Electricity: current, circuits, Ohm's law, power and batteries
Conductors and insulators, what a circuit needs, Ohm's law, power in watts, polarity and batteries, with simple calculations.
Key points
Summary
| Quantity | Unit | Formula |
|---|---|---|
| Current (I) | amp (A) | I = V ÷ R |
| Voltage (V) | volt (V) | V = I × R |
| Resistance (R) | ohm (Ω) | R = V ÷ I |
| Power (P) | watt (W) | P = V × I |
| Series | — | Same current; voltages add |
| Parallel | — | Same voltage; currents add |
Quick check
Which of these is a good conductor?
- Brass
- Glass
- Rubber
- Ceramic
What current flows when 12 V is applied across 4 Ω?
- 48 A
- 3 A
- 0.33 A
- 16 A
A transceiver draws 2 A from a 12 V supply. What power does it use?
- 6 W
- 14 W
- 24 W
- 0.17 W
Work it out
Try each one first, then open it to see the working.
What current flows when 12 V is applied across 6 Ω?
- I = V ÷ R
- I = 12 ÷ 6
A lamp draws 0.5 A from a 12 V battery. What is its resistance?
- R = V ÷ I
- R = 12 ÷ 0.5
A radio draws 2 A from 13.8 V. What power does it use?
- P = V × I
- P = 13.8 × 2
A 25 W transmitter runs from 12.5 V. What current does it draw (ignoring losses)?
- P = V × I, so I = P ÷ V
- I = 25 ÷ 12.5
On air
- Read the voltage and current rating on your power supply and work out its maximum power.
- Check your radio's power lead: which colour is positive?
- Find out where your local tip or shop recycles batteries.
Video transcript
Every radio runs on electricity, so this is where the technical side starts. In this lesson: what current is, what a circuit needs, Ohm's law, power in watts, and batteries.
An electric current is a flow of electrons. Materials that let electrons flow easily are conductors. Metals such as copper and brass are good conductors, and so is carbon. Materials that don't are insulators, such as plastic, rubber, glass and ceramics. Take care: water on an insulator can let current flow across it. Current is measured in amps, and the push that drives it, the potential difference, is measured in volts.
Resistance opposes the flow of current, and it's measured in ohms. The current through a resistor is proportional to the voltage across it. That gives Ohm's law: voltage equals current times resistance. Rearrange it to find what you need. Current is voltage divided by resistance, so twelve volts across six ohms drives two amps. Resistance is voltage divided by current, so twelve volts pushing three amps means four ohms.
Current needs a complete loop to flow, called a circuit, and a source of energy, such as a battery. In a series circuit, the same current flows through every item, and the voltages across the items add up to the supply voltage. In a parallel circuit, every item has the same voltage across it, and the total current is the sum of the currents in each item. Whatever the circuit, connect the supply the right way round. Reversed polarity can stop equipment working, or damage it.
Power is measured in watts. It's the voltage times the current, so a radio drawing two amps from thirteen point eight volts uses twenty seven point six watts. When current flows through a resistor, electrical energy turns into heat. A battery is a group of cells, usually connected in series. It stores chemical energy and gives a voltage across its terminals. In a rechargeable battery, the chemical process can be reversed by charging. Always dispose of old batteries properly, never in household waste.
This is one of 16 lessons in the Foundation course
The full course adds drills, flashcards, a revision sheet for every lesson and 5 full mock exams.