Capacitors and inductors are essential components in electronics that frequently appear on the ASVAB test. Understanding how they store and release energy helps you answer questions accurately and confidently.
Capacitors and Inductors Explained: Boost Your ASVAB Electronics Score with Confidence
Whether you're aiming for a technical job in the military or simply want to master electronics basics, this guide will walk you through everything you need to know.
In this article, you'll learn:
- The fundamental principles behind capacitors and
inductors
- Key formulas you must memorize
- Real-world examples and ASVAB-style practice questions
1. What Is a Capacitor?
A capacitor is a passive
electronic component that stores electrical energy in an electric field.
It consists of two conductive plates separated by an insulating material called
a dielectric.
Key Function: It resists changes in voltage by charging and discharging.
Capacitance
(C)
Capacitance is measured in farads
(F) and represents the amount of charge a capacitor can store per volt.
Formula:
C = Q / V
Where:
- C
= capacitance in farads (F)
- Q
= charge in coulombs (C)
- V
= voltage in volts (V)
Example:
If a capacitor stores 0.002 C of charge at 5 V:
C = 0.002 / 5 = 0.0004 F or 400 µF
2. Capacitors in Series and Parallel
Series
Configuration
When capacitors are connected in
series, the total capacitance decreases.
Formula:
1 / C_total = 1 / C1 + 1 / C2 + 1 /
C3 + ...
Parallel
Configuration
In parallel, the capacitances add
directly.
Formula:
C_total = C1 + C2 + C3 + ...
3. Charging and Discharging a Capacitor
Capacitors charge and discharge over
time, depending on the resistor value in the circuit.
The time constant (τ) is an
important concept:
Formula:
τ = R × C
Where:
- τ
(tau) = time constant in seconds
- R
= resistance in ohms (Ω)
- C
= capacitance in farads (F)
After one time constant, the
capacitor charges up to about 63% of the supply voltage.
4. What Is an Inductor?
An inductor is a component
that stores energy in a magnetic field when current flows through it. It
resists changes in current.
Construction: Typically a coil of wire.
Inductance
(L)
Inductance is measured in henries
(H) and depends on the number of coil turns, core material, and coil size.
Formula:
V = L × (ΔI / Δt)
Where:
- V
= voltage (volts)
- L
= inductance in henries (H)
- ΔI = change in current (amperes)
- Δt = change in time (seconds)
✅ ASVAB Practice Question 1:
Q:
What is the total capacitance of three capacitors connected in series with
values 4 µF, 6 µF, and 12 µF?
A.
22 µF
B. 2 µF
C. 3 µF
D. 5 µF
Correct Answer: C
Explanation:
1 / C_total = 1 / 4 + 1 / 6 + 1 / 12
1 / C_total = 0.25 + 0.1667 + 0.0833
= 0.5
C_total = 1 / 0.5 = 2 µF
Oops! Actually, 0.25 + 0.1667 +
0.0833 = 0.5 ⇒ 1 / 0.5 = 2 µF,
so the correct answer is B.
5. Inductors in Series and Parallel
Inductors in Series
L_total = L1 + L2 + L3 + ...
Example:
Imagine you have three inductors connected in series with the following values:
2 henries, 3 henries, and 5 henries.
L_total = 2 + 3 + 5 = 10 H
Inductors in Parallel
1 / L_total = 1 / L1 + 1 / L2 + 1 / L3 + ...
6. Capacitor vs. Inductor: Key Differences
Property |
Capacitor |
Inductor |
Energy Stored |
Electric field |
Magnetic field |
Unit |
Farad (F) |
Henry (H) |
Opposes |
Voltage change |
Current change |
Stores Energy As |
Charge (Q) |
Magnetic flux |
Main Formula |
Q = C × V |
V = L × (ΔI / Δt) |
✅ ASVAB Practice Question 2:
Q:
What is the time constant for a circuit with R = 1.5 kΩ and C = 470 µF?
A.
0.705 s
B. 0.47 s
C. 705 s
D. 0.0007 s
Correct Answer: A
Explanation:
τ = R × C = 1500 × 0.00047 = 0.705
seconds
✅ ASVAB Practice Question 3:
Q:
Which of the following components stores energy in a magnetic field?
A.
Capacitor
B. Battery
C. Inductor
D. Diode
Correct Answer: C
Explanation: Inductors store energy in magnetic fields, while capacitors
store energy in electric fields.
✅ ASVAB Practice Question 4:
Q:
What is the voltage across a 0.5 H inductor when the current changes at a rate
of 4 A/s?
A.
2 V
B. 0.125 V
C. 8 V
D. 4 V
Correct Answer: A
Explanation:
V = L × (ΔI / Δt) = 0.5 × 4 = 2 V
7. How Capacitors and Inductors React to AC vs. DC
Understanding how these components
respond to different types of current is crucial for the ASVAB:
Capacitors
- In DC circuits, capacitors charge up and then
act as an open circuit.
- In AC circuits, capacitors continuously charge
and discharge.
Inductors
- In DC circuits, inductors eventually act as
short circuits after initial resistance.
- In AC circuits, they resist changes in current
flow and create impedance.
✅ ASVAB Practice Question 5:
Q:
In a DC circuit, what happens to a capacitor after it is fully charged?
A.
It acts as a short circuit
B. It acts as an open circuit
C. It stores magnetic energy
D. It continues to conduct current
Correct Answer: B
Explanation: Once fully charged, a capacitor blocks further DC current,
behaving like an open circuit.
8. Practical Applications in Military Electronics
To understand how capacitors and
inductors are used in real life, it's helpful to connect theoretical concepts
to military scenarios.
Capacitors Are Used In:
- Power Supply Circuits: To smooth voltage fluctuations.
- Timing Circuits:
To create specific delays through controlled discharge.
- Radios & Communication Devices: For signal filtering and tuning.
Inductors Are Used In:
- Radar and Radio Systems: To form resonant circuits.
- Transformers and Power Grids: For voltage transformation and energy transfer.
- Motors and Coils:
To store energy and induce magnetic fields.
9. Summary Formulas Review
Capacitor Key Formulas
- Q = C × V → Charge = Capacitance × Voltage
- C_total (series) = 1 / (1/C1 +
1/C2 + ...)
- C_total (parallel) = C1 + C2 +
...
Inductor Key Formulas
- V = L × (ΔI / Δt) → Voltage = Inductance × Rate of Change of Current
- L_total (series) = L1 + L2 +
...
- L_total (parallel) = 1 / (1/L1
+ 1/L2 + ...)
✅ ASVAB Practice Question 6:
Q:
In which configuration does a capacitor offer the least total capacitance?
A. Parallel
B. Series
C. Diagonal
D. AC configuration
✅ Correct Answer: B
Explanation: Capacitors in series produce a lower total capacitance than
any individual capacitor.
✅ ASVAB Practice Question 7:
Q:
A current through an inductor is increasing at 3 A/s. If the inductor value is
0.25 H, what voltage is induced?
A. 0.75 V
B. 1.5 V
C. 0.25 V
D. 3 V
✅ Correct Answer: A
Explanation:
V = L × (ΔI / Δt) =
0.25 × 3 = 0.75 V
✅ ASVAB Practice Question 8:
Q:
What happens to the impedance of a capacitor as frequency increases?
A. It increases
B. It stays constant
C. It decreases
D. It becomes infinite
✅ Correct Answer: C
Explanation: The impedance of a capacitor is inversely proportional to
frequency:
X_C = 1 / (2πfC)
10. Study Tips for Capacitors and Inductors Section on ASVAB
- Use flashcards to memorize formulas.
- Draw circuits to understand series and parallel setups.
- Watch explainer videos with real-world military
electronics examples.
- Solve at least 5 practice questions daily about RC and
RL circuits.
- Learn unit conversions (e.g., microfarads to farads,
millihenries to henries).
11. Conclusion: Build Confidence with Concepts
Understanding how capacitors and
inductors work—both individually and together—is essential to mastering the
Electronics Information section of the ASVAB.
With the formulas, practice
questions, and real-world context in your toolkit, you're well prepared to face
the exam confidently.
Keep practicing. Stay focused.
You’re on your way to a successful military career backed by knowledge and
preparation.