Current, Voltage and Resistance

Electronics Information is the most learnable subtest on the ASVAB for someone starting from nothing, because it is a small set of relationships rather than a body of facts. This lesson establishes the three quantities everything else uses.

The three quantities

Enlarge

Zoom in

The letter for current is I, not C. That surprises people and it matters, because Ohm's law is written with it.

Voltage

Voltage is a difference in electrical potential between two points, which is why it is also called potential difference or electromotive force.

Voltage does not flow. It exists between two points, which is why a voltmeter has to be connected across something - you measure a difference, and a difference needs two places.

A 12-volt battery maintains a 12-volt difference between its terminals whether or not anything is connected.

Current

Current is the rate at which charge flows, and one ampere is one coulomb of charge per second.

An ammeter goes in line with the circuit, because the current has to pass through it to be measured. That is the practical difference from a voltmeter and it is asked directly.

Conventional current is taken to flow from positive to negative. Electrons, the things actually moving, flow from negative to positive. The convention predates the discovery of the electron, it was never corrected, and the test uses the conventional direction - so a question about current direction wants positive to negative, and a question about electron flow wants the opposite.

Resistance

Resistance opposes current and converts electrical energy into heat. A resistor is a component made to have a set resistance; its main job is to limit current. A variable resistor can be adjusted: a potentiometer has three terminals and divides a voltage (a volume knob), a rheostat has two and controls a current.

Four things determine a conductor's resistance:

Longer and thinner is more resistance; shorter and thicker is less. That is the pair to hold, and it is the whole reason wire gauge exists.

The water analogy carries it: a long narrow pipe restricts flow more than a short wide one.

How the three relate

More voltage pushes more current through the same resistance. More resistance lets less current through at the same voltage.

That is Ohm's law stated in words, and the next lesson gives it as a formula.

A short circuit is the extreme case: almost no resistance, so a very large current, so a great deal of heat. An open circuit is the other extreme: effectively infinite resistance, so no current at all.

Other quantities worth naming

Power is the rate of energy use, measured in watts, and it is voltage times current.

Energy is power times time, measured in kilowatt-hours on a bill or joules in physics.

Capacitance is measured in farads, inductance in henries, and frequency in hertz. Those appear in their own lessons, and recognizing the unit is often enough to answer a question about which quantity is being described.

Matching quantity to unit is a question type of its own, and this table is the whole answer to it.

Reading a resistor

Enlarge

Zoom in

Resistors are too small to print a value on, so the value is painted on in colored bands. The color code is asked in 18 questions, most of them simply "what digit does this color represent", so it is worth learning once:

The easy way to hold it: black is nothing, brown is one, and from red to violet the colors run in rainbow order - red, orange, yellow, green, blue, violet - for 2 through 7. Gray and white finish it at 8 and 9.

On a four-band resistor:

1. First band: first digit.
2. Second band: second digit.
3. Third band: the multiplier - how many zeros to add.
4. Fourth band: the tolerance - gold is 5 percent, silver is 10 percent.

Yellow, violet, red, gold.

4 and 7, then two zeros: 4,700 ohms, within 5 percent.

Gold in the multiplier position means multiply by 0.1, so green, blue, gold reads 56 x 0.1 = 5.6 ohms.

Tolerance gives a range. A 4,700-ohm resistor at 5 percent may measure anywhere within 235 ohms of that: 4,465 to 4,935 ohms. So a reading of 4,900 is within tolerance. Precision resistors use five bands, adding a third digit before the multiplier.

Meters

Why the connections matter: a voltmeter has a very high internal resistance so it draws almost no current from the circuit; an ammeter has a very low one so it does not add resistance. Connect an ammeter in parallel across a source and that low resistance makes it a short circuit, which damages the meter.

An ohmmeter supplies its own small current, which is why the circuit must be off

• an outside voltage would damage it and spoil the reading.

• A reading of 0 ohms across something that should have resistance means it is shorted.

• An infinite reading across a wire means it is open - broken.

A clamp-on ammeter measures current without breaking the circuit: its jaws close around one wire and read the magnetic field the current makes. Insulation is tested with a megohmmeter because good insulation has resistance in the millions of ohms.

Charge, static and cells

Static electricity is a buildup of electric charge on a surface, usually by friction rubbing electrons from one material to another. The spark from a doorknob after walking across carpet is that charge discharging. Like charges repel and unlike charges attract, which is what an electroscope's spreading leaves demonstrate.

A primary cell is used once; a secondary cell is rechargeable. An alkaline flashlight battery is primary; a car battery or a phone battery is secondary.

A transducer converts energy from one form to another, such as a microphone turning sound into an electrical signal.

Prefixes

Electronics uses metric prefixes constantly.

Milliamperes are the usual unit for small currents, and converting to amperes before using Ohm's law is where people slip.

What you can skip

Across all 1,358 Electronics Information questions in our bank:

• The temperature coefficient of resistance comes up once, at the hardest difficulty. Know that resistance changes a little with temperature.
• Five-band and six-band resistor reading. One question asks why precision resistors have five bands; none asks you to read one.
• Resistivity calculations. Resistivity comes up as a word, not as a formula with lengths and cross-sections.

Where people lose points

Saying voltage flows. Current flows; voltage is a difference.

Connecting an ammeter across a component. It goes in line.

Using C for current. It is I.

Saying a thicker wire has more resistance. Thicker is less.

Forgetting to convert milliamperes to amperes before a calculation.

Mixing up conventional current and electron flow.

Work one in under a minute

Two copper wires carry the same current. One is twice as long and half as thick as the other. Which has more resistance, and roughly how much more?

Doubling the length doubles the resistance.

Halving the thickness - that is, halving the cross-sectional area - doubles it again.

So the long thin wire has about four times the resistance.

Both factors push the same way, which is why this pairing is the standard version of the question.

Where this leads

Every other lesson in this subtest uses these three quantities, and the next one puts them into a single equation.