Electronic Devices Questions and Answers

Answer: A

The source voltage is 12 V.

In the upper branch the two diodes face opposite ways, so one of them is always reverse-biased and no current flows there. In the lower branch both ideal diodes are forward-biased and have zero resistance, so all 5 mA flows through the lower 2.4 kΩ resistor.

\[ V = 5 \times 10^{-3} \times 2.4 \times 10^{3} = 12\ \text{V} \]

6 V treats the two resistors as being in parallel (1.2 kΩ), and 24 V as being in series (4.8 kΩ); both ignore the blocked upper branch.

Answer: C

Solar cells are made of semi-conductors.

A solar (photovoltaic) cell is a p-n junction, usually of silicon. Light falling on the junction frees electrons and holes, which the junction's electric field separates, producing a current. This photovoltaic effect needs a semiconductor's band gap.

Metals have no band gap, so absorbed light only heats them; insulators have a band gap too wide for sunlight to bridge; and glass is an insulator, used only as a protective cover over the cells.

Answer: A

A solar cell is based on the photovoltaic effect.

When light falls on a p-n junction, photons create electron-hole pairs that the junction's electric field separates, producing a voltage and a current in an external circuit. This direct conversion of light into electricity is the photovoltaic effect.

The thermoelectric effect converts heat differences into voltage (as in a thermocouple), the piezoelectric effect produces voltage from mechanical stress, and electromagnetic induction produces voltage from changing magnetic fields.

Answer: A

Solar cells are made of silicon.

Silicon is a semiconductor. In a solar cell, layers of n-type and p-type silicon form a junction, and light falling on it frees electrons that flow as an electric current, the photovoltaic effect. Most solar panels use crystalline silicon.

Iron and copper are metals, good conductors but not semiconductors, so they cannot produce the photovoltaic effect. Wood is an insulator.

Answer: A

The device that converts AC into DC is the Rectifier.

A rectifier uses diodes (or controlled devices) that conduct in one direction only, so an alternating input produces a unidirectional output, which is then smoothed by filters.

An inverter does the opposite: it converts DC into AC. A thyristor is a switching device that can be used inside a controlled rectifier or an inverter, but it is a component, not the converter itself. A dynamo is a DC generator that converts mechanical energy into electrical energy.