ASVAB Electronics Information: Ohm's Law, Circuits, and Current

What the subtest is

The official ASVAB site describes Electronics Information as knowledge of electricity and electronics. On the computer test (CAT-ASVAB), the official table lists:

That's about 40 seconds a question, so EI rewards knowing the rule on sight more than working it out from scratch.

EI isn't part of your AFQT, the score that decides basic enlistment eligibility. It matters for jobs. The Air Force lists its Electronics composite as Arithmetic Reasoning, Electronics Information, General Science, and Mathematics Knowledge. Each service sets its own job composites, so ask your recruiter which subtests count for the jobs you want; the line scores guide explains how composites work.

The core ideas, with the formulas

These come from OpenStax College Physics, a free peer-reviewed textbook.

Current, voltage, resistance. Current is the flow of charge, measured in amperes. Voltage pushes it, measured in volts. Resistance is the property that impedes current, measured in ohms (Ω). One ohm equals one volt per ampere.

Ohm's law. V = I × R, or I = V / R. Double the voltage across the same resistor and the current doubles. Double the resistance at the same voltage and the current halves.

Power. P = I × V, measured in watts. Two equivalent forms: P = I²R and P = V²/R. Electric bills charge for energy in kilowatt-hours.

Series circuits. Resistances add: R_total = R1 + R2 + R3. The same current flows through every resistor.

Parallel circuits. 1/R_total = 1/R1 + 1/R2 + 1/R3. Each branch gets the full source voltage, and the total resistance is smaller than the smallest single resistor.

AC and DC. Direct current flows in one direction only, as from a battery. Alternating current periodically reverses direction. Most U.S. household electricity is 120 V AC at 60 Hz.

Conductors and insulators. Metals and salty water let charge move easily, so they're conductors. Glass, rubber, and plastic don't, so they're insulators; pure water is an insulator too. Electrons carry negative charge and protons positive charge.

Worked examples

1. Ohm's law. A 12-volt battery drives current through a 4-ohm resistor. What's the current?

I = V / R = 12 / 4 = 3 amperes.

2. Series. Resistors of 2, 3, and 5 ohms are in series across 20 volts. What's the current?

R_total = 2 + 3 + 5 = 10 ohms. I = 20 / 10 = 2 amperes, the same through each resistor.

3. Parallel. Two 6-ohm resistors are in parallel. What's the total resistance?

1/R = 1/6 + 1/6 = 2/6, so R = 3 ohms. Two equal resistors in parallel always give half of one.

4. Power. A device draws 2 amperes at 120 volts. What power does it use?

P = I × V = 2 × 120 = 240 watts.

Traps to watch for

• Mixing up series and parallel. Adding resistors always increases the total in series and always decreases it in parallel. If your parallel answer is bigger than a single resistor, recheck.
• Using the wrong power form. If you know I and R, use P = I²R; if you know V and R, use P = V²/R. All three give the same answer when the numbers are consistent.
• Calling water a conductor without a qualifier. Salty or impure water conducts; the textbook treats pure water as an insulator.
• Rushing the units. Kilo means thousand and milli means thousandth: 2 kΩ is 2,000 ohms, and 500 mA is 0.5 amperes.

How to study EI in a week

1. Memorize V = IR and P = IV, and practice rearranging them until you can solve for any letter.
2. Do five series and five parallel problems by hand. There's no calculator on the ASVAB, so keep the arithmetic clean.
3. Learn the vocabulary: current, voltage, resistance, ampere, volt, ohm, watt, AC, DC, conductor, insulator.
4. Review the General Science guide, because the Air Force Electronics composite counts GS too, and the math formulas guide for unit and fraction work.

Practice ASVAB Electronics Information questions with explanations → and keep a list of the formula you reached for when you missed one.