Senior 3 practice book · Unit 10 of 10

Basic Electronic Components

50 questions that complete the Senior 3 quiz for this unit: 28 core and 22 challenge. Easy and hard questions are mixed. Try each question, then tap “Show answer”.

Common misconceptions

A capacitor and an inductor are active components because they store energy.Storing energy does not make a component active. Active components (diodes, transistors) can control or switch current; resistors, capacitors and inductors are passive.
An LED limits its own current, so it can be connected straight to any battery.Once an LED conducts, a little extra voltage makes its current rise sharply. A series resistor is needed to take the extra voltage and limit the current.
A transistor amplifier creates energy, because the output signal is bigger than the input.The extra energy comes from the power supply. The small input signal only controls how much of the supply's energy reaches the output.
The resistance of an LDR increases when more light falls on it.An LDR's resistance FALLS in bright light and rises in the dark; a thermistor (NTC) similarly has a lower resistance when it is hot.

What this unit covers

Topics marked new are not tested much in the quiz, so this book gives them extra questions.

Passive and active components: meaning and classification
Resistors: fixed and variable, units and use in circuits
Resistor colour code and tolerance
Capacitors: storing charge, the farad, Q = CV, smoothing and timing, polarity
Inductors: energy in a magnetic field, the henry, opposing changes of current
Diodes: forward and reverse bias, the 0.7 V drop, rectification
Light-emitting diodes and their series resistors
Sensors (LDR, thermistor) and the potential divider
Transistors: terminals, switch and amplifier, current gain
Circuit symbols of electronic components and reading circuit diagrams
Testing components with a multimeter (ohms, diode test, ranges)
Handling and replacing components safely: static, soldering, ratings, e-waste

Go to the questions

Questions (1–50)

Easier and harder questions are mixed, just like in a real exam. The 22 harder ones are marked ★ Challenge.

1True or false
A diode connected in reverse bias has a very high resistance.
Show answer
Answer: True
In reverse bias almost no current flows, so the diode behaves like a very large resistance (an open switch).
Common mistakeLearners may think a diode's resistance is the same both ways, like a resistor's.
2True or false · ★ Challenge
In the circuit symbol for a cell, the longer line is the negative terminal.
Show answer
Answer: False
The longer line is the positive terminal; the shorter, thicker line is the negative terminal.
Common mistakeLearners often guess that the bigger line is negative; remember 'long is plus'.
3True or false
A passive component can still store energy, as a capacitor does.
Show answer
Answer: True
Capacitors and inductors store energy but cannot amplify or switch a signal, so they are passive.
Common mistakeLearners often think storing energy makes a component active; the test is whether it can control or amplify current.
4True or false
The resistance of an LDR falls as the light falling on it gets brighter.
Show answer
Answer: True
More light frees more charge carriers in the LDR, so it conducts better.
Common mistakeLearners often reverse this, thinking light makes it 'resist more'.
5True or false · ★ Challenge
An electrolytic capacitor connected the wrong way round can overheat and even burst.
Show answer
Answer: True
Electrolytic capacitors are polarised: their + leg must go to the more positive side, or the insulating layer breaks down.
Common mistakeLearners think capacitors, like resistors, can go either way; electrolytic capacitors are marked with a stripe for the negative leg.
6True or false · ★ Challenge
When a transistor is used as a switch and its base current is zero, it is 'off' and almost no collector current flows.
Show answer
Answer: True
With no base current the collector–emitter path does not conduct, so the transistor acts like an open switch.
Common mistakeLearners sometimes think the collector current flows anyway because it comes from the supply; the base current must be present to turn it on.
7True or false
Adding more turns to a coil, or putting an iron core inside it, increases its inductance.
Show answer
Answer: True
Both make a stronger magnetic field for the same current, so the coil opposes changes in current more strongly.
Common mistakeLearners sometimes think an iron core lowers inductance because iron has resistance; its magnetic effect raises the inductance.
8True or false
Holding a hot soldering iron on a component's leg for a very long time can damage the component.
Show answer
Answer: True
Heat travels along the leg into the component; diodes, transistors and ICs can be ruined by overheating, so joints should be made quickly.
Common mistakeLearners believe a longer heating time always gives a better joint; a good joint is made quickly with the right temperature.
9Multiple choice · ★ Challenge
Which description matches the circuit symbol for a capacitor rather than a cell?
1. AOne long line beside one short line
2. BA circle containing a cross
3. CTwo parallel lines of equal length
4. DA zigzag line between two wires
Show answer
Answer: C. Two parallel lines of equal length
A capacitor is drawn as two equal plates; a cell has a long (positive) and a short (negative) line.
Common mistakeChoosing 'one long line and one short line' describes a cell; the plates of a capacitor are drawn the same length.
10Fill in the blank
A variable resistor with three terminals and a sliding contact, used as a volume control, is called a ______.
Show answer
Answer: potentiometer
Turning the knob moves the sliding contact along a resistance track, changing the resistance in the circuit.
Common mistakeSome learners write 'rheostat' only; a potentiometer with three terminals can be used both ways, as a variable resistor or a potential divider.
11Multiple choice · ★ Challenge
A phone charger uses a bridge of four diodes instead of a single diode to rectify a.c. What is the advantage?
1. AThe output becomes alternating at twice the voltage
2. BBoth halves of each a.c. cycle are used (full-wave)
3. CNo smoothing capacitor is ever needed after it
4. DThe diodes then let current flow both ways
Show answer
Answer: B. Both halves of each a.c. cycle are used (full-wave)
In a bridge, two diodes conduct on each half-cycle and send the current through the load in the same direction, so no half-cycle is wasted.
Common mistakeChoosing 'no smoothing capacitor is needed' is wrong: full-wave output is still bumpy and needs a capacitor to smooth it.
12Fill in the blank
When the anode of a diode is connected to the positive side so that current flows, the diode is said to be forward ______.
Show answer
Answer: biased
Forward bias: the diode conducts with about 0.7 V (silicon) across it. Reverse bias: it blocks the current.
Common mistakeSome learners write 'charged'; the correct term for the connection direction is bias.
13Multiple choice · ★ Challenge
A bulging electrolytic capacitor on a motherboard must be replaced. Which choice of new capacitor is correct?
1. AAny capacitance, as long as it fits into the same holes
2. BSame capacitance, voltage rating at least as high, fitted the same way round
3. CSame capacitance but a lower voltage rating, to save some money
4. DSame capacitance, fitted either way round, since polarity never matters
Show answer
Answer: B. Same capacitance, voltage rating at least as high, fitted the same way round
The capacitance keeps the circuit working as designed; a lower voltage rating could fail again; electrolytic capacitors are polarised and must match the board's markings.
Common mistakeChoosing a lower voltage rating is tempting because it is cheaper, but a capacitor run above its rated voltage can fail or burst.
14True or false
A multimeter switched to its 20 V d.c. range can safely be used to measure the 230 V mains supply.
Show answer
Answer: False
The voltage is far above the range and is a.c., not d.c.; the meter could be damaged and the user put in danger. Mains testing is for trained electricians with suitable meters.
Common mistakeLearners may think any voltage range is fine as long as the probes are insulated; the range and type (a.c./d.c.) must suit the voltage.
15Fill in the blank
In a transistor, the emitter current is equal to the base current plus the ______ current.
Show answer
Answer: collector
IE = IB + IC; because IB is small, IE is only slightly larger than IC.
Common mistakeSome learners write IC = IB + IE; the emitter carries the largest current, the sum of the other two.
16Multiple choice · ★ Challenge
A radio amplifier's loudspeaker gives out far more energy each second than the weak signal arriving at the transistor's input. Where does the extra energy come from?
1. AFrom the power supply, controlled by the transistor
2. BFrom the transistor itself, which creates new energy
3. CFrom the loudspeaker, which stores energy
4. DFrom the aerial, which collects it from the air
Show answer
Answer: A. From the power supply, controlled by the transistor
A transistor is active: the small input signal controls a much larger current drawn from the battery or mains supply, so the energy comes from the supply.
Common mistakeChoosing 'the transistor creates energy' breaks the law of conservation of energy; it only controls energy from the supply.
17Multiple choice
The three terminals of a bipolar transistor are called:
1. ABase, collector and emitter
2. BAnode, cathode and gate
3. CLive, neutral and earth
4. DInput, output and common earth
Show answer
Answer: A. Base, collector and emitter
A small current into the base controls a larger current between the collector and the emitter.
Common mistakeChoosing 'anode, cathode and gate' mixes in diode terminal names; a bipolar transistor has base, collector and emitter.
18Multiple choice · ★ Challenge
A multimeter on its diode-test setting reads 0.62 V with the probes one way round on a diode and 'OL' with the probes reversed. What does this show?
1. AThe diode is short-circuited
2. BThe diode is open-circuited (broken)
3. CThe diode is working normally
4. DThe diode is a resistor, not a diode
Show answer
Answer: C. The diode is working normally
A good diode conducts one way (showing about 0.6–0.7 V) and blocks the other way ('OL'). A short circuit reads near 0 both ways; an open circuit reads OL both ways.
Common mistakeChoosing 'open-circuited' looks only at the OL reading; a broken diode would show OL in BOTH directions.
19Multiple choice
An inductor (choke) is placed in the power line of a radio. Its main effect is to:
1. ABlock all of the steady direct current
2. BLet current flow in one direction
3. COppose sudden changes in the current
4. DStore charge on two metal plates
Show answer
Answer: C. Oppose sudden changes in the current
When the current tries to change, the changing magnetic field of the coil induces a voltage that opposes the change, smoothing out spikes and interference.
Common mistakeChoosing 'block all steady d.c.' describes a capacitor; an inductor lets steady d.c. through but opposes changes.
20True or false · ★ Challenge
Two identical 10 kΩ resistors connected in parallel on a circuit board have a combined resistance of 5 kΩ.
Show answer
Answer: True
1/R = 1/10 + 1/10 = 2/10, so R = 5 kΩ: two equal resistors in parallel give half the value of one.
Common mistakeLearners sometimes add them to get 20 kΩ; adding is the rule for series, not parallel.
21Fill in the blank
A gold fourth band on a resistor means that its tolerance is ±______ %.
Show answer
Answer: 5
Gold = ±5 %, silver = ±10 %: the true value may differ from the coded value by up to this percentage.
Common mistakeSome learners think gold is the multiplier; in a four-band resistor the gold or silver band at the end gives the tolerance.
22Multiple choice
To measure the resistance of a resistor soldered onto a circuit board accurately, you should:
1. ASwitch off and disconnect at least one end of the resistor
2. BLeave the power on so current flows through it
3. CMeasure it while the other parts stay connected in parallel
4. DPut the meter on the current range and connect it in series
Show answer
Answer: A. Switch off and disconnect at least one end of the resistor
With power on, the meter can be damaged; with other parts still connected in parallel, the meter measures the whole combination, not just the resistor.
Common mistakeChoosing to measure with other parts connected forgets that parallel paths lower the reading.
23Fill in the blank · ★ Challenge
A multimeter set to its 2 kΩ range shows 1.47. The resistance being measured is ______ Ω.
Show answer
Answer: 1470
On a kilohm range the display is in kΩ: 1.47 kΩ = 1.47 × 1000 = 1470 Ω.
Common mistakeLearners often read the display as 1.47 Ω; the range tells you the display unit.
24Multiple choice · ★ Challenge
In a transistor, a base current of 50 µA gives a collector current of 6.0 mA. What is the current gain?
1. A0.0083
2. B120
3. C300
4. D12
Show answer
Answer: B. 120
Gain = IC ÷ IB = 6.0 × 10⁻³ ÷ 50 × 10⁻⁶ = 120.
Common mistakeChoosing 0.0083 divides the base current by the collector current; the gain is the large current divided by the small one.
25Multiple choice
In the circuit symbol for a diode, a triangle points towards a short bar. What does this show?
1. ACurrent flows in the arrow's direction; the bar is the cathode
2. BCurrent flows towards the triangle, and the bar is the anode
3. CThe diode conducts equally well in both directions
4. DThe bar shows where the diode gives out light
Show answer
Answer: A. Current flows in the arrow's direction; the bar is the cathode
The triangle (anode side) points in the direction of conventional current; the bar marks the cathode, matching the stripe on a real diode.
Common mistakeChoosing 'the bar is the anode' reverses the symbol; the stripe or bar always marks the cathode.
26Multiple choice
A thermistor (NTC type) is used as the sensor in a fire alarm. As the room gets hotter, its resistance:
1. ARises
2. BStays the same
3. CFalls
4. DBecomes infinite
Show answer
Answer: C. Falls
An NTC thermistor's resistance decreases as its temperature increases, which changes the voltage in the alarm circuit.
Common mistakeChoosing 'rises' is the behaviour of a metal wire; an NTC thermistor behaves the opposite way.
27Multiple choice · ★ Challenge
A voltmeter shows 6.6 V across a 2.2 kΩ resistor. What current flows through it?
1. A0.33 mA
2. B3.0 A
3. C3.0 mA
4. D14.5 A
Show answer
Answer: C. 3.0 mA
I = V ÷ R = 6.6 ÷ 2200 = 0.0030 A = 3.0 mA.
Common mistakeChoosing 3.0 A forgets the 'kilo': 2.2 kΩ is 2200 Ω, not 2.2 Ω.
28Fill in the blank
1 microfarad (1 µF) is equal to ______ F.
Show answer
Answer: 10⁻⁶ (0.000 001)
Micro means one millionth, so 1 µF = 1 × 10⁻⁶ F.
Common mistakeSome learners write 10⁻³; that is milli. Micro is 10⁻⁶ and nano is 10⁻⁹.
29Short answer · ★ Challenge
An LDR and a fixed resistor look similar in a box of parts. Describe how to use a multimeter to tell which is the LDR.
Show answer
Model answer: Set the multimeter to a resistance (ohms) range and connect it to the component. Cover the component with your hand, then uncover it or shine a torch on it. If the reading changes a lot (much higher in the dark, lower in bright light), it is the LDR; a fixed resistor gives the same reading.
Common mistakeLearners sometimes suggest measuring the voltage across a part in the box; with no supply there is no voltage, so the resistance range must be used.
30Multiple choice
Why do technicians who repair computer boards wear an earthed wrist strap?
1. ATo protect themselves from the mains voltage in the board
2. BTo make the solder melt at a lower temperature
3. CTo measure the current flowing in the board
4. DTo stop static charge on their body damaging the chips
Show answer
Answer: D. To stop static charge on their body damaging the chips
Static charge on a person can reach thousands of volts; a spark to a chip can destroy its tiny transistors. The strap keeps the body at earth potential.
Common mistakeChoosing 'protect themselves from the mains' is wrong: the board should be unplugged, and the strap is to protect the components.
31Multiple choice
Why does replacing filament bulbs with LED bulbs reduce a family's electricity bill?
1. ALEDs need no current at all to produce light
2. BLEDs work on d.c., so they use no energy from a.c.
3. CLEDs store energy in the day and give it out at night
4. DLEDs turn more of the electrical energy into light
Show answer
Answer: D. LEDs turn more of the electrical energy into light
A filament bulb wastes most of its energy as heat; an LED changes a much larger fraction into light, so a lower power gives the same brightness.
Common mistakeChoosing 'need no current' is wrong: an LED still needs current and energy, just much less for the same light.
32Multiple choice · ★ Challenge
A resistor is coded yellow, violet, orange, gold (yellow 4, violet 7, orange 3, gold ±5 %). Which measured value shows that it is within its tolerance?
1. A4.7 kΩ
2. B50 kΩ
3. C43 kΩ
4. D46 kΩ
Show answer
Answer: D. 46 kΩ
Value = 47 × 10³ = 47 kΩ. ±5 % is ±2.35 kΩ, so the allowed range is 44.65 kΩ to 49.35 kΩ; only 46 kΩ is inside it.
Common mistakeChoosing 4.7 kΩ uses orange as 2 zeros instead of 3; orange means multiply by 10³.
33True or false
On a four-band resistor, the third band tells you how many zeros to write after the first two digits.
Show answer
Answer: True
The third band is the multiplier, a power of ten.
Common mistakeLearners sometimes read the third band as a third digit; that is true only for five-band resistors.
34Short answer · ★ Challenge
An LDR sensor circuit can supply only a few microamperes, but a security buzzer needs 50 mA. Explain how a transistor allows the sensor to control the buzzer.
Show answer
Model answer: The sensor circuit is connected to the base, and the buzzer is in the collector circuit, powered by the supply. When the sensor gives a small base current, the transistor switches on and a much larger collector current (enough for the buzzer) flows from the supply. The sensor only controls the current; the energy comes from the supply.
Common mistakeLearners often connect the buzzer straight to the sensor; the sensor cannot provide enough current, which is exactly why the transistor is needed.
35Multiple choice
A resistor is marked 4.7 kΩ. What is its resistance in ohms?
1. A470 Ω
2. B47 000 Ω
3. C4700 Ω
4. D0.0047 Ω
Show answer
Answer: C. 4700 Ω
k means kilo = 1000, so 4.7 kΩ = 4.7 × 1000 = 4700 Ω.
Common mistakeChoosing 0.0047 Ω divides by 1000; kilo makes the number bigger, not smaller.
36Short answer · ★ Challenge
In a potential divider, an LDR is connected to the positive supply and a fixed resistor to the negative supply. The output is taken across the fixed resistor. How does the output voltage change as it gets dark? Is this arrangement suitable for switching ON a night light? Explain.
Show answer
Model answer: In the dark the LDR's resistance rises, so it takes a larger share of the supply voltage and the voltage across the fixed resistor falls. Since the output FALLS in the dark, it is not suitable for switching a light on at night (it would suit a daylight detector). Swapping the LDR and the fixed resistor makes the output rise in the dark.
Common mistakeLearners often say the output rises because 'the LDR's resistance rises'; the output here is across the fixed resistor, whose share falls.
37Short answer
Classify each of these as active or passive, giving one reason in each case: potentiometer, LED, thermistor, transistor.
Show answer
Model answer: Potentiometer: passive (it only opposes current). LED: active (a diode, a semiconductor device that controls the direction of current). Thermistor: passive (its resistance changes with temperature but it cannot amplify or switch). Transistor: active (a small base current controls a large collector current).
Common mistakeLearners sometimes call the thermistor active because its resistance changes; changing resistance in response to heat is not amplifying or switching.
38Multiple choice · ★ Challenge
A shop sign has 10 identical branches in parallel across a 12 V battery. Each branch is an LED with its own resistor and takes 15 mA. What power does the sign take from the battery?
1. A0.18 W
2. B18 W
3. C0.30 W
4. D1.8 W
Show answer
Answer: D. 1.8 W
Total current = 10 × 15 mA = 150 mA = 0.15 A. P = VI = 12 × 0.15 = 1.8 W.
Common mistakeChoosing 0.18 W uses only one branch (12 × 0.015); in parallel the branch currents add.
39Multiple choice
A capacitor is connected in series with a lamp to a battery. After a short time the capacitor is fully charged. What happens then?
1. ANo more current flows, so the lamp goes out
2. BThe lamp glows brighter and brighter
3. CThe capacitor lets steady current pass freely
4. DThe capacitor reverses the current direction
Show answer
Answer: A. No more current flows, so the lamp goes out
A capacitor's plates are separated by an insulator, so steady d.c. cannot pass once it is charged; current flows only while it charges.
Common mistakeChoosing 'lets steady current pass freely' describes an inductor (or a wire); a charged capacitor blocks d.c.
40Short answer
Describe the circuit symbol for an LED and explain how it differs from the symbol for a photodiode.
Show answer
Model answer: An LED is drawn as a diode symbol (triangle pointing to a bar) with two small arrows pointing OUTWARDS, showing light given out. A photodiode has the same diode symbol but with the arrows pointing INWARDS, showing light falling on it.
Common mistakeLearners often draw the arrows the same way for both; the direction of the arrows shows whether light leaves or arrives.
41Multiple choice · ★ Challenge
In a sensor circuit, a 4.0 kΩ resistor and a 2.0 kΩ resistor are joined in series across a 6.0 V cell pack. What voltage is across the 2.0 kΩ resistor?
1. A4.0 V
2. B2.0 V
3. C3.0 V
4. D6.0 V
Show answer
Answer: B. 2.0 V
Current I = 6.0 ÷ 6000 = 1.0 mA; V = IR = 0.0010 × 2000 = 2.0 V. The smaller resistor takes the smaller share of the voltage.
Common mistakeChoosing 4.0 V gives the share of the 4.0 kΩ resistor; the voltage divides in proportion to the resistances.
42Multiple choice
A resistor has the colour bands brown, black, red (then gold). What is its resistance? (black 0, brown 1, red 2)
1. A1 kΩ
2. B102 Ω
3. C10 Ω
4. D100 kΩ
Show answer
Answer: A. 1 kΩ
First two bands give the digits 1 and 0; the third band (red = 2) means add 2 zeros: 10 followed by 00 = 1000 Ω = 1 kΩ.
Common mistakeChoosing 102 Ω reads all three bands as digits; the third band is the multiplier (number of zeros).
43Multiple choice · ★ Challenge
A 470 µF capacitor in a radio is charged to 9.0 V. How much charge does it store? (Q = CV)
1. A5.2 × 10⁻⁵ C
2. B4.2 × 10⁻³ C
3. C4.2 × 10³ C
4. D1.9 × 10⁻² C
Show answer
Answer: B. 4.2 × 10⁻³ C
Q = CV = 470 × 10⁻⁶ × 9.0 = 4.2 × 10⁻³ C.
Common mistakeChoosing 4.2 × 10³ C forgets to change microfarads into farads; 1 µF = 10⁻⁶ F.
44Short answer
Before connecting an LED, how can you tell which leg is the cathode (negative), and why does it matter?
Show answer
Model answer: The cathode is usually the shorter leg, and the plastic rim is flat on the cathode side. It matters because an LED lights only when forward biased: the cathode must be connected towards the negative terminal, or no current flows and it stays dark.
Common mistakeLearners often believe an LED works either way round like a filament bulb; it is a diode and conducts in one direction only.
45Short answer · ★ Challenge
A learner needs a 220 Ω resistor. Which colours should the first three bands be? (black 0, brown 1, red 2) Explain.
Show answer
Model answer: Red, red, brown. The first two bands give the digits 2 and 2, and the third band (brown = 1) means one zero: 22 followed by 0 = 220 Ω.
Common mistakeA common error is red, red, red, which would be 2200 Ω (2.2 kΩ); the third band is the number of zeros, not another digit.
46Multiple choice
In a circuit diagram, a resistor symbol (a rectangle) with a diagonal arrow drawn through it represents a:
1. AFixed resistor
2. BLight-emitting diode
3. CFuse
4. DVariable resistor
Show answer
Answer: D. Variable resistor
The arrow through the symbol means the value can be changed, as with a rheostat or volume control.
Common mistakeChoosing 'light-emitting diode' confuses this arrow with the small outward arrows drawn beside an LED symbol.
47Short answer · ★ Challenge
A capacitor and an inductor are each connected in series with a lamp to a battery. Compare what the lamps do a few seconds after the switch is closed.
Show answer
Model answer: With the capacitor, the lamp flashes briefly and then goes out, because once the capacitor is charged it blocks steady d.c. With the inductor, the lamp is slow to reach full brightness (the coil opposes the change in current) but then stays fully lit, because steady d.c. passes through the coil's wire.
Common mistakeLearners often expect both to behave alike because both store energy; they respond to steady d.c. in opposite ways.
48Short answer
A technician warns that the power supply of an old TV can still give a shock several minutes after it has been unplugged. Explain why.
Show answer
Model answer: Large capacitors in the power supply stay charged after the plug is pulled out. They can hold a high voltage until they are discharged, so touching their terminals can drive a current through the body.
Common mistakeLearners often think all danger stops at the moment of unplugging; stored charge in capacitors can remain.
49Short answer · ★ Challenge
A forward-biased silicon diode (0.7 V across it when conducting) is in series with a 470 Ω resistor and a 5.0 V supply. Calculate the current.
Show answer
Model answer: Voltage across resistor = 5.0 − 0.7 = 4.3 V. I = V ÷ R = 4.3 ÷ 470 = 0.0091 A = 9.1 mA.
Common mistakeA common slip is to use the full 5.0 V across the resistor (10.6 mA); the diode takes 0.7 V of the supply.
50Short answer
Give two reasons why old phones and circuit boards should be taken to an e-waste collection point instead of being burned or thrown in ordinary rubbish.
Show answer
Model answer: They contain harmful substances such as lead, mercury and cadmium, which can poison soil and water, and burning them releases toxic fumes. They also contain valuable materials (copper, gold, silver) that can be recovered and reused by recycling.
Common mistakeLearners sometimes think small devices make too little waste to matter; millions of discarded devices add up to serious pollution.