⚡ Current, Voltage, Resistance and Ohm's Law

Part 1 of 7 — Electricity on the ASVAB

Topic Overview

What Electronics Information (EI) Is

Electronics Information is the 6th subtest on the computer-adaptive CAT-ASVAB, right after Mathematics Knowledge. You get 15 scored questions in 10 minutes, which works out to about 40 seconds per question. Every question has 4 answer choices.

You do not need to be an electrician. EI asks about a small set of ideas that come up again and again:

• the three basic quantities: current, voltage and resistance
• Ohm's law, with numbers simple enough to work by hand (no calculator is allowed)
• which materials conduct electricity and which insulate
• how meters measure a circuit
• series and parallel circuits, components and their symbols, and AC vs DC (the other two EI topics in this course)

Key Fact: EI is not part of your AFQT score (that comes from Arithmetic Reasoning, Mathematics Knowledge, Word Knowledge and Paragraph Comprehension). EI matters for job line scores: for example the Air Force Electronics score (E = GS + AR + MK + EI), the Navy and Marine Corps Electronics score (EL = GS + AR + MK + EI), and the Marine Corps Mechanical Maintenance score (MM = AR + MC + EI + AS). In the Army, EI is one of the subtests that matter most for the Electronics, General Maintenance, Mechanical Maintenance and Surveillance and Communications scores. Ask your recruiter which line scores your target job needs.

If you take the paper version at a test site, EI has 20 questions in 9 minutes. The high-school CEP computer version has 16 EI questions. The content is the same.

⚡ Current, Voltage, Resistance and Ohm's Law

Part 2 of 7 — Charge, Current and Voltage

Where Electricity Comes From

Everything is made of atoms. An atom has a center (the nucleus) holding protons, which carry a positive (+) charge, and neutrons, which carry no charge. Around the nucleus move electrons, which carry a negative (−) charge.

• Like charges repel (two negatives push apart).
• Opposite charges attract (a negative is pulled toward a positive).

In a metal like copper, the outermost electron of each atom is held loosely. These free electrons can drift from atom to atom. When something pushes them all in the same direction, you have an electric current.

Electric charge is measured in coulombs (C). One coulomb is the charge of a huge number of electrons (about 6 billion billion), so you never count electrons one at a time.

⚡ Current, Voltage, Resistance and Ohm's Law

Part 3 of 7 — Resistance, Conductors and Insulators

Resistance

Resistance is the opposition a material gives to the flow of current. It is measured in ohms, written with the Greek letter omega, Ω. When charge is forced through a resistance, some electrical energy turns into heat. That is how a toaster, a space heater and an old-style light bulb filament work.

Every material falls into one of three groups:

The best conductors, in order

1. Silver: the lowest resistance of any common metal, but expensive
2. Copper: almost as good and far cheaper, so it is the most commonly used wire metal
3. Gold: does not corrode, so it coats connectors and contacts
4. Aluminum: lighter and cheaper, used in overhead power lines

Key Fact: "Best conductor" = silver. "Most commonly used conductor in wiring" = copper. The ASVAB likes this distinction.

Pure water is actually a poor conductor, but real water (tap water, sweat, puddles) has dissolved minerals and conducts well enough to be dangerous. Treat any wet surface near electricity as a conductor.

⚡ Current, Voltage, Resistance and Ohm's Law

Part 4 of 7 — Ohm's Law

One Formula, Three Forms

Ohm's law says the voltage across a resistance equals the current through it times the resistance:

$V = I \times R$

Rearranged, it gives all three quantities:

The triangle trick

Draw the triangle in Figure 4 on your scratch paper at the start of EI. Cover the letter you want to find:

• Cover V: $I$ and $R$ sit side by side, so multiply: $V = I \times R$
• Cover I: $V$ sits over $R$, so divide: $I = V \div R$
• Cover R: $V$ sits over $I$, so divide: $R = V \div I$

Some books write $E = I \times R$ (with $E$ for EMF). It is the same law.

⚡ Current, Voltage, Resistance and Ohm's Law

Part 5 of 7 — Measuring Electricity: Meters and Units

Three Meters, Three Jobs

A multimeter is one handheld tool that switches between all three jobs (and usually more). A galvanometer is a very sensitive meter that detects tiny currents; it is the heart of older needle-style meters. A wattmeter measures power.

Why the hookup matters

• An ammeter connected across a part (in parallel) offers a nearly zero-resistance path, so a huge current rushes through it. That can blow the meter's fuse or damage it.
• An ohmmeter connected to a live circuit gives a false reading and can be damaged, because the circuit's voltage adds to the meter's own battery.

Reading an ohmmeter

Key Fact: Checking whether a fuse or bulb is good is a continuity test with an ohmmeter: near 0 Ω means good, infinite means open (bad).

⚡ Current, Voltage, Resistance and Ohm's Law

Part 6 of 7 — EI Test Strategy and Common Traps

Your Time Budget

Most EI questions are recall: you either know that resistance is measured in ohms or you don't. Those should take 10 to 15 seconds. That banks time for the few calculation questions (Ohm's law, unit conversions), which may need 45 to 60 seconds.

How the CAT works for you (and against you)

• You cannot go back to a question once you answer it, and you must answer to move on. Settle each one before you click.
• Some questions may be unscored tryout items. You can't tell which, so treat every question as if it counts.
• If time runs out, unanswered questions are scored as if you guessed at random, and a run of wrong answers at the end of a subtest is penalized heavily. If you are running short, keep reading and genuinely answering rather than clicking randomly.
• No calculator. The numbers are chosen to work by hand.

A four-step routine for calculation questions

1. Name the unknown and its unit (amps? ohms? volts?).
2. Cover it on the triangle you drew on your scratch paper: multiply or divide.
3. Convert prefixes first (kΩ, mA) so the units match.
4. Sanity check: more resistance should mean less current; the answer's unit must match the question.

⚡ Current, Voltage, Resistance and Ohm's Law

Part 7 of 7 — Mixed Mastery Set

Ten questions at real EI difficulty: a mix of recall, reasoning and hand calculation, in no particular order, just like the test. Give yourself about 40 seconds each. Use scratch paper, no calculator.