Electrical Knowledge — Apply principles of electricity, circuits, and electronics. - Armed Services Vocational Aptitude Battery ASVAB

SECTION 1

Historical Context & Motivation

Every military branch depends on electronics — from the radar systems guiding aircraft to the communication equipment that keeps units connected on the battlefield. Understanding electricity is not just an academic exercise; it is a foundational skill for dozens of military occupational specialties. The journey to our modern understanding of electrical principles spans centuries, driven by curious scientists and practical inventors who unlocked the secrets of nature's most versatile form of energy.

1752 • Franklin's Kite Experiment

Benjamin Franklin demonstrated that lightning is electrical in nature, establishing the concept of positive and negative charges and inventing the lightning rod.

1800 • Volta's Battery

Alessandro Volta created the first true battery (the voltaic pile), providing a steady source of electrical current and enabling systematic experiments with electricity.

1827 • Ohm's Law Published

Georg Simon Ohm published his law relating voltage, current, and resistance — the single most important equation you will encounter on the ASVAB EI subtest.

1882 • Edison's Power Station

Thomas Edison opened the Pearl Street Station in New York City, delivering direct current (DC) power to customers and launching the age of electrical utilities.

1947 • Invention of the Transistor

Bell Labs engineers invented the transistor, replacing bulky vacuum tubes and enabling the compact, reliable electronics used in modern military equipment.

These milestones built upon each other to create the electrical world you interact with every day. The ASVAB Electronics Information subtest asks you to apply these principles — not just memorize dates. The core question is straightforward: How does electrical energy move through circuits, and how can we predict and control its behavior?

SECTION 2

Core Principles & Definitions

Before diving into circuit calculations, you need a solid grasp of the fundamental quantities that describe electrical behavior. Think of these as the vocabulary of electronics — without them, you cannot read the language of circuits. Each quantity has a unit of measurement and a letter symbol used in equations.

Voltage (V)

Voltage is the electrical pressure that pushes charge through a circuit. Measured in volts (V), it represents the energy difference between two points. A 9 V battery has more "push" than a 1.5 V battery.

Current (I)

Current is the flow rate of electric charge past a given point. Measured in amperes (A), often shortened to "amps." One amp equals one coulomb of charge flowing per second.

Resistance (R)

Resistance is the opposition to current flow. Measured in ohms (Ω). A thin wire has more resistance than a thick one, just as a narrow hose restricts water more than a wide one.

Power (P)

Power is the rate at which electrical energy is consumed or produced. Measured in watts (W). A 100 W light bulb converts more energy per second than a 60 W bulb.

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KEY TAKEAWAY

Think of electricity like water flowing through a pipe. Voltage is the water pressure from a pump. Current is how much water actually flows through the pipe. Resistance is a narrow section in the pipe that restricts the flow. Increase the pressure and more water flows; narrow the pipe and less water gets through. This "water pipe" analogy is the single most useful mental model for understanding every ASVAB circuit problem.

SECTION 3

Visual Explanation — Basic Circuit Diagram

A complete electrical circuit requires a power source, a conductive path, and a load (a device that uses the energy). If any part of the path is broken, no current flows — this is called an open circuit. The diagram below shows a simple DC circuit with a battery, a switch, and a resistor (the load).

A simple DC circuit: the battery (left) provides 12 V, the switch (top) controls current flow, and the resistor (right) is the load that uses the energy. When the switch is closed, current flows clockwise from the positive terminal.

Notice that the switch in the diagram is shown in the open position, meaning the circuit is incomplete and no current flows. When you close the switch (making physical contact), you complete the loop and current begins to flow. This is an important ASVAB concept: a closed circuit allows current to flow, while an open circuit stops it. A blown fuse or a broken wire creates an unintentional open circuit.

SECTION 4

Mathematical Framework — Ohm's Law & Power

The relationship between voltage, current, and resistance is captured in the most important formula on the ASVAB EI subtest: Ohm's Law. If you know any two of the three quantities, you can solve for the third. You should also know the power equation, which tells you how much energy a device uses per second.

OHM'S LAW

V = I × R

V = voltage in volts (V), I = current in amperes (A), R = resistance in ohms (Ω). Rearranged: I = V ÷ R or R = V ÷ I.

ELECTRICAL POWER

P = V × I

P = power in watts (W), V = voltage in volts, I = current in amperes. Alternative forms: P = I² × R or P = V² ÷ R.

SERIES RESISTANCE

R_total = R₁ + R₂ + R₃ + …

In a series circuit, resistances add up directly. Total resistance is always greater than any individual resistor.

PARALLEL RESISTANCE

1 / R_total = 1 / R₁ + 1 / R₂ + 1 / R₃ + …

In a parallel circuit, total resistance is always less than the smallest individual resistor. For two resistors: R_total = (R₁ × R₂) ÷ (R₁ + R₂).

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ASVAB TIP

Memorize all three forms of Ohm's Law and both power equations. The ASVAB frequently gives you two known values and asks you to calculate the third. Write down V = I × R at the top of your scratch paper — it will be your most-used equation.

SECTION 5

Series vs. Parallel Circuits — Detailed Breakdown

The ASVAB tests your ability to distinguish between series circuits and parallel circuits. In a series circuit, components are connected end-to-end in a single path — the current must pass through every component. In a parallel circuit, components are connected across the same two nodes, providing multiple paths for current. Most real-world circuits (including military electronics) combine both configurations in what are called series-parallel circuits.

Side-by-side comparison: in the series circuit (left), resistors share the same current but divide the voltage. In the parallel circuit (right), resistors share the same voltage but divide the current.

Property	Series Circuit	Parallel Circuit
Current	Same through all components	Divides among branches
Voltage	Divides among components	Same across all branches
Total Resistance	Sum of all resistances (increases)	Less than the smallest resistor (decreases)
If one component fails	Entire circuit stops (open)	Other branches keep working
Common example	Old-style holiday string lights	Household outlets on a circuit

Key differences between series and parallel circuits

SECTION 6

Worked Example — Solving an ASVAB Circuit Problem

Let's walk through a typical ASVAB-style problem step by step. Imagine you see this question: "A 24-volt battery powers a series circuit containing a 4 Ω resistor and an 8 Ω resistor. What is the current flowing through the circuit, and how much power does the 8 Ω resistor consume?"

Series Circuit — Current & Power Calculation

1
Step 1 — Identify Given Values

From the problem: V = 24 V (battery voltage), R₁ = 4 Ω, R₂ = 8 Ω. The circuit is in series, so we will add the resistances.
2
Step 2 — Find Total Resistance

In a series circuit, total resistance is the sum: R_total = R₁ + R₂ = 4 Ω + 8 Ω

R_total = 12 Ω
3
Step 3 — Apply Ohm's Law to Find Current

Using I = V ÷ R: I = 24 V ÷ 12 Ω

I = 2 A
4
Step 4 — Find Voltage Drop Across the 8 Ω Resistor

In a series circuit, current is the same through every component. The voltage drop across R₂ is: V₂ = I × R₂ = 2 A × 8 Ω

V₂ = 16 V
5
Step 5 — Calculate Power Consumed by the 8 Ω Resistor

Using P = V × I (or equivalently P = I² × R): P = 16 V × 2 A = 32 W. Alternatively, P = (2)² × 8 = 4 × 8 = 32 W.

P = 32 watts

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CHECK YOUR WORK

Notice that the voltage drops must add up to the source voltage: V₁ = I × R₁ = 2 × 4 = 8 V, and V₂ = 16 V. Since 8 V + 16 V = 24 V, our answer checks out. This is Kirchhoff's Voltage Law — the sum of voltage drops around a closed loop equals the source voltage.

SECTION 7

Key Electronic Components

Beyond resistors and batteries, the ASVAB expects you to recognize common electronic components and understand their functions. Military electronics use all of these components in everything from radio transceivers to night-vision goggles. Here is a quick-reference comparison of the most frequently tested components.

Component	Function	Key Fact for ASVAB
Resistor	Limits current flow; converts electrical energy to heat	Color bands indicate resistance value; measured in ohms (Ω)
Capacitor	Stores electrical energy in an electric field; releases it quickly	Measured in farads (F); blocks DC but passes AC
Inductor	Stores energy in a magnetic field; resists changes in current	Measured in henrys (H); passes DC but resists AC
Diode	Allows current to flow in only one direction	Acts as a one-way valve; LEDs are a type of diode
Transistor	Amplifies signals or acts as an electronic switch	Foundation of all modern digital electronics; replaced vacuum tubes
Fuse / Circuit Breaker	Protects the circuit by breaking if current exceeds a safe level	Fuse burns out (replace it); breaker trips (reset it)
Transformer	Steps AC voltage up or down using electromagnetic induction	Works only with AC; used in power distribution and chargers

Electronic components commonly tested on the ASVAB EI subtest

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KEY TAKEAWAY

Think of a capacitor as a rechargeable water balloon — it fills up quickly, stores the water (charge), and can release it in a fast burst. An inductor is more like a heavy flywheel — it resists sudden changes in motion (current) but keeps things flowing steadily once they get going. A diode is simply a one-way street sign for electrons.

SECTION 8

AC vs. DC — Connecting to Advanced Concepts

So far we have focused mostly on direct current (DC), where electrons flow steadily in one direction — like the current from a battery. However, the electricity that powers your home and most military installations is alternating current (AC), where the direction of electron flow reverses many times per second. In the United States, AC power alternates at 60 hertz (Hz), meaning it completes 60 full cycles every second. The ASVAB expects you to know the key differences.

Feature	Direct Current (DC)	Alternating Current (AC)
Direction of flow	One direction only	Reverses direction periodically
Common sources	Batteries, solar cells, DC power supplies	Wall outlets, generators, power grid
Voltage	Constant (e.g., 12 V battery)	Varies sinusoidally (120 V RMS in the US)
Transmission efficiency	Loses energy over long distances	Easily stepped up/down by transformers; efficient for long distances
Military use	Portable radios, vehicle electronics, drones	Base power systems, shipboard electrical systems

DC vs. AC comparison for ASVAB preparation

As you advance in your military career or pursue further electronics training, you will encounter AC circuit analysis involving impedance (the AC equivalent of resistance, combining resistance with the effects of capacitors and inductors), reactance, and phase angles. For the ASVAB, focus on understanding the basic differences outlined above and knowing that standard US household voltage is 120 V AC at 60 Hz.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

A string of holiday lights is wired in series. When one bulb burns out, the entire string goes dark. Explain why this happens, and describe how wiring the lights in parallel would change this behavior.

Show answer

In a series circuit, there is only one path for current to flow. When one bulb burns out, it creates an open circuit — a break in that single path — so current stops flowing to all the other bulbs and the entire string goes dark. If the lights were wired in parallel, each bulb would have its own independent path from the power source. A burned-out bulb would only break its own branch, while the other branches would continue to carry current and remain lit. This is why modern holiday lights and household outlets use parallel wiring.

Question 2: BASIC CALCULATION

A flashlight uses a 6 V battery and has a bulb with a resistance of 12 Ω. How much current flows through the bulb?

Show answer

Using Ohm's Law: I = V ÷ R = 6 V ÷ 12 Ω = 0.5 A. The flashlight bulb draws 0.5 amperes (or 500 milliamperes) of current. This is a straightforward single-step Ohm's Law problem — identify the voltage (6 V) and resistance (12 Ω), then divide. On the ASVAB, always check that your units make sense: volts divided by ohms gives amperes.

Question 3: INTERMEDIATE

Two resistors, R₁ = 10 Ω and R₂ = 15 Ω, are connected in parallel across a 30 V source. Find the total resistance and the total current drawn from the source.

Show answer

For two resistors in parallel, use the product-over-sum formula: R_total = (R₁ × R₂) ÷ (R₁ + R₂) = (10 × 15) ÷ (10 + 15) = 150 ÷ 25 = 6 Ω. Now apply Ohm's Law: I_total = V ÷ R_total = 30 V ÷ 6 Ω = 5 A. Notice that the total resistance (6 Ω) is less than either individual resistor, which is always the case in a parallel circuit. The source supplies a total of 5 amperes, which splits between the two branches: I₁ = 30 ÷ 10 = 3 A through R₁ and I₂ = 30 ÷ 15 = 2 A through R₂, and 3 + 2 = 5 A confirms our answer.

Question 4: APPLIED

A military field radio operates at 24 V and draws 3 A of current. The radio is connected to a portable generator through 50 meters of wire that has a total resistance of 0.8 Ω (for both the outgoing and return wires combined). How much power is wasted as heat in the wiring, and what voltage actually reaches the radio?

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First, find the power lost in the wire using P = I² × R = (3)² × 0.8 = 9 × 0.8 = 7.2 W. Next, find the voltage drop across the wire: V_wire = I × R_wire = 3 × 0.8 = 2.4 V. The voltage that actually reaches the radio is the generator voltage minus the wire voltage drop: V_radio = 24 − 2.4 = 21.6 V. This means 7.2 watts of power is wasted heating the cable, and the radio only receives 21.6 V instead of the full 24 V. In practice, this is why military field installations use heavier-gauge wire for long runs — thicker wire has lower resistance and wastes less power.

Question 5: CRITICAL THINKING

A technician has three identical 12 Ω resistors and needs to create a circuit with a total resistance of exactly 8 Ω. How should the resistors be arranged? Explain your reasoning and verify your answer.

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Place one 12 Ω resistor in series with a parallel combination of the other two 12 Ω resistors. The two parallel resistors have a combined resistance of R_parallel = (12 × 12) ÷ (12 + 12) = 144 ÷ 24 = 6 Ω. Adding the series resistor: R_total = 12 + 6 = 18 Ω — that is too much. Instead, try placing two resistors in parallel (giving 6 Ω) and then that combination in series with... wait, we need exactly 8 Ω. The correct approach: put two resistors in series (12 + 12 = 24 Ω) and then put that series combination in parallel with the third 12 Ω resistor. R_total = (24 × 12) ÷ (24 + 12) = 288 ÷ 36 = 8 Ω. This works because the parallel formula reduces the total resistance below the smallest branch (12 Ω) while the series branch (24 Ω) pulls the effective value up to exactly 8 Ω. This problem demonstrates that series-parallel combinations let you create custom resistance values — a real-world skill for electronics technicians.

SUMMARY

Lesson Summary

Summary

This lesson covered the essential electrical concepts tested on the ASVAB Electronics Information subtest. Voltage is electrical pressure measured in volts, current is the flow of charge measured in amperes, and resistance is the opposition to flow measured in ohms. Ohm's Law (V = I × R) connects all three quantities and is the single most important equation for the test. Power (P = V × I) tells you how fast energy is used, measured in watts.

In series circuits, current is the same everywhere and resistances add up; in parallel circuits, voltage is the same across all branches and total resistance decreases. Key components include resistors, capacitors, diodes, and transistors. Remember that DC flows in one direction (batteries) while AC alternates direction (wall outlets at 120 V, 60 Hz). Master these fundamentals, and you will be well prepared for the EI portion of the ASVAB.