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Physics questions, chapter 5
Nama / NameKelas / ClassTarikh / Date

Electronics

44 marks. Use Practice to check each answer as you go, or Exam to answer everything against the clock first.

Section A: objective questions

12 marks
Choose the best answer for each question. Charge of an electron e = 1.6 × 10⁻¹⁹ C; mass of an electron m = 9.11 × 10⁻³¹ kg.
1

Question 1

What is thermionic emission?
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A
Heating a metal gives its free electrons enough kinetic energy to leave the surface. This is thermionic emission.
For the teacher: Remembering, low difficulty, standard 5.1.1
Textbook p. 174
2

Question 2

What are cathode rays?
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B
Cathode rays are fast electron beams in a vacuum. They are negatively charged, so they are attracted to the anode.
For the teacher: Remembering, low difficulty, standard 5.1.2
Textbook p. 176
3

Question 3

A Maltese cross tube produces a sharp shadow of the cross on the fluorescent screen. What does this show about cathode rays?
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C
An object that blocks the rays gives a sharp shadow because the rays travel in straight lines.
For the teacher: Understanding, low difficulty, standard 5.1.2
Textbook p. 178
4

Question 4

An electron is accelerated from rest through a potential difference of 250 V in a vacuum. What is its maximum velocity?
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D
eV = ½mv², so v = √(2 × 1.6 × 10⁻¹⁹ × 250 / 9.11 × 10⁻³¹) = √(8.78 × 10¹³) = 9.4 × 10⁶ m s⁻¹.
For the teacher: Applying, medium difficulty, standard 5.1.3
Textbook p. 179
5

Question 5

What is the function of a semiconductor diode?
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D
A diode conducts when forward biased and blocks when reverse biased.
For the teacher: Remembering, low difficulty, standard 5.2.1
Textbook p. 182
6

Question 6

What happens to the depletion layer of a diode when it is forward biased?
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B
In forward bias the holes and electrons move towards the junction, so the depletion layer becomes thinner and the diode has very small resistance.
For the teacher: Understanding, low difficulty, standard 5.2.1
Textbook p. 182
7

Question 7

An a.c. supply is connected in series with a single diode and a resistor. Which describes the potential difference across the resistor?
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C
The diode conducts only in the positive half cycle (half-wave rectification), so the output is pulsating d.c. with gaps.
For the teacher: Understanding, medium difficulty, standard 5.2.2
Textbook p. 184
8

Question 8

In a bridge rectifier, diodes D₁ and D₂ conduct during the positive half cycle. Which diodes conduct during the negative half cycle?
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A
The pairs alternate: D₁ and D₂ in the positive half cycle, D₃ and D₄ in the negative half cycle. The current through the load keeps the same direction.
For the teacher: Understanding, low difficulty, standard 5.2.2
Textbook p. 184
9

Question 9

A capacitor is connected in parallel with the load of a rectifier. How does it make the output smoother?
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B
The capacitor charges as the potential difference rises and discharges as it falls, so the output does not drop to zero.
For the teacher: Understanding, low difficulty, standard 5.2.2
Textbook p. 186
10

Question 10

Which terminal of a transistor is a thin layer that controls the flow of charge carriers from the emitter to the collector?
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A
The base is the thin middle layer. A small base current controls the large collector current.
For the teacher: Remembering, low difficulty, standard 5.3.1
Textbook p. 187
11

Question 11

A transistor circuit is changed from an npn transistor to a pnp transistor. What else must change for the circuit to work?
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D
For a pnp transistor the polarity of the base and collector batteries is reversed, so the directions of IB, IC and IE are also reversed.
For the teacher: Applying, low difficulty, standard 5.3.1
Textbook p. 188
12

Question 12

A light-controlled switch uses an npn transistor. A fixed resistor and an LDR form a potential divider, with the LDR as the lower resistor, and the voltage across the LDR is applied to the base. Why does the lamp light up in the dark?
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C
In the dark the LDR has a high resistance, so it takes a larger share of the supply voltage. When VLDR is more than VBE, IB flows and turns on the collector circuit.
For the teacher: Analysing, medium difficulty, standard 5.3.2
Textbook p. 191

Section B: structured questions

32 marks
Answer all questions. Show your working for calculations.
13

Question 13

Diagram 1 shows a vacuum tube in which an electron beam is produced. The extra high tension (E.H.T.) supply is 2 000 V.
Vacuum tube with a heated filament, cathode, anode and E.H.T. supply
Diagram 1 (textbook Figure 5.1, p. 174)
[1 mark] (a)Name the process by which electrons leave the hot filament.
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[1 mark] (b)Why must the tube be in a vacuum?
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[1 mark] (c)Calculate the kinetic energy of an electron when it reaches the anode.
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[2 marks] (d)Calculate the maximum velocity of the electron. [m = 9.11 × 10⁻³¹ kg]
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For the teacher: Applying, medium difficulty, standard 5.1.3
Textbook p. 179
14

Question 14

A student uses a deflection tube and a Maltese cross tube to study cathode rays.
[1 mark] (a)When the plates of a deflection tube are connected to an E.H.T. supply, the beam bends towards the positive plate. What is the charge on cathode rays?
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Negative
[2 marks] (b)In a Maltese cross tube, a bar magnet is brought near the tube. State what happens to the shadow and what this shows.
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Tick each point you got.
For the teacher: Understanding, low difficulty, standard 5.1.2
Textbook p. 177
15

Question 15

Diagram 2 shows a rectifier circuit during the positive half cycle of the a.c. input.
Bridge rectifier with four diodes D1 to D4 and a load R
Diagram 2 (textbook Figure 5.9, p. 184)
[1 mark] (a)Name the arrangement of four diodes in this circuit.
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[1 mark] (b)Which diodes are forward biased in the positive half cycle?
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[1 mark] (c)State one advantage of this circuit over a single-diode circuit.
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Any 1 points
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[1 mark] (d)Name the component that should be connected in parallel with R to give a smooth output.
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For the teacher: Applying, low difficulty, standard 5.2.2
Textbook p. 184
16

Question 16

Diagram 3 shows the output of a half-wave rectifier and of a full-wave rectifier, each with a capacitor in parallel with the load.
Half-wave and full-wave rectifier circuits with a capacitor and the smoothed output waveforms
Diagram 3 (textbook Figure 5.12, p. 186)
[1 mark] (a)What does the capacitor do while the potential difference across R is falling?
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[2 marks] (b)Which output is smoother, half-wave or full-wave? Explain.
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[1 mark] (c)State one way to make the output smoother still.
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For the teacher: Evaluating, medium difficulty, standard 5.2.2
Textbook p. 186
17

Question 17

A student sets up an npn transistor circuit as an amplifier and records the base current IB and collector current IC.
IB / mA0.100.200.300.400.50
IC / mA15.631.246.862.478.0
[1 mark] (a)State the relationship between IC and IB.
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[2 marks] (b)Calculate the amplification factor β of the transistor.
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[1 mark] (c)Predict IC when IB = 0.35 mA.
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[1 mark] (d)State the role of the transistor in this circuit.
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For the teacher: Applying, low difficulty, standard 5.3.1
Textbook p. 190
18

Question 18

Diagram 4 shows a light-controlled switch for a street lamp. The supply is 6 V and the fixed resistor is 10 kΩ. In the dark the LDR is 100 kΩ; in bright light it is 0.5 kΩ. The transistor needs VBE of at least 0.7 V.
Light-controlled switch circuit with a 10 kilohm resistor above an LDR, a 1 kilohm base resistor, an npn transistor and a lamp on 6 V
Diagram 4 (textbook Figure 5.18, p. 191)
[2 marks] (a)Calculate the voltage across the LDR in the dark.
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[1 mark] (b)Calculate the voltage across the LDR in bright light.
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[2 marks] (c)Does the lamp light up in bright light? Explain.
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For the teacher: Applying, medium difficulty, standard 5.3.2
Textbook p. 191
19

Question 19

Diagram 5 shows a temperature-controlled alarm circuit. The supply is 6 V, the resistor R is 10 kΩ and the transistor is silicon.
Temperature-controlled alarm circuit with a thermistor above a 10 kilohm resistor, a 1 kilohm base resistor and an npn transistor
Diagram 5 (textbook Figure 5.19, p. 191)
[3 marks] (a)Explain how the circuit switches on the alarm when the temperature rises.
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[2 marks] (b)Calculate the largest resistance of the thermistor at which the transistor turns on. Ignore the base current.
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[1 mark] (c)Suggest a change to make this a frost alarm that rings when the temperature falls.
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For the teacher: Creating, high difficulty, standard 5.3.2
Textbook p. 191