to the band gap of the semiconductor, and in materials such as gallium arsenide phosphide that energy falls in the visible range. The colour therefore depends on the band gap of the material rather than on any coating. Because no filament is heated, an LED wastes far less energy than an incandescent lamp.

275. A photodiode is used to detect light because incident photons

• A. heat the junction
• B. create extra electron hole pairs, increasing the reverse current
• C. reduce the leakage current to zero
• D. change the doping of the material

Explanation: Photons with enough energy free electrons across the band gap in the depletion region, so the reverse current rises in proportion to the light intensity and gives a measurable signal. The device is therefore operated in reverse bias, the opposite of an LED. Solar cells work on the same principle but are designed to deliver power rather than a signal.

276. Compared with a conductor, a pure semiconductor at room temperature has

• A. far fewer free charge carriers, and a conductivity that rises with temperature
• B. more free carriers
• C. the same number of carriers
• D. no carriers at any temperature

Explanation: A pure semiconductor has a small energy gap, so only a few electrons have enough thermal energy to cross it at room temperature, but heating promotes many more and conductivity climbs sharply. In a metal the carrier number is fixed and heating only increases scattering, so resistance rises. Doping raises the carrier concentration enormously without needing heat.

277. The energy gap between the valence band and the conduction band is largest in

• A. a conductor
• B. a semiconductor
• C. an insulator
• D. all three equally

Explanation: An insulator has a gap of several electron volts, far more than thermal energy can supply, so almost no electrons reach the conduction band. A semiconductor's gap is about 1 eV, small enough for a useful number to cross, and in a conductor the bands overlap so there is effectively no gap at all. This single picture explains the huge range of electrical behaviour in solids.

278. In an n type semiconductor, the minority carriers are

• A. electrons
• B. holes
• C. protons
• D. positive ions

Explanation: Doping supplies a large number of electrons, but thermal generation still creates a small population of holes, and these minority carriers are what produce the leakage current in a reverse biased junction. Their number rises steeply with temperature, which is why semiconductor devices become unreliable when they overheat. In p type material the roles are exactly reversed.

279. The ripple frequency at the output of a full wave rectifier fed from a 50 Hz supply is

• A. 25 Hz
• B. 50 Hz
• C. 100 Hz
• D. 200 Hz

Explanation: Both halves of each cycle produce an output pulse, so there are two pulses per input cycle and the ripple appears at twice the supply frequency. A half wave rectifier would give ripple at 50 Hz instead. The higher ripple frequency is one reason full wave output is easier to smooth.

280. Silicon is preferred to germanium in most modern devices because silicon

• A. has a lower barrier potential
• B. tolerates higher temperatures and has a much smaller leakage current
• C. is a better conductor
• D. requires no doping

Explanation: Silicon's larger band gap means far fewer thermally generated carriers, so leakage is low and devices work reliably at higher temperatures, and it also forms a stable oxide that makes integrated circuits possible. Germanium's lower barrier of 0.3 V is an advantage in some low voltage detector circuits, which is why it has not vanished entirely. Silicon is also abundant and cheap.