Correct answer: D — ohm

Impedance is the total opposition a circuit presents to alternating current, combining resistance, capacitive reactance, and inductive reactance into a single complex quantity. Like resistance, impedance is measured in ohms (Ω), named after Georg Simon Ohm.

• A. Ampere — the ampere is the unit of electric current, not impedance.
• B. Farad — the farad is the unit of capacitance.
• C. Henry — the henry is the unit of inductance.

Therefore, impedance, being a measure of opposition to current flow, is expressed in ohms, the same unit as resistance.

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Correct answer: D — 1000 ohm

The prefix "kilo-" (symbol k) means one thousand (10³) in the SI system of units. Therefore, one kilohm (1 kΩ) equals 1000 ohms. This prefix is used throughout electronics — kilohms for resistance, kilohertz for frequency, and so on — to avoid writing large numbers of zeros.

• A. 10 ohm — This is incorrect; 10 ohms has no standard single-prefix name and is far less than 1 kΩ.
• B. 0.01 ohm — This is incorrect; 0.01 Ω would be 10 milliohms (10 mΩ), a very small resistance.
• C. 0.001 ohm — This is incorrect; 0.001 Ω equals 1 milliohm (1 mΩ), even smaller still.

Therefore, one kilohm is simply 1000 ohms, following the standard SI "kilo-" prefix meaning ×1000.

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Just like a kilometer equates to 1000 meters, a kilovolt would equate to 1000 volts.

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Correct answer: 250 milliampere

One quarter of one ampere is:

\[ \frac{1}{4} \times 1\ \mathrm{A} = 0.25\ \mathrm{A} \]

Convert to milliamps:

\[ 0.25\ \mathrm{A} = 250\ \mathrm{mA} \]

• 250 microampere = 0.00025 A (too small)
• 0.5 ampere = half an ampere
• 0.25 milliampere = 0.00025 A (too small)

Therefore, one quarter of one ampere is 250 milliampere.

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Correct answer: A — power

The watt (W) is the SI unit of power, which is the rate at which energy is transferred or work is done. In electrical circuits, power is calculated as the product of voltage and current.

\[ P = V \times I \]

For example, a circuit with 12 V and 2 A dissipates:

\[ P = 12 \times 2 = 24\ \mathrm{W} \]

• B. magnetic flux — magnetic flux is measured in webers (Wb), not watts.
• C. electromagnetic field strength — electric field strength is measured in volts per metre (V/m); magnetic field strength in amperes per metre (A/m).
• D. breakdown voltage — breakdown voltage is a voltage threshold, measured in volts (V), not a unit of power.

Therefore, the watt is correctly defined as the unit of power, representing one joule of energy transferred per second.

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Correct answer: A — 2000 mV

The prefix "milli" (m) means one-thousandth (1/1000), so there are 1000 millivolts in one volt. Two volts therefore equals 2 × 1000 = 2000 mV.

\[ 2\ \mathrm{V} = 2 \times 1000\ \mathrm{mV} = 2000\ \mathrm{mV} \]

• B — 2000 kV: The prefix "kilo" (k) means 1000, so 2000 kV = 2,000,000 V — enormously larger than 2 V.
• C — 2000 µV: The prefix "micro" (µ) means one-millionth (1/1,000,000), so 2000 µV = 0.002 V — far smaller than 2 V.
• D — 2000 MV: The prefix "mega" (M) means 1,000,000, so 2000 MV = 2,000,000,000 V — an impossibly large voltage for any normal application.

Therefore, 2 volts is exactly equal to 2000 millivolts, since milli denotes a factor of 10⁻³.

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Correct answer: volt

Potential difference (voltage) between two points is measured in volts (V).

• Ampere measures current.
• Ohm measures resistance.
• Coulomb measures electric charge.

Therefore, the unit is the volt.

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Correct answer: C — resistance with reactance

Impedance (Z) is the total opposition to alternating current in a circuit. It combines two distinct components: resistance (R), which dissipates energy as heat and is frequency-independent, and reactance (X), which stores and returns energy and arises from capacitors and inductors. Because resistance and reactance are 90° out of phase with each other, they combine as vectors:

\[ Z = \sqrt{R^2 + X^2} \]

For example, a circuit with R = 3 Ω and X = 4 Ω has an impedance of:

\[ Z = \sqrt{3^2 + 4^2} = \sqrt{9 + 16} = \sqrt{25} = 5\ \Omega \]

• A — reactance with reluctance: Reluctance is a property of magnetic circuits (the opposition to magnetic flux), not a component of electrical impedance.
• B — resistance with conductance: Conductance is simply the reciprocal of resistance (G = 1/R); combining them does not define impedance.
• D — reactance with radiation: Radiation resistance is a real (resistive) quantity used in antenna analysis — it is not a separate category that pairs with reactance to define impedance.

Therefore, impedance is always the vector combination of resistance and reactance.

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Correct answer: B — one thousandth of one ampere

A milliampere (mA) uses the SI prefix milli-, which always means one thousandth (1/1000, or 10⁻³) of the base unit. Applied to the ampere, 1 mA = 0.001 A. This prefix appears throughout electronics — millivolts, milliwatts, millihenries — and always carries the same meaning.

• A — one millionth of one ampere is incorrect; one millionth of an ampere is a microampere (µA), using the prefix micro- (10⁻⁶).
• C — one tenth of one ampere is incorrect; one tenth of an ampere would use the prefix deci- (dA), which is rarely used in electronics practice.
• D — one millionth of admittance is incorrect; admittance is measured in siemens (S), not amperes, and "millionth" again describes micro- not milli-.

Therefore, a milliampere is precisely one thousandth of one ampere, consistent with the standard SI prefix milli- (10⁻³).

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Correct answer: ohm

The unit of electrical resistance is the ohm (\(\Omega\)).

It is defined by Ohm’s Law:

\[ R = \frac{V}{I} \]

where:

• \(R\) is resistance in ohms

• \(V\) is voltage in volts

• \(I\) is current in amperes

• The farad is the unit of capacitance.

• The watt is the unit of power.

• A resistor is a component, not a unit.

Therefore, the unit of resistance is the ohm.

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