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WBBSE Madhyamik Electronics (Vocational) study materials
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WBBSE Madhyamik Electronics (Vocational) practice questions

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Question 1Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

In an electric circuit, what does an electric current actually consist of in a copper wire?

  1. A flow of protons moving between atoms
  2. A flow of free electrons through the metal lattice Correct answer.
  3. A flow of neutrons passing through the conductor
  4. A flow of positive ions travelling along the wire

Explanation

In a metallic conductor such as copper, current is carried by free electrons that drift through the fixed lattice of positive ions under the influence of an applied voltage. The conventional current direction is taken opposite to this electron flow. Protons and neutrons stay bound in the nucleus and do not travel through the wire.

Question 2Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

The SI unit of electric current is the ampere. Which statement correctly defines one ampere?

  1. One joule of energy delivered in one second
  2. One coulomb of charge passing a point in one second Correct answer.
  3. One watt of power dissipated in one second
  4. One volt of potential difference across one ohm

Explanation

Current is the rate of flow of charge, so one ampere equals one coulomb of charge crossing a section of conductor per second. The joule per second is the watt, and the volt per ohm is also a current but not the definition of the ampere.

Question 3Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

A charge of 90 coulomb flows through a lamp in 3 minutes. What is the average current in the lamp?

  1. 30 A
  2. 1.8 A
  3. 0.5 A Correct answer.
  4. 0.03 A

Explanation

Convert the time to seconds: 3 minutes equals 180 seconds. Current equals charge divided by time, so 90 coulomb divided by 180 second gives 0.5 ampere. Always convert minutes to seconds before applying the formula.

Question 4Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

What is the standard meaning of the potential difference of 6 volt between two points of a circuit?

  1. 6 joule of work is done in moving 1 coulomb between the points Correct answer.
  2. 6 watt of power is dissipated between the points
  3. 6 ohm of resistance exists between the two points
  4. 6 coulomb of charge flows between the points each second

Explanation

Potential difference is defined as work done per unit charge, so one volt means one joule of work per coulomb moved between the two points. Six volt therefore means six joule per coulomb. The other options describe current, resistance and power, which are different quantities.

Question 5Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

Which instrument is connected in parallel with a component to measure the potential difference across it?

  1. Rheostat
  2. Voltmeter Correct answer.
  3. Galvanometer placed in series
  4. Ammeter

Explanation

A voltmeter measures potential difference and must be connected in parallel across the component so that it experiences the same voltage. It is designed with very high resistance so that it draws negligible current. An ammeter, by contrast, is placed in series to measure current.

Question 6Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

A resistor of 12 ohm carries a current of 0.25 A. What is the potential difference across it?

  1. 12.25 V
  2. 0.02 V
  3. 48 V
  4. 3 V Correct answer.

Explanation

By Ohm's law, potential difference equals current multiplied by resistance. Multiplying 0.25 ampere by 12 ohm gives 3 volt. The other values come from dividing or adding the given numbers instead of multiplying.

Question 7Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

A 9 volt battery is connected across a resistor and a current of 0.3 A flows. What is the resistance?

  1. 2.7 ohm
  2. 30 ohm Correct answer.
  3. 9.3 ohm
  4. 0.033 ohm

Explanation

Rearranging Ohm's law gives resistance equal to voltage divided by current. Dividing 9 volt by 0.3 ampere gives 30 ohm. Multiplying the two numbers or adding them does not give resistance.

Question 8Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

Which property of a conductor is measured by its resistance?

  1. Its ability to store electric charge
  2. Its tendency to magnetise when current flows
  3. Its opposition to the flow of electric current Correct answer.
  4. Its capacity to generate electromotive force

Explanation

Resistance quantifies how strongly a material opposes the drift of charge carriers, converting electrical energy into heat. It is measured in ohm. Charge storage is capacitance, and generation of emf is a property of sources, not resistance.

Question 9Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

A wire of uniform material has resistance 8 ohm. If it is stretched uniformly so that its length doubles while its volume stays constant, what is the new resistance?

  1. 4 ohm
  2. 16 ohm
  3. 32 ohm Correct answer.
  4. 8 ohm

Explanation

Resistance equals resistivity times length divided by area. Doubling the length at constant volume halves the cross-sectional area, so the resistance rises by a factor of four. Thus 8 ohm becomes 32 ohm.

Question 10Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

Two resistors of 6 ohm and 3 ohm are joined in series across a 9 volt supply. What current flows in the circuit?

  1. 1 A Correct answer.
  2. 0.67 A
  3. 1.5 A
  4. 3 A

Explanation

In series the resistances add, giving a total of 9 ohm. Dividing the supply voltage 9 volt by 9 ohm gives 1 ampere. The same current passes through both resistors in a series circuit.

Question 11Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

A 4 ohm and a 12 ohm resistor are connected in parallel. What is the equivalent resistance of the combination?

  1. 16 ohm
  2. 8 ohm
  3. 3 ohm Correct answer.
  4. 0.33 ohm

Explanation

For two resistors in parallel the product divided by the sum gives the equivalent resistance. Here 4 times 12 is 48, and 4 plus 12 is 16, so the equivalent resistance is 3 ohm. It must be smaller than the smallest resistor, which 3 ohm satisfies.

Question 12Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

Three identical resistors are connected in parallel across a battery. How does the total current compare with the current through one resistor?

  1. It equals the current through one resistor
  2. It is three times the current through one resistor Correct answer.
  3. It is one third of the current through one resistor
  4. It is nine times the current through one resistor

Explanation

Each parallel branch has the full battery voltage across it, so each carries the same current when the resistors are identical. The supply current is the sum of the branch currents, hence three times one branch current.

Question 13Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

A 60 watt lamp operates on a 240 volt supply. What current does it draw?

  1. 14400 A
  2. 0.25 A Correct answer.
  3. 4 A
  4. 0.5 A

Explanation

Electrical power equals voltage multiplied by current, so current equals power divided by voltage. Dividing 60 watt by 240 volt gives 0.25 ampere. Multiplying the two values instead of dividing gives a very large wrong answer.

Question 14Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

A heater of resistance 20 ohm carries a current of 2 A. What power does it dissipate?

  1. 80 W Correct answer.
  2. 160 W
  3. 10 W
  4. 40 W

Explanation

Power can be found from current squared multiplied by resistance. Two squared is four, and four times 20 ohm gives 80 watt. Using voltage times current would need the voltage, which here is 40 volt from Ohm's law.

Question 15Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

An electric iron rated 1000 W at 230 V is used for 2 hours. How much electrical energy does it consume?

  1. 0.5 kilowatt-hour
  2. 2 kilowatt-hour Correct answer.
  3. 2000 kilowatt-hour
  4. 230 kilowatt-hour

Explanation

Energy in kilowatt-hour equals power in kilowatt multiplied by time in hours. One thousand watt is one kilowatt, and one kilowatt for two hours gives two kilowatt-hour. This is the unit commonly used for domestic electricity billing.

Question 16Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

Why is a series circuit unsuitable for connecting domestic lamps in a house?

  1. All lamps go out when one lamp fails and they cannot be switched separately Correct answer.
  2. The total resistance becomes too small and the current becomes excessive
  3. The current divides unequally between the lamps
  4. Each lamp receives more voltage than the supply provides

Explanation

In a series circuit the same current passes through every element, so a break anywhere stops the whole circuit. Also, all lamps share the supply voltage and cannot be controlled independently. Domestic wiring therefore uses parallel connection.

Question 17Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

In a parallel circuit containing three branches, which quantity is the same for every branch?

  1. The power dissipated in each branch
  2. The current through each branch
  3. The resistance of each branch
  4. The potential difference across each branch Correct answer.

Explanation

Parallel branches are connected across the same two nodes, so they all experience the same potential difference. The currents, resistances and powers may differ depending on each branch. This shared voltage is the defining feature of a parallel connection.

Question 18Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

A cell of emf 1.5 V has an internal resistance of 0.5 ohm and is connected to a 2.5 ohm resistor. What current flows?

  1. 0.75 A
  2. 0.6 A
  3. 0.5 A Correct answer.
  4. 3 A

Explanation

The internal resistance is in series with the external resistor, so the total resistance is 3 ohm. Dividing the emf 1.5 volt by 3 ohm gives 0.5 ampere. Ignoring the internal resistance would wrongly give 0.6 ampere.

Question 19Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

A uniform wire of resistance 24 ohm is cut into four equal parts. What is the resistance of each part?

  1. 1.5 ohm
  2. 6 ohm Correct answer.
  3. 96 ohm
  4. 24 ohm

Explanation

Resistance is proportional to length for a wire of uniform cross-section. Cutting the wire into four equal lengths divides the resistance by four, giving 6 ohm per piece. The material and area are unchanged.

Question 20Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

Which statement correctly describes the relation between the resistance of a metallic conductor and its temperature, over normal ranges?

  1. Resistance increases as temperature rises because lattice vibrations hinder electron drift Correct answer.
  2. Resistance falls to zero at room temperature in all metals
  3. Resistance decreases as temperature rises because electrons move faster
  4. Resistance is unaffected by temperature in any metal

Explanation

In metals, higher temperature increases the amplitude of lattice vibrations, which scatter drifting electrons more often. This raises resistivity and hence resistance. The decrease of resistance with temperature is characteristic of semiconductors, not ordinary metals.

Question 21Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

A 12 volt supply is connected to two resistors of 4 ohm and 6 ohm in parallel. What is the total current drawn from the supply?

  1. 1.2 A
  2. 2.4 A
  3. 0.5 A
  4. 5 A Correct answer.

Explanation

Each parallel resistor has the full 12 volt across it, so the branch currents are 12 divided by 4, which is 3 ampere, and 12 divided by 6, which is 2 ampere. Their sum gives a total supply current of 5 ampere.

Question 22Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

What is the correct relation between the kilowatt-hour and the joule?

  1. One kilowatt-hour equals 60 joule
  2. One kilowatt-hour equals 1000 joule
  3. One kilowatt-hour equals 3600 joule
  4. One kilowatt-hour equals 3.6 million joule Correct answer.

Explanation

One kilowatt-hour is 1000 watt multiplied by 3600 second, which gives 3,600,000 joule. This is why domestic energy meters record energy in kilowatt-hour rather than in joule. The joule is the SI unit, but the kilowatt-hour is more practical for large amounts.

Question 23Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

A resistor of 100 ohm is rated at 0.5 W. What is the maximum current it can safely carry?

  1. 0.005 A
  2. 50 A
  3. 2 A
  4. 0.0707 A Correct answer.

Explanation

From power equal to current squared times resistance, the current is the square root of power divided by resistance. Dividing 0.5 watt by 100 ohm gives 0.005, and its square root is about 0.0707 ampere. Exceeding this current would overheat the resistor.

Question 24Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

Which combination gives an equivalent resistance larger than any individual resistor used?

  1. Resistors connected across a common supply
  2. Resistors connected in parallel
  3. Two equal resistors connected in parallel
  4. Resistors connected in series Correct answer.

Explanation

In series, resistances add directly, so the total is greater than the largest individual value. In parallel the equivalent resistance is always smaller than the smallest resistor. Therefore only the series combination satisfies the condition.

Question 25Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

A 10 ohm resistor and an unknown resistor are in series with a 6 volt battery, and the current is 0.4 A. What is the unknown resistance?

  1. 2.4 ohm
  2. 25 ohm
  3. 15 ohm
  4. 5 ohm Correct answer.

Explanation

The total resistance is 6 volt divided by 0.4 ampere, which is 15 ohm. Subtracting the known 10 ohm leaves 5 ohm for the unknown resistor. Adding the known resistance instead of subtracting would give a wrong value.

Question 26Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

In an electric circuit, what does an ammeter measure and how is it connected?

  1. It measures power and is connected in parallel with the component.
  2. It measures voltage and is connected in parallel with the component.
  3. It measures current and is connected in series with the component. Correct answer.
  4. It measures resistance and is connected in series with the component.

Explanation

An ammeter is designed to measure the flow of electric charge, that is current, and it must carry that same current, so it is connected in series. A voltmeter, by contrast, measures potential difference and is connected in parallel across the component. Connecting an ammeter in parallel would divert current and give a wrong, possibly damaging, reading.

Question 27Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

A copper wire and a nichrome wire of the same length and the same cross-sectional area are compared. Which statement about their resistance is correct?

  1. The resistance of each wire depends only on the current passing through it.
  2. The nichrome wire has greater resistance because its resistivity is higher. Correct answer.
  3. The copper wire has greater resistance because it is the better conductor.
  4. Both wires have equal resistance because their dimensions are identical.

Explanation

Resistance depends on the material through its resistivity, and nichrome has a much higher resistivity than copper. With length and area fixed, the wire with the higher resistivity has the higher resistance. Copper is used for connecting wires precisely because its low resistivity gives low resistance.

Question 28Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

A charge of 60 coulomb flows through a lamp in 2 minutes. What is the steady current in the lamp?

  1. 0.5 A Correct answer.
  2. 30 A
  3. 120 A
  4. 2 A

Explanation

Current is the rate of flow of charge, so I = Q / t. The time must be in seconds: 2 minutes is 120 s. Dividing 60 C by 120 s gives 0.5 A.

Question 29Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

Three resistors of 2 ohm, 3 ohm and 6 ohm are joined in parallel. What is the equivalent resistance of the combination?

  1. 11 ohm
  2. 3.67 ohm
  3. 0.5 ohm
  4. 1 ohm Correct answer.

Explanation

For a parallel combination, 1/R = 1/2 + 1/3 + 1/6. Taking sixths gives 3/6 + 2/6 + 1/6 = 6/6 = 1, so R = 1 ohm. The equivalent resistance is always smaller than the smallest branch resistance, which is 2 ohm here, confirming the answer.

Question 30Basic Electrical and Electronic Fundamentals (current, voltage, resistance, Ohm's law, power, series and parallel circuits)

A 12 V battery of negligible internal resistance feeds two resistors in parallel, 6 ohm and 3 ohm, and the 6 ohm resistor carries 2 A. A student claims the total power delivered by the battery is 24 W. Which analysis of this claim is correct?

  1. The claim is right because power equals voltage times the current in one branch.
  2. The claim is wrong; the parallel equivalent is 9 ohm, so the power is 16 W.
  3. The claim is wrong; the total current is 6 A, so the battery delivers 72 W. Correct answer.
  4. The claim is right because the 3 ohm resistor dissipates no power in parallel.

Explanation

In parallel both resistors have the full 12 V across them, so the 3 ohm branch carries 12/3 = 4 A. The total current is 2 + 4 = 6 A, and total power is P = VI = 12 x 6 = 72 W. Using only the 6 ohm branch current underestimates the power, so the 24 W claim is incorrect.

Question 31Passive Components (resistors, capacitors, inductors: types, colour code, testing and applications)

In the four-band colour code of a carbon film resistor, what does the third band represent?

  1. The tolerance of the resistor in percent
  2. The temperature coefficient in ppm per degree Celsius
  3. The working voltage rating of the resistor
  4. The decimal multiplier applied to the first two digits Correct answer.

Explanation

The first two bands give the significant digits, the third band gives the power-of-ten multiplier, and the fourth band gives tolerance. For example brown-black-red-gold means 1, 0, times 100, that is 1 kilo-ohm with 5 percent tolerance. The third band is therefore a multiplier, not a tolerance or rating.

Question 32Passive Components (resistors, capacitors, inductors: types, colour code, testing and applications)

A resistor is marked with the bands yellow, violet, orange and gold. What is its nominal value and tolerance?

  1. 470 kilo-ohm, plus or minus 10 percent
  2. 47 ohm, plus or minus 5 percent
  3. 47 kilo-ohm, plus or minus 5 percent Correct answer.
  4. 4.7 kilo-ohm, plus or minus 5 percent

Explanation

Yellow is 4, violet is 7, and orange is a multiplier of 1000, giving 47 times 1000 equals 47 kilo-ohm. Gold in the fourth band means plus or minus 5 percent. So the correct reading is 47 kilo-ohm at 5 percent.

Question 33Passive Components (resistors, capacitors, inductors: types, colour code, testing and applications)

Why is a wire-wound resistor preferred over a carbon composition resistor in a high-current power supply bleeder circuit?

  1. It has higher power dissipation capability and better stability under heating Correct answer.
  2. It provides a precisely defined capacitance across the supply
  3. It can be trimmed to a finer tolerance during manufacture
  4. It has a much larger negative temperature coefficient

Explanation

Wire-wound resistors use resistive wire on a ceramic former, so they handle large currents and dissipate heat well while remaining stable. Carbon composition types drift with temperature and age, which suits them to low-power, non-critical work. High current capability is the deciding property here.

Question 34Passive Components (resistors, capacitors, inductors: types, colour code, testing and applications)

Which resistor type is the most suitable choice for a precision measuring instrument requiring 0.1 percent tolerance and low temperature drift?

  1. Carbon composition resistor
  2. Carbon film resistor
  3. Metal film resistor Correct answer.
  4. Ceramic wire-wound resistor

Explanation

Metal film resistors are deposited by evaporation or sputtering, giving tight tolerances such as 0.1 percent and low temperature coefficients. Carbon composition and carbon film parts are cheaper but drift more with heat and time. Precision instruments therefore use metal film types.

Question 35Passive Components (resistors, capacitors, inductors: types, colour code, testing and applications)

A carbon film resistor is measured at 980 ohm while its marked value is 1000 ohm. The fourth band is gold. What conclusion is correct?

  1. The resistor is within tolerance because the deviation is 2 percent Correct answer.
  2. The resistor is out of tolerance because gold allows only 1 percent
  3. The resistor is faulty because any deviation from the marked value is a defect
  4. The measured value proves the colour code was read wrongly

Explanation

Gold denotes plus or minus 5 percent, so the acceptable range is 950 ohm to 1050 ohm. A reading of 980 ohm is 2 percent low, which lies inside that band. The component is therefore acceptable and need not be replaced.

Question 36Passive Components (resistors, capacitors, inductors: types, colour code, testing and applications)

Which instrument connection is used when testing an individual resistor in a live circuit board?

  1. Measure across the resistor in circuit with the power on, using the volt range
  2. Connect the meter in series with the resistor while the supply is energised
  3. Measure across the resistor in circuit with the power off, without lifting any lead
  4. Isolate the resistor by desoldering one lead, then measure across it with a multimeter on the ohm range Correct answer.

Explanation

Parallel paths on a board can shunt current and make an in-circuit ohm reading falsely low. Lifting one lead removes those paths and the power must be off before using the ohm range. This gives a true measurement of the component.

Question 37Passive Components (resistors, capacitors, inductors: types, colour code, testing and applications)

A resistor shows a cracked body and a burnt smell after a fault. Which statement about the correct repair approach is valid?

  1. Leave it in place because a burnt resistor still limits current
  2. Replace it with a part of the same resistance and equal or higher power rating, then check the cause Correct answer.
  3. Replace it with a part of lower power rating to limit fault current
  4. Replace it with any resistor of the same physical size regardless of value

Explanation

A burnt resistor usually indicates excessive dissipation caused by an upstream fault, so the cause must be found. The replacement must match the resistance and carry at least the original power rating. Fitting a lower-rated part would simply fail again.

Question 38Passive Components (resistors, capacitors, inductors: types, colour code, testing and applications)

What is the effect of connecting a resistor in series with an LED in a display panel?

  1. It rectifies the supply so the LED works on alternating current
  2. It limits the LED current to a safe value for the applied supply voltage Correct answer.
  3. It increases the light output of the LED by raising the current
  4. It stores charge to keep the LED lit after power is removed

Explanation

An LED has a nearly fixed forward voltage, so without a series resistor the current would rise until the device is destroyed. The resistor drops the surplus voltage and sets the current by Ohm's law. This is its standard protective role.

Question 39Passive Components (resistors, capacitors, inductors: types, colour code, testing and applications)

In a potential divider using two equal resistors across a 12 volt supply, what is the voltage across each resistor?

  1. 12 volt
  2. 6 volt Correct answer.
  3. 3 volt
  4. 24 volt

Explanation

Equal resistors split the supply equally because the same current flows through both and the drops are proportional to resistance. Half of 12 volt is 6 volt. This assumes no significant load is drawn from the midpoint.

Question 40Passive Components (resistors, capacitors, inductors: types, colour code, testing and applications)

Which property makes a capacitor useful for coupling an audio signal between two amplifier stages?

  1. It holds a constant voltage regardless of charge
  2. It blocks alternating current while passing direct current
  3. It converts alternating current into direct current
  4. It blocks direct current while passing alternating current Correct answer.

Explanation

A capacitor charges to the direct voltage difference between the stages and then blocks further direct current, while its reactance falls with frequency so the audio signal passes. This isolates the bias points of the two stages. The reactance relation is Xc equals one over two pi f C.

Question 41Passive Components (resistors, capacitors, inductors: types, colour code, testing and applications)

A capacitor is marked 474K. What is its value and tolerance?

  1. 470 picofarad, plus or minus 10 percent
  2. 470 nanofarad, plus or minus 10 percent Correct answer.
  3. 4.7 microfarad, plus or minus 10 percent
  4. 47 nanofarad, plus or minus 10 percent

Explanation

In the three-digit code the first two digits are significant and the third is the number of zeros in picofarad, so 47 followed by four zeros gives 470000 picofarad, that is 470 nanofarad. The letter K denotes plus or minus 10 percent. This is a common ceramic disc marking.

Question 42Passive Components (resistors, capacitors, inductors: types, colour code, testing and applications)

Which capacitor type is normally chosen for the tuning circuit of a radio receiver because of its stability?

  1. Aluminium electrolytic capacitor
  2. Silvered mica or ceramic capacitor Correct answer.
  3. Paper dielectric capacitor
  4. Tantalum bead capacitor

Explanation

Tuned circuits need a capacitance that stays constant with temperature and time, and mica or ceramic dielectrics offer that stability with low loss. Electrolytic and paper types drift and have higher losses. The stable dielectric is therefore the deciding factor.

Question 43Passive Components (resistors, capacitors, inductors: types, colour code, testing and applications)

What happens if an aluminium electrolytic capacitor is connected with reversed polarity in a direct current circuit?

  1. The capacitor behaves as an inductor and blocks the current
  2. The oxide dielectric breaks down and the capacitor may overheat and vent Correct answer.
  3. Nothing changes because electrolytic capacitors are non-polarised
  4. The capacitance doubles and the circuit works normally

Explanation

The thin oxide layer on the anode foil acts as the dielectric and is maintained only by correct polarity. Reverse voltage destroys that layer, causing heavy leakage current, heating and possible rupture of the case. Polarity must therefore be observed during assembly.

Question 44Passive Components (resistors, capacitors, inductors: types, colour code, testing and applications)

A capacitor is tested with a multimeter on the ohm range and the reading rises steadily towards infinity. What does this indicate?

  1. The meter is reading its inductance
  2. The capacitor has lost its dielectric completely
  3. The capacitor is short-circuited
  4. The capacitor is charging and is probably good Correct answer.

Explanation

The meter applies a small direct voltage, so current flows initially to charge the capacitor and then falls as it charges, making the apparent resistance climb. A steady rise towards infinity is the expected healthy response for a capacitor of reasonable value. A short would read near zero and stay there.

Question 45Passive Components (resistors, capacitors, inductors: types, colour code, testing and applications)

Why must the stored charge be removed from a large capacitor before handling it during servicing?

  1. It prevents the capacitor from being tested at all
  2. It will change the colour code of nearby resistors
  3. It will permanently magnetise the service tools
  4. It can deliver a dangerous shock even after the supply is switched off Correct answer.

Explanation

A large capacitor can hold charge for a long time after power is removed and can discharge through the technician. The safe practice is to discharge it through a suitable resistor and verify with a meter. This protects both the person and the test equipment.

Question 46Passive Components (resistors, capacitors, inductors: types, colour code, testing and applications)

What is the role of a capacitor connected across the output of a bridge rectifier?

  1. It increases the ripple content of the output
  2. It converts the alternating input into direct current
  3. It smooths the pulsating direct voltage by charging and discharging Correct answer.
  4. It limits the current drawn by the load

Explanation

The capacitor charges near the peak of each rectified pulse and discharges into the load between pulses, filling the gaps and reducing ripple. Rectification itself is done by the diodes, not the capacitor. A larger capacitance gives a smoother output for a given load.

Question 47Passive Components (resistors, capacitors, inductors: types, colour code, testing and applications)

A capacitor of 10 microfarad is connected across a 50 hertz supply. What is its approximate capacitive reactance?

  1. About 3180 ohm
  2. About 318 ohm Correct answer.
  3. About 500 ohm
  4. About 31.8 ohm

Explanation

Reactance is one divided by two pi times frequency times capacitance. With 50 hertz and 10 microfarad, two pi f C is about 0.00314, so the reactance is roughly 318 ohm. The value falls if frequency or capacitance is increased.

Question 48Passive Components (resistors, capacitors, inductors: types, colour code, testing and applications)

Which combination of capacitors gives a total of 30 microfarad from three 10 microfarad capacitors?

  1. All three connected in parallel Correct answer.
  2. Two in series with the third in parallel
  3. Two in parallel with the third in series
  4. All three connected in series

Explanation

Capacitances in parallel add directly, so three 10 microfarad units give 30 microfarad. Series connection divides, giving about 3.3 microfarad for three equal parts. The parallel arrangement is the one that sums the values.

Question 49Passive Components (resistors, capacitors, inductors: types, colour code, testing and applications)

Which statement correctly describes the behaviour of an inductor in a direct current circuit that is switched on?

  1. It converts the current into a magnetic charge
  2. It blocks the current permanently
  3. It opposes the rise of current by inducing a back emf Correct answer.
  4. It instantly allows the full steady current to flow

Explanation

A changing current creates a changing magnetic flux, and by Faraday's law the coil induces an emf that opposes the change. The current therefore rises gradually towards its steady value. Once the current is steady, the inductor behaves like a low-resistance winding.

Question 50Passive Components (resistors, capacitors, inductors: types, colour code, testing and applications)

What is the main reason for using a laminated iron core in a transformer or choke?

  1. To reduce eddy current losses in the core Correct answer.
  2. To increase the resistance of the winding
  3. To raise the inductance to an infinite value
  4. To eliminate the magnetic field entirely

Explanation

Thin insulated laminations interrupt the paths of eddy currents induced in the core, greatly reducing heating losses. The magnetic flux still passes along the core. This improves efficiency at mains frequencies.