guide
ASVAB Electronics Information study guide
The Ohm's law box that covers every calculation, the four-step method for a term you don't know, the confusable pairs this subtest is built on, and a chart of the circuit symbols it shows you.
What this subtest actually asks of you
Electronics Information is 15 of the 135 scored questions on the computer ASVAB — 11% of the scored test. Like General Science, it is not one of the four AFQT subtests, so it does not move the number that decides whether you can enlist — but it does feed the line scores that decide which jobs you qualify for (how the ASVAB is scored).
Most of it is vocabulary and recognition: what a component does, what a unit measures, what a symbol means, which way current flows. A smaller part is relationships you calculate — and because that part is so small, it is the part people skip and then lose points on.
Two consequences:
- Learn the terms and the symbols first. The questions are built out of them, and a question you cannot read is a question you cannot answer.
- Do not skip the arithmetic. Three relationships cover every calculation on this subtest (below), and they are the only place where a point is available for knowing a formula rather than recognizing a word.
The one box that is not recall: Ohm's law
Everything calculable on this subtest comes from three relationships. Learn them as one triangle, not as three formulas.
V = I × R — voltage equals current times resistance. Rearranged: I = V ÷ R, and R = V ÷ I.
- V is voltage, in volts — the push.
- I is current, in amperes — the flow.
- R is resistance, in ohms — what opposes the flow.
P = V × I — power, in watts, is voltage times current.
P = I²R — and this third form is the one questions ask for most often, because it is what you reach for when the voltage was never given. Put V = I × R into P = V × I and the voltage disappears: power is the current squared times the resistance. Two amperes through five ohms is 2² × 5 = 20 W.
The three forms are one box, not three facts. Any two of voltage, current and resistance give you the third, and then any pair gives you power.
How to use the box
- Name the two you know, with their units. "Twelve volts, four ohms."
- Pick the arrangement that leaves the unknown on its own. Want current from volts and ohms? I = V ÷ R. Want power from current and resistance, with no voltage in the question? P = I²R — the same move, and the reason the third form is worth memorizing separately.
- Compute, then check the unit and the size. In the example above, 12 ÷ 4 = 3, and the unit is amperes. A sanity check: more resistance at the same voltage must give less current, so if you got a bigger number than the voltage, you divided the wrong way.
That last check is the whole game — the wrong-answer options in these questions are almost always the multiplication you would get by inverting the formula.
The four-step method for a term you do not know
For the recall majority of this subtest, this is what stops a blank from becoming a guess:
1. Name the quantity or the class. Is the question about voltage, current, resistance or power? Or about a component — a source, a conductor, a control, a load, a protection device? Naming the class narrows the field to a few candidates.
2. If it is a relationship, use the box above rather than memory.
3. Eliminate the swapped pair. This subtest's favorite wrong answer is the neighboring concept: voltage for current, series for parallel, a motor for a generator, AC for DC, a conductor for an insulator. If two choices are a known pair, the question is about the boundary between them.
4. For "what happens if" questions, follow the chain physically. More resistance at the same voltage means the current decreases. Add a second path in parallel and the total resistance falls. A fuse blows because the current got too large, not because the voltage did. One physical step is all any of these questions asks.
The confusables
| Question | The boundary |
|---|---|
| How do voltage and current differ? | the push vs the flow |
| How do current and resistance differ? | how much moves vs what opposes it |
| How do series and parallel differ? | one path vs several paths |
| How do AC and DC differ? | reverses direction vs one direction |
| How do motor and generator differ? | electricity in, motion out vs motion in, electricity out |
| How do conductor, insulator and semiconductor differ? | flows freely vs does not flow vs flows under conditions |
| How do cell and battery differ? | one unit vs several cells together |
| How do fuse and circuit breaker differ? | a wire that melts, replaced vs a switch that trips, reset |
| How do ammeter and voltmeter differ? | measures current, wired in series vs measures voltage, wired across |
| How do power and energy differ? | the rate of using energy (watts) vs the total used (watt-hours) |
| How do a conductor and a resistor differ? | lets current through vs deliberately limits it |
| How do farad and henry differ? | how much charge a capacitor holds vs how much a coil opposes a change in current |
| How do diode and rectifier differ? | one one-way valve vs a one-way valve used to turn AC into DC — four of them in a bridge rectifier |
| How do step-up and step-down differ? | more turns on the secondary, so a higher voltage vs fewer turns, so a lower one |
The symbols are worth a minute
A question may show you a symbol and ask what it is. The drawing below is the seven that turn up most; the table carries those, and the two rows under them are the pair the questions build a wrong answer from.
| Question | What it is | The one fact to know |
|---|---|---|
| What does the battery symbol mean? | the source that pushes the current | alternating long and short plates — the long plate is the positive terminal |
| What does the resistor symbol mean? | the part that opposes the current | the zigzag |
| What does the lamp symbol mean? | a load that gives off light | a circle with a cross through it |
| What does the switch symbol mean? | a control that makes or breaks the circuit | an arm over a gap: closed when the arm touches down |
| What does the ground symbol mean? | the common reference point for voltage | three shrinking bars, widest at the top |
| What does the diode symbol mean? | a one-way valve for current | a triangle pointing into a bar — current passes the way the triangle points and not the other way |
| What does the capacitor symbol mean? | a part that stores charge | two parallel plates with a gap, so a steady current cannot cross it |
| What does the LED symbol mean? | a light-emitting diode | a diode symbol with arrows pointing away, because it gives off light |
| What does the rectifier symbol mean? | a diode, or four in a bridge, arranged to turn AC into DC | a single diode is a half-wave rectifier; four in a bridge is a full-wave bridge rectifier, which is the one questions usually mean |
Reading a resistor: the color code
A resistor is marked with colored bands rather than printed with a number, because the value has to stay readable when the part is small and hot. The code is a decimal spelling: one color is one digit, and the bands are read from the end where they start.
The ten colors, in order
| Question | The digit it spells | The multiplier it says |
|---|---|---|
| What digit does black stand for? | 0 | ×1 |
| What digit does brown stand for? | 1 | ×10 |
| What digit does red stand for? | 2 | ×100 |
| What digit does orange stand for? | 3 | ×1,000 |
| What digit does yellow stand for? | 4 | ×10,000 |
| What digit does green stand for? | 5 | ×100,000 |
| What digit does blue stand for? | 6 | ×1,000,000 |
| What digit does violet stand for? | 7 | ×10,000,000 |
| What digit does gray stand for? | 8 | ×100,000,000 |
| What digit does white stand for? | 9 | ×1,000,000,000 |
How the bands are read
| Question | What it says | The one fact to know |
|---|---|---|
| What does the first band say? | the first digit of the value | read from the end where the bands start, and the two digit bands run in the order they are painted |
| What does the second band say? | the second digit of the value | brown and black is 10; green and blue is 56 |
| What does the third band say? | the multiplier — the number of zeros to add, in the same ten colors | orange, orange, brown is 33 × 10 = 330 ohms |
| What does the fourth band say? | the tolerance | gold is ±5%, silver is ±10%, and a resistor with no fourth band is ±20% — it says how far the real part may stray, so read it before trusting a measurement |
| What is the third-band shortcut? | a three-band read ends in the multiplier, so the third band tells you the size | brown–black–red is 10 × 100 = 1,000 ohms, which we write as 1 kΩ — the third band being red is why so many ordinary resistors are the kilo-ohms kind |
| How do you spot the tolerance band? | the band is almost always gold or silver rather than a digit color | if one band looks different from the other three, that is the one it is |
The transformer, and the turns ratio
A transformer is two coils sharing a core, and it moves energy between them by magnetism rather than by a wired connection — which is why it works on alternating current only. A steady direct current builds a field that never changes, and a field that never changes induces nothing in the second coil.
Almost every transformer question is one relationship, the turns ratio:
primary turns ÷ secondary turns = primary voltage ÷ secondary voltage
- Read the turns off the question on each side: "1,000 turns on the primary, 100 on the secondary". If it gives you the ratio instead — "a 4:1 transformer" — that is this step already done.
- Reduce it. 1,000 ÷ 100 = 10, so this transformer changes the voltage by a factor of ten. A 200:50 transformer reduces to 4:1.
- Decide which way the change goes. More turns on the secondary means more voltage — a step-up transformer. Fewer turns means less voltage — a step-down. Ten to one down takes a 120 V primary to 120 ÷ 10 = 12 V out.
The catch that makes it a real question: the turns trade voltage for current, so the power is not multiplied. A transformer passes nearly all the power through, so when the voltage comes down by ten the available current goes up by ten. A transformer that raised the voltage and the current would be making energy, which is the wrong answer a careless reader picks.
The core facts, by idea
The units
| Question | What it measures | The one fact to know |
|---|---|---|
| What does the volt measure? | voltage, the push | the quantity the battery provides |
| What does the ampere measure? | current, the flow | a milliamp is a thousandth of an amp, so 0.5 A is 500 mA |
| What does the ohm measure? | resistance, what opposes the flow | the quantity a resistor provides |
| What does the watt measure? | power, the rate of using energy | power is volts times amps |
| What does the hertz measure? | frequency, cycles per second | 60 Hz means 60 cycles per second |
Which way the current flows
| Question | What it says | The one fact to know |
|---|---|---|
| Which way does conventional current flow? | drawn flowing from the positive terminal, through the circuit, back to the negative terminal | this is the direction questions use |
| Which way do electrons actually drift? | the electrons actually drift the opposite way | both statements are true, and they never conflict |
| What is direct current? | flows one way only | what a battery supplies |
| What is alternating current? | reverses direction many times a second | what the mains supplies, which is why a transformer is used on it and a diode is used to make it into DC |
Series and parallel
| Question | What it is | The one fact to know |
|---|---|---|
| What is a series circuit? | one path, so the same current passes through every component | the resistances add (R = R1 + R2) |
| What happens when a second identical lamp is added in series? | Each of the two lamps glows dimmer than one alone — the supply voltage is now shared between them | the total resistance has doubled |
| What is a parallel circuit? | several paths, so the same voltage sits across each branch | the total resistance is less than the smallest single resistor |
| What happens when a series circuit is broken? | in series it stops everything | in parallel the other branches keep working |
Conductors, insulators and semiconductors
| Question | What it is | The one fact to know |
|---|---|---|
| What are copper, aluminum and salt water examples of? | conductors | charge flows through them freely, which is why they are what a wire is made of |
| What are rubber, glass, plastic and dry wood examples of? | insulators | charge does not flow, which is why one of them wraps a wire |
| What is silicon an example of? | a semiconductor | it conducts only under the right conditions, which is what makes it the material diodes and transistors are built from |
The component classes
| Question | What it does | Examples |
|---|---|---|
| What does the source class do? | pushes current | a battery, a generator |
| What does the conductor class do? | carries the current | copper wire |
| What does the load class do? | uses the current | a lamp, a motor, a resistor |
| What does the control class do? | starts and stops the current | a switch, a relay |
| What does the protection class do? | stops the current when it gets too high | a fuse, a breaker |
Transformer and relay
| Question | What it does | The one fact to know |
|---|---|---|
| What does a transformer do? | changes AC voltage up or down | it works on alternating current only, which is what the section above turns on |
| What does a relay do? | a switch operated by an electromagnet | a small control current switches a much larger one |
Magnetism and motion
| Question | What it is | The one fact to know |
|---|---|---|
| What is an electromagnet? | a current in a coil produces a magnetic field | it is a magnet only while the current flows |
| What are magnetic poles? | like poles repel and unlike poles attract | reversing the current reverses the poles |
| What is a motor? | turns electrical energy into motion | the machine a generator runs in reverse |
| What is a generator? | converts motion into electricity — a motor run backwards | they are the same machine run in opposite directions, which is why the pair is such a common trap |
Safety and measurement
| Question | What it does | The one fact to know |
|---|---|---|
| What does a fuse do? | melts to break a circuit that is drawing too much current | a wire that has to be replaced |
| What does a circuit breaker do? | trips to break the circuit | and can be reset |
| What does a multimeter do? | measures voltage, current and resistance in one instrument | the standard fault-finding tool |
| What does an ammeter do? | measures current | wired in series, because the current must pass through it |
| What does a voltmeter do? | measures voltage | wired across the component, because it is comparing two points |
How to practice this
- Work the free Electronics Information practice set with the symbol chart beside you.
- Write the Ohm's-law triangle on your scratch paper before you start. The calculation questions are few and easy once it is in front of you — and it costs you four seconds.
- Keep a swapped-pair list from the questions you get wrong. On this subtest the pairs are the questions.
- Read the explanation on every question, including the ones you got right.
How to study for the ASVAB has the rest of the method, and what the pass adds.
Where to go next
- Next topic — Auto Information
- The exam itself — how the ASVAB is scored, how many questions, and what makes the ASVAB hard.
Electronics Information carries 15 of the 135 scored questions on the computer test — the published subtest table's count — and the AFQT score is not computed from it: that score comes from Arithmetic Reasoning, Mathematics Knowledge, Word Knowledge and Paragraph Comprehension.
Elk Ridge Academy is independent and is not affiliated with or endorsed by the Department of Defense, USMEPCOM, or MEPS.
Source: Official ASVAB — subtest descriptions, with CAT-ASVAB scored-question counts and the AFQT composition (accessed 2026-09)