Introduction to Electronics

14 Grade 10 Physics questions on Introduction to Electronics, with worked marking schemes. 14 questions are available on this topic in full.

1. Question 1

   1 mark

   A solar panel on a homestead in Kitui charges a 12 V battery. The technician fits a diode between the panel and the battery so that at night the battery cannot drive current back through the panel. Which property of the diode is being used?

   A. It changes the direct current produced by the panel into alternating current.

   B. It raises the voltage of the panel so that it is always above that of the battery.

   C. It stores the charge produced during the day and releases it slowly at night.

   D. It allows current to pass through it in one direction only.

   (1 mark)

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2. Question 2

   3 marks

   A learner connects the n-side of a silicon diode to the positive terminal of a battery and the p-side to the negative terminal, with a torch bulb in series. Explain why the bulb does not light.

   (3 marks)

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3. Question 3

   2 marks

   Distinguish between an intrinsic semiconductor and an extrinsic semiconductor.

   (2 marks)

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4. Question 4

   1 mark

   A shopkeeper replaces the filament bulbs in his solar lanterns with light-emitting diodes of the same brightness, and finds that the lanterns now run for much longer on one charge. The main reason is that:

   A. an LED converts a much larger fraction of the electrical energy supplied to it into light.

   B. an LED heats its filament to a lower temperature, so less energy is wasted as heat.

   C. an LED passes current in both directions, so the battery is recharged during part of the time.

   D. an LED works on a much higher voltage, so it draws a smaller current from the battery.

   (1 mark)

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5. Question 5

   1 mark

   In a p-type semiconductor, conduction is described as the movement of holes. A hole moves through the crystal when:

   A. a tiny empty space between the atoms is pushed along the crystal by the applied voltage.

   B. an electron in a neighbouring bond shifts into the vacancy, leaving a new vacancy behind it.

   C. a proton hops from one atom to the next in the direction opposite to that of the electrons.

   D. an atom of the doping element itself drifts slowly towards the negative terminal.

   (1 mark)

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6. Question 6

   4 marks

   Describe what happens at the junction when a piece of p-type silicon is joined to a piece of n-type silicon to form a diode.

   (4 marks)

   Marking scheme

   - Free electrons from the n-side diffuse across the junction into the p-side, while holes from the p-side diffuse into the n-side (1 mark) - The electrons and holes recombine near the junction, leaving a thin region with no mobile charge carriers, called the depletion layer (1 mark) - The impurity atoms left behind are fixed, giving positive ions on the n-side of the junction and negative ions on the p-side (1 mark) - This separated charge sets up a potential difference (barrier p.d., about 0.6 V for silicon) across the junction, which opposes and finally stops further diffusion (1 mark)

7. Question 7

   2 marks

   Explain why a resistor must be connected in series with a light-emitting diode when it is operated from a 6 V battery.

   (2 marks)

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8. Question 8

   3 marks

   Explain how doping pure silicon with boron, a Group III element, produces a p-type semiconductor.

   (3 marks)

   Marking scheme

   - A boron atom has only three valence electrons, one fewer than silicon (1 mark) - It therefore forms complete bonds with only three of the four neighbouring silicon atoms, so one bond is short of an electron, leaving a vacancy called a hole (1 mark) - Every boron atom added creates such a hole, so holes become the majority charge carriers and conduction takes place mainly by the movement of holes; the material is described as p-type (1 mark)

9. Question 9

   1 mark

   A silicon diode is connected in series with a torch bulb and a 3 V battery, with the p-side of the diode joined to the positive terminal. The bulb lights. The best explanation is that:

   A. heat from the battery melts the depletion layer and frees the electrons trapped inside it.

   B. the depletion layer becomes wider, giving the charge carriers more energy to cross it.

   C. the battery changes the n-type material into p-type material so that the junction disappears.

   D. the depletion layer becomes narrower, so electrons and holes cross the junction and current flows.

   (1 mark)

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10. Question 10

    1 mark

    A learner adds a very small amount of phosphorus, a Group V element, to a crystal of pure silicon. The doped silicon is found to conduct electricity much better than the pure silicon. Which statement explains this?

    A. The phosphorus atoms break the silicon bonds so that protons are set free to move through the crystal.

    B. The phosphorus atoms give the crystal an overall negative charge which pushes electrons along it.

    C. Each phosphorus atom uses four of its valence electrons in bonds with silicon, leaving one electron free to move.

    D. The phosphorus atoms lower the melting point of the silicon so that its atoms are able to drift.

    (1 mark)

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11. Question 11

    4 marks

    Describe how a single diode is used to obtain a direct current from an alternating supply, and state one disadvantage of the arrangement.

    (4 marks)

    Marking scheme

    - The diode is connected in series with the load (resistor) across the alternating supply (1 mark) - During the half cycle in which the diode is forward biased it conducts, and current passes through the load (1 mark) - During the next half cycle the diode is reverse biased and blocks the current, so no current passes through the load; the current in the load is therefore in one direction only, that is a pulsating direct current (half-wave rectification) (1 mark) - Disadvantage: only half of each cycle is used, so the average current and power delivered to the load are low, and the output is very uneven (1 mark for any one)

12. Question 12

    1 mark

    A sample of pure germanium and a length of copper wire are each warmed gently. Which statement about the sample of germanium is correct?

    A. Its resistance stays constant until the germanium begins to melt.

    B. Its resistance changes in exactly the same way as that of the copper wire.

    C. Its resistance rises because its atoms vibrate more strongly and obstruct the carriers.

    D. Its resistance falls because more electrons gain enough energy to break free from their bonds.

    (1 mark)

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13. Question 13

    1 mark

    A copper wire, a silicon bar and a rubber rod of the same length and thickness are each connected in turn in a circuit containing a battery and an ammeter. The reading is large for copper, very small for silicon and too small to detect for rubber. The best explanation is that:

    A. only metals are able to carry an electric current, so the readings for silicon and rubber mean nothing.

    B. copper has very many free electrons, silicon has far fewer, and rubber has almost none.

    C. silicon and rubber conduct only when the connections to the battery are reversed.

    D. the current depends only on the density of the material, and copper is the densest of the three.

    (1 mark)

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14. Question 14

    3 marks

    Suggest three ways in which the spread of electronic devices has improved the work of a small-scale farmer in Kenya.

    (3 marks)

    Marking scheme

    Any three, 1 mark each: - A mobile phone lets the farmer find out market prices and contact buyers without travelling to town - Weather forecasts and extension advice received by radio or SMS help in deciding when to plant, spray or harvest - Mobile money makes it safe to pay for seed and fertiliser and to receive payment for produce - Electronic sensors and timers control irrigation, incubators or greenhouse conditions automatically - Electronic weighing scales and milk-testing meters give accurate measurements when selling produce

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