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BASIC NEC 2023

Inductors & Inductance Practice Test

Free online inductors and inductance practice test. 25 basic electrical questions on how inductors store energy, inductive reactance, inductive kick, and flyback diodes. Good for electrician exam prep.

TOPICS COVERED

Inductor Basics Inductance Unit Inductor Voltage-Current Relationship Inductance Factors Turns Effect on Inductance Core Material Effect

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An inductor is just a coil of wire that stores energy in a magnetic field. Its defining behavior: it opposes SUDDEN CHANGES in current — the electrical mirror image of a capacitor, which opposes sudden changes in voltage. Inductance is measured in henries (H) and depends on the number of turns, the core material, and the coil's size. In an AC circuit, an inductor creates inductive reactance (X_L = 2πfL), which goes UP as frequency goes UP — the opposite of a capacitor. Watch for the dangerous "inductive kick": when current through a coil is suddenly interrupted, the collapsing magnetic field can produce a brief, high-voltage spike.

25

Questions

38 min

Time

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Inductors & Inductance Practice Test — Question List

Question 1: An inductor is a device that stores electrical energy:

In a magnetic field created by current flowing through a coil of wire
In an electric field between two plates
As heat inside a resistor
As a chemical reaction, like a battery

Question 2: Inductance is measured in which unit?

Henry (H)
Farad (F)
Ohm (Ω)
Tesla (T)

Question 3: The voltage induced across an inductor is proportional to:

The rate at which the current through it is changing
The exact value of the steady current flowing through it
The physical color of the wire insulation
The ambient room temperature only

Question 4: Which of the following factors affects the inductance of a coil?

Number of turns, core material, and the coil's physical dimensions
Only the color of the wire insulation
Only the length of the connecting leads
Only the ambient room lighting

Question 5: Increasing the number of turns in a coil, with everything else unchanged, will:

Increase its inductance
Decrease its inductance
Have no effect on inductance
Turn the coil into a capacitor

Question 6: Adding an iron core to an air-core inductor, with everything else unchanged, will:

Increase its inductance
Decrease its inductance
Have no effect on inductance
Convert it into a resistor

Question 7: In a DC circuit that has reached a steady, unchanging current, an inductor:

Behaves like a plain piece of wire (essentially zero opposition, aside from its wire resistance)
Completely blocks all current, like an open circuit
Converts the DC supply into AC
Immediately overheats and fails

Question 8: In an AC circuit, an inductor:

Opposes the continually changing current, creating an opposition called inductive reactance
Provides absolutely no opposition to AC current
Converts the AC supply into steady DC
Only works properly on DC circuits

Question 9: The formula for inductive reactance is X<sub>L</sub> = 2πfL. What happens to inductive reactance as frequency increases?

It increases
It decreases
It stays exactly the same
It drops to zero

Question 10: What is the inductive reactance of a 0.1 H inductor at 60 Hz? (X<sub>L</sub> = 2πfL)

About 37.7 Ω
About 6 Ω
About 600 Ω
About 0.377 Ω

Question 11: For inductors connected in SERIES (with no mutual coupling), the total inductance is found by:

Simply adding the individual inductances together
Using a reciprocal (product over sum) formula
Multiplying all the inductances together
Subtracting the smallest from the largest

Question 12: For inductors connected in PARALLEL (with no mutual coupling), the total inductance is found using a formula that looks like the formula for:

Resistors in PARALLEL (a reciprocal formula)
Capacitors in SERIES only
There is no valid formula for inductors in parallel
Voltage sources in series

Question 13: What is the energy stored in a 2 H inductor carrying 5 A of current? (E = ½LI²)

25 J
10 J
50 J
5 J

Question 14: The "L/R time constant" of an inductive circuit (τ = L / R) represents:

The time it takes for current to rise to about 63% of its final steady-state value
The exact time it takes to reach 100% current, always
The total resistance of the circuit only
The maximum current the inductor can handle

Question 15: When the current through an inductor is suddenly interrupted (such as a switch opening), the inductor:

Produces a brief, high-voltage "inductive kick" as it resists the sudden change in current
Instantly drops to zero voltage with no effect
Converts immediately into a capacitor
Only responds to changes in voltage, never current

Question 16: A flyback (freewheeling) diode is often placed across a relay coil or solenoid to:

Protect the switching device from the voltage spike caused by the coil's inductive kick
Increase the coil's inductance permanently
Convert the coil into a capacitor
Prevent the coil from ever energizing

Question 17: A "choke" is a common name for:

An inductor used to block or limit AC current while allowing DC to pass
A type of fuse used only in lighting circuits
A capacitor used for power factor correction
A tool used to strip wire insulation

Question 18: The key defining property of an inductor — similar to how a capacitor opposes sudden changes in voltage — is that an inductor opposes:

Sudden changes in current
Sudden changes in resistance
Sudden changes in temperature
Sudden changes in wire color

Question 19: In a purely inductive AC circuit, the current:

Lags the voltage by 90 degrees
Leads the voltage by 90 degrees
Is always exactly in phase with the voltage
Has no fixed relationship with the voltage

Question 20: Self-inductance is the property where a coil's own changing current induces a voltage in:

Itself
A completely separate, unrelated circuit
The power source only
Nothing — self-inductance does not produce a voltage

Question 21: Mutual inductance, by contrast, is when a changing current in one coil induces a voltage in:

A separate, nearby coil
Itself only
A resistor only
Nothing — mutual inductance does not exist

Question 22: Air-core inductors are generally used instead of iron-core inductors in applications such as:

High-frequency radio circuits, where iron cores would introduce excessive core losses
Only in DC circuits below 5 volts
Only in circuits with no current flow at all
Never — iron core is always superior

Question 23: In simple terms, an inductor's basic behavior in a circuit can be summarized as:

It opposes sudden changes in current, resisting AC more than steady DC
It opposes steady DC more than it opposes AC
It blocks both AC and DC current at all times
It has no effect on either AC or DC current

Question 24: Switching off a large inductive load, such as a motor contactor coil, can cause dangerous arcing at the switch contacts because:

The collapsing magnetic field induces a high voltage spike trying to keep the current flowing
The coil instantly loses all its resistance
The switch contacts become better insulators
Inductive loads never cause any arcing

Question 25: Compared to a capacitor, an inductor behaves in the opposite way in one key respect: a capacitor opposes changes in voltage, while an inductor opposes changes in:

Current
Voltage
Frequency only
Resistance only

EXAM CARD BASIC

Questions25

Time limit38 min

Pass score75%

Code editionNEC 2023

DifficultyIntermediate

Last reviewedJun 2026

Timer starts immediately. Progress is auto-saved.

Source: NEC 2026 (NFPA 70)