Water in a pipe
Picture water in a hose. A pump pushes it. A wide hose lets a lot through. A narrow, kinked hose lets only a little through.
Electricity in a wire behaves in much the same way. The push, the flow and the narrowness all have names, and one simple rule joins them.
Key words
| Word | What it means |
|---|---|
| Voltage | the electrical push, measured in volts (V) |
| Current | the flow of electricity, measured in amps (A) |
| Resistance | how much a wire holds back the flow, measured in ohms |
| Ohm's law | the rule that links voltage, current and resistance |
| Proportional | changing in step: double one and the other doubles |
| Conductor | the copper wire inside a cable |
| Frequency | how fast a signal changes each second |
| Gauge | the thickness of a wire |
The cells students fill in are left blank in this preview.
By the end of this lesson
Note
I can use Ohm's law with whole numbers and explain why resistance matters in copper cabling.
Starter: copper
Have a go
Starter: copper
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
Ohm's law in words
Voltage is the push. Current is the flow. Resistance is how much the wire holds back.
Ohm's law: the voltage equals the current times the resistance. So if the resistance stays the same, more voltage gives more current. If the voltage stays the same, more resistance gives less current.
Worked example: paired values
| Voltage (volts) | Resistance (ohms) | Current (amps) | What changed |
|---|---|---|---|
| 6 | 3 | 2 | Start |
| 12 | 3 | 4 | Voltage doubled, so current doubled |
| 12 | 6 | 2 | Resistance doubled, so current halved |
| 12 | 12 | 1 | Resistance doubled again, so current halved again |
The cells students fill in are left blank in this preview.
Ohm's law in a cable
A signal goes from point A down a copper cable to point B. It is a tiny change in voltage that the receiver reads as a bit. The cable has resistance, so it takes some of the signal on the way.
If the resistance is too high, too little arrives to read. So installers keep resistance low.
What changes resistance
| Factor | Effect | What the installer does |
|---|---|---|
| Length of cable | Longer means more resistance | Keep to the 90 metre and 100 metre limits |
| Thickness (gauge) of the wire | Thinner wire means more resistance | Use the cable the specification names |
| Frequency of the signal | Resistance to the signal changes at different frequencies, and fast signals feel more | Use a category rated for the speed |
| Resistors inside hardware | Parts inside devices add resistance on purpose | Use devices rated for the cable |
The cells students fill in are left blank in this preview.