What are current, voltage and resistance, and what do we measure them in?
Every radio you will ever use is a circuit: a loop that electrons can travel round. Three ideas describe what is going on in that loop. Get these straight and the rest of the electricity unit is just arithmetic.
Current: the flow
Current is the flow of electrons in an electric circuit. Picture standing in the middle of a stream. You can feel the water pushing past you, and that push is the current.
Current is measured in amperes, usually shortened to "amps" and written A. See Ampere.
Small currents are often given in milliamperes (mA). There are 1000 mA in 1 A, so:
Voltage: the push
Voltage is the force that causes electrons to flow through a circuit. Another name for it is electromotive force, or EMF.
Think of the pressure in a water pipe. Pressure is a readiness to push water along before you open the tap. Voltage is the same kind of thing: a readiness to push electrons before the circuit is complete.
A circuit has to connect to two terminals of a voltage source, the positive one and the negative one. A battery is the everyday example. Voltage is measured in volts, written V. See Volt.
Conductors and insulators
Materials differ in how easily electrons pass through them.
| What it does | Examples | |
|---|---|---|
| Conductor | Lets electrons pass easily | copper, gold, silver, aluminium |
| Insulator | Does not let electrons pass easily | glass, air, plastic, porcelain |
That is why wire is metal inside and plastic outside: the metal carries the current and the plastic keeps it where it belongs.
Resistance: the brake
Resistance is the current-limiting property of a material. It opposes electron movement through a circuit. It is measured in ohms, written with the Greek letter omega, . See Ohm.
A resistor is the electrical component built to oppose electron movement. In the picture at the top, it is the zigzag.
The fuse
A fuse is a protective device. It is useful for protecting radio equipment because it limits how much current can pass through the equipment. If something goes wrong and too much current tries to flow, the fuse stops it before the equipment is damaged.
You now know
- Current is the flow of electrons, measured in amperes (amps). .
- Voltage is the force that pushes electrons round a circuit, measured in volts. Think pipe pressure.
- Conductors (copper, gold, silver, aluminium) pass electrons easily; insulators (glass, air, plastic, porcelain) do not.
- Resistance opposes the flow and is measured in ohms. A resistor is the component that does it.
- A fuse protects equipment by limiting how much current can pass.