T2 · Published 23 September 2026 · By The Ham Desk

Ohm's Law in the Shack, Prefixes and Component Behaviour

The Technician electrical questions are the ones you will actually use: working out a current so you choose the right fuse, understanding why a power supply is rated in amps, and knowing what a capacitor or a diode does in a circuit rather than on a diagram.

Band none
Study time 16
Level Core
questions 10

Waveform and antenna diagram for Electrical Principles and Components — Electrical Principles and Components — waveform overlay and radiation pattern

Pre-flight briefing

Ohm's law and the power law together

Ohm's law says that voltage equals current times resistance, so 12 volts across a 100 ohm resistor drives 0.12 amperes, which is 120 milliamperes. The power law says that power equals voltage times current, so the same resistor dissipates 12 times 0.12, about 1.44 watts, and you would choose at least a 2 watt part to be safe.

The two laws combine usefully. Substituting one into the other gives power equals current squared times resistance, which is how a feed line's loss is computed, and power equals voltage squared divided by resistance. The squared terms explain why doubling the voltage across a fixed resistance quadruples the heat, and why a small increase in current through a long cable has an outsized effect on what the cable wastes.

Prefixes, batteries and what a rating means

Metric prefixes are part of everyday amateur speech. One kilohertz is a thousand hertz, one megahertz a million, one gigahertz a thousand million; below one hertz, milli means a thousandth, micro a millionth, nano a thousand-millionth and pico a million-millionth. A 0.5 ampere current is 500 milliamperes, and a 2 metre signal at 146 MHz is 146,000,000 hertz.

A battery rated 12 volts at 7 ampere-hours can deliver roughly 7 amperes for one hour, or 1 ampere for seven hours, before it is exhausted. Ampere-hours are a capacity, not a current limit, and drawing more current than the internal resistance allows generates heat inside the battery rather than usable power.

How the common parts behave

A resistor converts electrical energy to heat, which is why a fuse is simply a resistor designed to melt: it opens the circuit when current exceeds the rating and protects everything downstream. A capacitor stores energy in an electric field and therefore blocks direct current while passing alternating current, since the constant voltage of DC charges it once and then stops. An inductor stores energy in a magnetic field and does the opposite, passing DC and opposing changes in current.

A diode conducts in one direction and blocks the other, so it turns AC into pulsating DC and protects a circuit from a reversed supply. A transistor is a three-terminal device in which a small current or voltage at the control terminal regulates a much larger current through the other two: that is both how a signal is amplified and how a modern transceiver switches its output power.

Terms you must be able to define
Term	What the examiner wants to hear
Ohm's law	Voltage equals current times resistance.
Ampere-hour	A capacity rating: the current a battery can supply for a stated time.
Capacitor	Stores energy in an electric field; blocks DC and passes AC.
Diode	A two-terminal device that conducts in one direction only.
Fuse	A deliberately weak resistor that opens the circuit when current is excessive.

Question deck 10 questions

T2B01 Core

What current flows through a 100 ohm resistor with 12 volts across it?

T2B02 Core

What is the total resistance of two 100 ohm resistors connected in parallel?

T2B03 Core

A power supply delivers 12 volts at 2 amperes. How much power is it supplying?

T2B04 Core

What happens to the power dissipated by a fixed resistor if the voltage across it is doubled?

T2B05 Recall

What does the prefix 'pico' represent?

T2B06 Core

What is the primary purpose of a fuse in a DC power lead?

T2B07 Core

How does a capacitor behave in a circuit carrying direct current compared with alternating current?

T2B08 Core

Why does a wire carrying a large current become warm?

T2B09 Stretch

How does a transistor differ from a diode?

T2B10 Stretch

A battery is marked 12 volts, 7 ampere-hours. What does this mean in practice?

Reference sheet

The explanations for all ten questions sit on this sheet. Pick an answer on the left and the matching card lights up.

T2B01 Explanation 1

Key D — 120 milliamperes

Current equals voltage divided by resistance: 12 / 100 = 0.12 A, or 120 mA. The 8.3 mA answer comes from dividing 100 by 12 the wrong way round.

Rule Fundamentals - Ohm's law

T2B02 Explanation 2

Key C — 50 ohms

Two equal resistors in parallel give half the value of one, so 50 ohms. In series they would add to 200 ohms. Parallel paths always reduce the total below the smallest individual resistance.

Rule Fundamentals - series and parallel circuits

T2B03 Explanation 3

Key B — 24 watts

Power is voltage times current: 12 x 2 = 24 W. A 100 watt transceiver drawing 20 amperes on transmit is the same calculation in reverse - 13.8 V x 20 A = 276 W of input, most of which leaves as heat.

Rule Fundamentals - power

T2B04 Explanation 4

Key A — It quadruples

Power equals voltage squared divided by resistance, so doubling the voltage multiplies the power by four. This is why a resistor run at twice its rated voltage fails well before its wattage rating suggests it should.

Rule Fundamentals - power law

T2B05 Explanation 5

Key D — One million-millionth (10 to the minus 12)

Pico is 10^-12, which is why a capacitor of 100 picofarads is a very small capacitor. Milli is 10^-3, micro 10^-6 and nano 10^-9, and confusing nano with pico is a classic mistake when ordering parts.

Rule Fundamentals - metric prefixes

T2B06 Explanation 6

Key C — To open the circuit if current becomes excessive and prevent damage or fire

A fuse protects the wiring and everything downstream by melting open before the current can do damage. It must be sized to the actual current drawn plus reasonable margin, and it should be fitted in the hot lead as close to the battery as practical.

Rule 47 CFR §97.13(a) - safe station installation

T2B07 Explanation 7

Key B — It blocks direct current but passes alternating current

A capacitor charges to the applied DC voltage and then stops conducting, but an alternating voltage keeps charging and discharging it, so current flows continuously. The higher the frequency, the lower the reactance and the more the capacitor looks like a short circuit.

Rule Fundamentals - capacitance

T2B08 Explanation 8

Key A — The resistance of the wire dissipates power as heat in proportion to the square of the current

Every conductor has resistance, and dissipated power is current squared times resistance. Because the current term is squared, doubling the current through a power cable quadruples the heat and doubles the voltage drop - which is why long power runs need heavier wire.

Rule 47 CFR §97.13(a) - wiring practice

T2B09 Explanation 9

Key D — A transistor has three terminals and can control a large current with a small one; a diode has two and conducts one way

A diode is a two-terminal one-way valve. A transistor adds a third control terminal, which is what makes amplification and switching possible - the small base or gate current modulates the much larger collector or drain current.

Rule Fundamentals - active devices

T2B10 Explanation 10

Key D — It can supply about 7 amperes for one hour, or 1 ampere for seven hours

Ampere-hours describe capacity, not a current ceiling. The 7 Ah figure is a product of current and time, so a 0.5 ampere receive-only load would run for roughly fourteen hours in theory, less in practice because capacity falls as discharge current rises.

Rule Fundamentals - battery capacity

How to use this unit

Work the ten questions in one sitting with the reference sheet covered, then uncover it and read every explanation - including the ones you got right. The wording of the real exam is different, so learn the reasoning rather than the answer letters.