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ACMA Standard Theory Exam study materials
ACMA Standard Theory Exam study resources

ACMA Standard Theory Exam practice questions

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Question 1Electrical and electronic fundamentals

A resistor of 100 ohms carries a current of 0.5 A. What is the voltage across the resistor?

  1. 0.005 V
  2. 200 V
  3. 20 V
  4. 50 V Correct answer.

Explanation

Ohm's law relates voltage, current and resistance as V = I x R. Substituting the values gives 0.5 A x 100 ohms = 50 V.

Question 2Electrical and electronic fundamentals

A 12 V supply is connected directly across a 4 ohm resistor. What power is dissipated in the resistor?

  1. 3 W
  2. 36 W Correct answer.
  3. 48 W
  4. 144 W

Explanation

Power can be found from P = V squared divided by R. That gives 144 divided by 4, which is 36 W.

Question 3Electrical and electronic fundamentals

Three resistors of 220 ohms, 330 ohms and 470 ohms are connected in series. What is the total resistance?

  1. 33 kilohms
  2. 1020 ohms Correct answer.
  3. 110 ohms
  4. 150 ohms

Explanation

In a series circuit the individual resistances simply add together. The sum of 220, 330 and 470 is 1020 ohms.

Question 4Electrical and electronic fundamentals

Two 100 ohm resistors are connected in parallel. What is the equivalent resistance?

  1. 50 ohms Correct answer.
  2. 200 ohms
  3. 25 ohms
  4. 150 ohms

Explanation

For two equal resistors in parallel the equivalent value is half of one resistor. So 100 ohms in parallel with 100 ohms gives 50 ohms.

Question 5Electrical and electronic fundamentals

The unit of capacitance is the:

  1. farad Correct answer.
  2. henry
  3. coulomb
  4. ohm

Explanation

Capacitance is measured in farads, defined as one coulomb of stored charge per volt of applied potential difference.

Question 6Electrical and electronic fundamentals

A signal has a frequency of 2 kHz. What is its period?

  1. 500 s
  2. 0.5 ms Correct answer.
  3. 0.2 ms
  4. 2 ms

Explanation

Period is the reciprocal of frequency, so T = 1 divided by f. One divided by 2000 Hz equals 0.0005 s, which is 0.5 ms.

Question 7Electrical and electronic fundamentals

The essential property of a semiconductor diode is that it:

  1. amplifies small signals
  2. conducts one way only Correct answer.
  3. converts DC to AC
  4. stores charge for long periods

Explanation

A diode is a PN junction that offers low resistance to current in the forward direction and very high resistance in the reverse direction.

Question 8Electrical and electronic fundamentals

In a steady DC circuit, a capacitor behaves as:

  1. an open circuit Correct answer.
  2. a fixed resistor
  3. a constant voltage source
  4. a short circuit

Explanation

Capacitive reactance falls as frequency rises and becomes infinite at zero frequency. Once charged, a capacitor therefore passes no further steady direct current and acts as an open circuit.

Question 9Electrical and electronic fundamentals

In a series circuit containing three different resistors, which quantity is identical for all three components?

  1. voltage across each
  2. resistance of each
  3. current through each Correct answer.
  4. power dissipated in each

Explanation

A series circuit provides only one path for charge, so the same current must flow through every component in that path.

Question 10Electrical and electronic fundamentals

A current of 0.2 A flows through a 470 ohm resistor. The power dissipated is approximately:

  1. 2.35 W
  2. 0.094 W
  3. 94 W
  4. 18.8 W Correct answer.

Explanation

Using P = I squared x R, the square of 0.2 A is 0.04, and 0.04 x 470 gives 18.8 W.

Question 11Electrical and electronic fundamentals

A 100 ohm resistor and a 150 ohm resistor are connected in parallel. What is the equivalent resistance?

  1. 250 ohms
  2. 30 ohms
  3. 60 ohms Correct answer.
  4. 125 ohms

Explanation

For two resistors in parallel the product divided by the sum applies: (100 x 150) divided by (100 + 150) equals 15000 divided by 250, which is 60 ohms.

Question 12Electrical and electronic fundamentals

A 1 kilohm resistor and a 3 kilohm resistor are connected in series across a 12 V supply. What is the voltage across the 3 kilohm resistor?

  1. 3 V
  2. 8 V
  3. 4 V
  4. 9 V Correct answer.

Explanation

In a series divider the voltage divides in proportion to resistance. The 3 kilohm resistor takes three quarters of the total 4 kilohm, so it drops 9 V of the 12 V.

Question 13Electrical and electronic fundamentals

A transformer has 240 V applied to its primary and delivers 12 V from its secondary. What is the primary-to-secondary turns ratio?

  1. 24 to 1
  2. 1 to 20
  3. 12 to 1
  4. 20 to 1 Correct answer.

Explanation

For an ideal transformer the voltage ratio equals the turns ratio. Dividing 240 by 12 gives 20, so the primary has twenty times as many turns as the secondary.

Question 14Electrical and electronic fundamentals

A sine wave has an RMS value of 240 V. Its peak value is approximately:

  1. 240 V
  2. 480 V
  3. 170 V
  4. 340 V Correct answer.

Explanation

For a sine wave the peak value equals the RMS value multiplied by the square root of two, about 1.414. Multiplying 240 V by 1.414 gives approximately 340 V.

Question 15Electrical and electronic fundamentals

A 10 microfarad capacitor and a 22 microfarad capacitor are connected in parallel. What is the total capacitance?

  1. 32 microfarads Correct answer.
  2. 6.9 microfarads
  3. 220 microfarads
  4. 16 microfarads

Explanation

Capacitances in parallel add directly, much as resistances do in series. Adding 10 and 22 gives 32 microfarads.

Question 16Electrical and electronic fundamentals

A 10 mH inductor is used at a frequency of 1 kHz. What is its inductive reactance?

  1. 62.8 ohms Correct answer.
  2. 628 ohms
  3. 6.28 kilohms
  4. 0.63 ohms

Explanation

Inductive reactance is given by XL = 2 x pi x f x L. Substituting 1000 Hz and 0.01 H gives 2 x 3.1416 x 1000 x 0.01, which equals about 62.8 ohms.

Question 17Electrical and electronic fundamentals

A series LC circuit is at resonance when:

  1. R equals XL
  2. the capacitor is fully charged
  3. XL equals XC Correct answer.
  4. current is at a minimum

Explanation

Resonance occurs at the frequency where the inductive and capacitive reactances are equal in magnitude, so they cancel and leave only resistance in the circuit.

Question 18Electrical and electronic fundamentals

An amplifier delivers 10 times as much power at its output as it receives at its input. What is its power gain in decibels?

  1. 1 dB
  2. 10 dB Correct answer.
  3. 3 dB
  4. 20 dB

Explanation

Power gain in decibels is 10 times the logarithm to base ten of the power ratio. The logarithm of 10 is 1, so the gain is 10 dB.

Question 19Electrical and electronic fundamentals

In an N-type semiconductor material, the majority charge carriers are:

  1. holes
  2. protons
  3. electrons Correct answer.
  4. positive ions

Explanation

N-type material is doped with donor impurities that contribute extra free electrons, so electrons are the majority carriers.

Question 20Electrical and electronic fundamentals

A light-emitting diode with a forward voltage of 2 V is to operate from a 12 V supply at a current of 20 mA. What series resistance is required?

  1. 1 kilohm
  2. 500 ohms Correct answer.
  3. 600 ohms
  4. 100 ohms

Explanation

The resistor must drop the difference between the supply and the diode forward voltage, which is 10 V. Dividing 10 V by 0.02 A gives 500 ohms.

Question 21Electrical and electronic fundamentals

A 30 ohm resistor is connected in series with a capacitor having a reactance of 40 ohms. What is the magnitude of the total impedance?

  1. 70 ohms
  2. 1200 ohms
  3. 10 ohms
  4. 50 ohms Correct answer.

Explanation

Resistance and capacitive reactance are ninety degrees out of phase, so they combine by the square root of the sum of squares. The square root of 900 plus 1600 is the square root of 2500, which is 50 ohms.

Question 22Electrical and electronic fundamentals

A 10 kilohm resistor is connected in series with a 100 microfarad capacitor. What is the time constant of the circuit?

  1. 1 s Correct answer.
  2. 0.01 s
  3. 10 s
  4. 0.001 s

Explanation

The time constant is the product of resistance and capacitance. Multiplying 10 000 ohms by 0.0001 farads gives 1 second.

Question 23Electrical and electronic fundamentals

A sine wave has a peak-to-peak value of 100 V. What is its RMS value?

  1. 71 V
  2. 25 V
  3. 35 V Correct answer.
  4. 50 V

Explanation

Peak-to-peak is twice the peak, so the peak value is 50 V. Dividing the peak by the square root of two gives an RMS value of about 35 V.

Question 24Electrical and electronic fundamentals

A transformer is rated at 240 V primary, 24 V secondary and 120 VA. What is the maximum current that can be drawn from the secondary?

  1. 2 A
  2. 0.5 A
  3. 12 A
  4. 5 A Correct answer.

Explanation

The volt-ampere rating applies to the secondary winding as well as the primary. Dividing 120 VA by 24 V gives 5 A.

Question 25Electrical and electronic fundamentals

A resistor is rated at 0.25 W and 100 ohms. What is the maximum current it can carry continuously without exceeding its rating?

  1. 25 mA
  2. 50 mA Correct answer.
  3. 2.5 mA
  4. 250 mA

Explanation

Rearranging P = I squared x R gives I equal to the square root of P divided by R. The square root of 0.0025 is 0.05 A, which is 50 mA.

Question 26Electrical and electronic fundamentals

Which of the following materials has the lowest resistivity at room temperature?

  1. Nichrome
  2. Silver Correct answer.
  3. Aluminium
  4. Copper

Explanation

Resistivity is an intrinsic property of a material, and among common conductors silver has the lowest room-temperature resistivity. Copper is a very good conductor but is slightly less conductive than silver. Aluminium is higher again, while nichrome is a resistance alloy with much higher resistivity.

Question 27Electrical and electronic fundamentals

In a purely inductive AC circuit, the current:

  1. Is in phase with the applied voltage
  2. Lags the applied voltage by 45 degrees
  3. Lags the applied voltage by 90 degrees Correct answer.
  4. Leads the applied voltage by 90 degrees

Explanation

In an inductor, voltage is proportional to the rate of change of current, so the current cannot change instantaneously. With a sinusoidal supply this causes the current to lag the applied voltage by a quarter cycle, or 90 degrees.

Question 28Electrical and electronic fundamentals

A resistor has colour bands brown, black, red and gold. What are its nominal resistance and tolerance?

  1. 1 kilohm, plus or minus 10 percent
  2. 100 ohms, plus or minus 5 percent
  3. 10 kilohms, plus or minus 5 percent
  4. 1 kilohm, plus or minus 5 percent Correct answer.

Explanation

The first two bands give the significant figures 1 and 0, and the third band red is a multiplier of 10 squared. This gives 10 times 100 ohms, or 1000 ohms, which is 1 kilohm. A gold fourth band indicates a tolerance of plus or minus 5 percent.

Question 29Electrical and electronic fundamentals

A current of 2 A flows for 5 hours. How much charge passes through the circuit?

  1. 7200 C
  2. 120 C
  3. 36000 C Correct answer.
  4. 10 C

Explanation

Charge is current multiplied by time, provided time is expressed in seconds. Five hours is 5 times 3600, or 18000 seconds, so the charge is 2 A times 18000 s, which equals 36000 C.

Question 30Electrical and electronic fundamentals

A coil of 200 turns has a cross-sectional area of 0.005 square metres. The magnetic flux density through it changes uniformly from 0.1 T to 0.3 T in 0.02 s. What average EMF is induced in the coil?

  1. 20 V
  2. 10 V Correct answer.
  3. 40 V
  4. 5 V

Explanation

Faraday's law gives the average induced EMF as the number of turns multiplied by the rate of change of flux. The flux change is 0.2 T times 0.005 square metres, or 0.001 Wb, and dividing by 0.02 s gives 0.05 Wb per second per turn. Multiplying by 200 turns gives 10 V.

Question 31Circuit theory and components

A 12 V DC supply is connected directly across a 470 Ω resistor. What current flows through the resistor?

  1. 39.2 mA
  2. 255 mA
  3. 25.5 mA Correct answer.
  4. 2.55 mA

Explanation

Ohm's law states that current equals voltage divided by resistance. Dividing 12 V by 470 Ω gives 0.02553 A, which is 25.5 mA.

Question 32Circuit theory and components

Resistors of 100 Ω, 220 Ω and 330 Ω are connected in series. What is the total resistance of the combination?

  1. 130 Ω
  2. 217 Ω
  3. 650 Ω Correct answer.
  4. 480 Ω

Explanation

In a series circuit the same current flows through every element, and the individual resistances simply add together. Adding 100 Ω, 220 Ω and 330 Ω gives 650 Ω.

Question 33Circuit theory and components

Two 100 Ω resistors are connected in parallel. What is the equivalent resistance of the pair?

  1. 100 Ω
  2. 25 Ω
  3. 50 Ω Correct answer.
  4. 200 Ω

Explanation

For parallel resistors 1/RT = 1/R1 + 1/R2, so RT = (100 × 100)/(100 + 100) = 50 Ω. Two equal resistors in parallel always give half the value of one of them.

Question 34Circuit theory and components

A current of 0.2 A flows through a 150 Ω resistor. What power is dissipated in the resistor?

  1. 0.03 W
  2. 750 W
  3. 30 W
  4. 6 W Correct answer.

Explanation

Power in a resistor can be calculated as P = I²R. Squaring 0.2 A gives 0.04, and multiplying by 150 Ω gives 6 W.

Question 35Circuit theory and components

How much energy is stored in a 100 µF capacitor charged to 50 V?

  1. 0.125 J Correct answer.
  2. 5 J
  3. 2.5 J
  4. 0.25 J

Explanation

The energy stored in a capacitor is W = ½CV². Substituting 100 × 10⁻⁶ F and 50 V gives ½ × 100 × 10⁻⁶ × 2500 = 0.125 J.

Question 36Circuit theory and components

A 10 kΩ resistor is connected in series with a 47 µF capacitor. What is the time constant of the circuit?

  1. 4.7 ms
  2. 0.47 s Correct answer.
  3. 47 ms
  4. 4.7 s

Explanation

For a series RC circuit the time constant is τ = RC. Multiplying 10 000 Ω by 47 × 10⁻⁶ F gives 0.47 s.

Question 37Circuit theory and components

The current through a 10 mH inductor rises steadily from 0 A to 2 A in 5 ms. What is the magnitude of the voltage induced across the inductor?

  1. 0.04 V
  2. 4 V Correct answer.
  3. 0.4 V
  4. 20 V

Explanation

The voltage across an inductor is v = L·di/dt. The current changes at 2 A in 5 ms, which is 400 A/s, so v = 0.01 H × 400 A/s = 4 V.

Question 38Circuit theory and components

What is the capacitive reactance of a 1 µF capacitor at a frequency of 1 kHz?

  1. 159 Ω Correct answer.
  2. 628 Ω
  3. 62.8 Ω
  4. 1.59 kΩ

Explanation

Capacitive reactance is given by XC = 1/(2πfC). With f = 1000 Hz and C = 1 × 10⁻⁶ F, XC = 1/(2π × 10⁻³) ≈ 159 Ω.

Question 39Circuit theory and components

What is the inductive reactance of a 10 mH inductor at a frequency of 1 kHz?

  1. 6.28 Ω
  2. 628 Ω
  3. 15.9 Ω
  4. 62.8 Ω Correct answer.

Explanation

Inductive reactance is given by XL = 2πfL. Using 1000 Hz and 0.01 H gives 2π × 1000 × 0.01 ≈ 62.8 Ω.

Question 40Circuit theory and components

A series LC circuit contains a 1 mH inductor and a 1 µF capacitor. At what frequency does the circuit resonate?

  1. 1.59 kHz
  2. 159 Hz
  3. 5.03 kHz Correct answer.
  4. 50.3 kHz

Explanation

Resonance occurs where XL equals XC, which gives f₀ = 1/(2π√(LC)). With L = 1 × 10⁻³ H and C = 1 × 10⁻⁶ F, √(LC) = 3.16 × 10⁻⁵ s and f₀ ≈ 5033 Hz.

Question 41Circuit theory and components

A 60 Ω resistor is connected in series with a coil that has a reactance of 80 Ω. What is the magnitude of the total impedance?

  1. 100 Ω Correct answer.
  2. 140 Ω
  3. 20 Ω
  4. 48 Ω

Explanation

Resistance and reactance are 90° apart in phase, so they combine as |Z| = √(R² + XL²). Here √(3600 + 6400) = 100 Ω.

Question 42Circuit theory and components

In a purely inductive AC circuit, how does the current behave relative to the voltage?

  1. Current leads the voltage by 90°
  2. Current leads the voltage by 45°
  3. Current lags the voltage by 90° Correct answer.
  4. Current is in phase with the voltage

Explanation

In an inductor the voltage is proportional to the rate of change of current, so the voltage peak occurs a quarter cycle before the current peak. The current therefore lags the voltage by 90°.

Question 43Circuit theory and components

At a circuit node, 5 A and 3 A enter along two branches while 2 A and 4 A leave along two others. What current leaves along the third branch?

  1. 6 A
  2. 4 A
  3. 10 A
  4. 2 A Correct answer.

Explanation

Kirchhoff's current law requires the total current entering a node to equal the total leaving it. The entering current is 8 A and the known leaving current is 6 A, so the third branch carries 2 A.

Question 44Circuit theory and components

A single series loop contains a 12 V source and voltage drops of 4 V and 3 V across two of its components. What is the voltage across the third component?

  1. 19 V
  2. 1 V
  3. 7 V
  4. 5 V Correct answer.

Explanation

Kirchhoff's voltage law states that the voltage drops around a closed loop must sum to the source voltage. Subtracting 4 V and 3 V from 12 V leaves 5 V.

Question 45Circuit theory and components

A 12 V supply is applied to a series combination of 1 kΩ and 3 kΩ. What voltage appears across the 3 kΩ resistor?

  1. 3 V
  2. 12 V
  3. 9 V Correct answer.
  4. 6 V

Explanation

The divider ratio is R2/(R1 + R2) = 3000/4000 = 0.75. Multiplying 0.75 by 12 V gives 9 V, so the larger resistor drops the larger share of the supply.

Question 46Circuit theory and components

A total current of 6 A divides between a 2 Ω and a 4 Ω resistor connected in parallel. What current flows through the 2 Ω resistor?

  1. 4 A Correct answer.
  2. 3 A
  3. 6 A
  4. 2 A

Explanation

In a current divider the larger share of current flows in the smaller resistance. Applying I = 6 A × 4/(2 + 4) gives 4 A.

Question 47Circuit theory and components

A 12 V source feeds a 6 Ω and a 3 Ω resistor in series, and the output terminals are taken across the 3 Ω resistor. What are the Thevenin voltage and resistance seen at those terminals?

  1. 9 V and 2 Ω
  2. 8 V and 3 Ω
  3. 4 V and 2 Ω Correct answer.
  4. 4 V and 9 Ω

Explanation

The open-circuit terminal voltage is the divider output, 12 × 3/9 = 4 V. With the source replaced by a short circuit, the resistance seen is 6 Ω in parallel with 3 Ω, which is 2 Ω.

Question 48Circuit theory and components

What is the typical forward voltage drop of a conducting silicon diode?

  1. 5.0 V
  2. 0.3 V
  3. 1.4 V
  4. 0.7 V Correct answer.

Explanation

A forward-biased silicon junction drops about 0.6 to 0.7 V before it conducts appreciable current. Germanium junctions are lower, at roughly 0.3 V.

Question 49Circuit theory and components

A zener diode is used as a voltage regulator. In which region is it operated?

  1. Zero bias
  2. Saturation
  3. Forward conduction
  4. Reverse breakdown Correct answer.

Explanation

A zener regulator relies on the controlled reverse breakdown voltage, which stays almost constant as the current varies. Operated in forward conduction it would behave like an ordinary diode dropping about 0.7 V.

Question 50Circuit theory and components

A transistor has a collector current of 20 mA and a base current of 0.2 mA. What is its DC current gain?

  1. 10
  2. 100 Correct answer.
  3. 20
  4. 40

Explanation

DC current gain is defined as β = IC/IB. Dividing 20 mA by 0.2 mA gives 100.