Where this is going
Everything in this course — antennas, propagation, the FCC’s power limits — eventually reduces to charge moving somewhere it wasn’t. Four quantities describe that movement, and the exams test whether you can name them, give their units, and keep them apart.
Current: charge on the move
Current is the rate at which electric charge flows past a point. Its unit is the ampere (amp, A). One amp is one coulomb of charge per second.
Two details the pool cares about:
- Current is the flow of electrons through a circuit. The pool asks this almost verbatim.
- Current is measured through a component. An ammeter goes in series, in the path the charge takes. Put one across a battery and you have built a short circuit — this is how most people learn to buy a new multimeter fuse.
Metals conduct well because their outer electrons are loosely bound and move freely between atoms. Glass, plastic, ceramic, and dry wood hold their electrons tightly, which is what makes them insulators.
Voltage: the pressure that makes it move
Charge does not move on its own. It moves when there is a difference in electric potential between two points — that difference is voltage, measured in volts (V). The exams and older texts often write it as E, for electromotive force; datasheets write V. Same quantity.
The word difference is doing real work. There is no such thing as “the voltage at a point,” only the voltage between two points. When someone says “12 volts at the antenna connector,” they mean 12 volts relative to chassis ground, and they are leaving the second point unstated because everyone in the room already knows it. A voltmeter has two leads for exactly this reason.
Resistance: what gets in the way
Resistance opposes current and converts electrical energy into heat. Its unit is the ohm. A conductor has low resistance, an insulator very high resistance, and everything useful sits somewhere in between.
Resistance opposes both alternating and direct current — worth noting, because the next module introduces reactance, which opposes only alternating current. That distinction is a favourite exam target.
Power: the rate of using energy
Power is the rate at which electrical energy is used, measured in watts (W). Note the wording carefully, because the pool tests it directly: power is a rate, energy per unit time. A joule is energy; a watt is a joule per second.
Frequency, while we are naming units
One more unit belongs in this list because it appears in the same question group. Frequency is the number of complete cycles per second that an alternating current makes, measured in hertz (Hz). You will meet it properly in the next module.
The five-line summary
| Quantity | Symbol | Unit | Measures |
|---|---|---|---|
| Current | I | ampere (A) | flow of electrons |
| Voltage | E or V | volt (V) | potential difference driving that flow |
| Resistance | R | ohm | opposition to flow, dissipating heat |
| Power | P | watt (W) | rate of energy use |
| Frequency | f | hertz (Hz) | cycles per second |
Check yourself
- A meter reads 3 A. What is being measured, and how was the meter connected?
- Why is “the voltage at this terminal” an incomplete statement?
- Which of resistance and reactance opposes direct current?
Answers
- Current — the rate of electron flow. The meter must be in series, in the path of the current.
- Voltage is a difference between two points; one point alone does not define it. The second point is usually an unstated ground reference.
- Resistance opposes both AC and DC. Reactance opposes only AC.
Where next
The next lesson deals with the arithmetic that trips people up more often than the physics: metric prefixes, and converting between them under exam pressure.