Electricity is hard because you can't see it. So picture water flowing round a pipe, and voltage, current and resistance fall into place.
Turn it into water
| Water | Electricity | Unit |
|---|---|---|
| The pump's push | Voltage (V) | volts, V |
| How much water flows past each second | Current (I) | amps, A |
| A narrow bit of pipe holding the flow back | Resistance (R) | ohms, Ω |
The rule
Push harder, and more water flows. Make the pipe narrower, and less flows. That's the whole rule: voltage equals current times resistance.
V = I × R
Worked example
A 12 volt battery and a 4 ohm resistor. Current = 12 ÷ 4 = 3 amps.
The VIR triangle
Write V at the top of a triangle, with I and R along the bottom. Cover the one you want to find:
- Cover I: you're left with V ÷ R.
- Cover R: you're left with V ÷ I.
- Cover V: you're left with I × R.
The water is a model, not the real thing, but it's the standard way to picture a circuit at GCSE.
10 volts across a 2 ohm resistor. How many amps?
Show the answer
I = V ÷ R = 10 ÷ 2 = 5 amps.
Save this for your next test
- Voltage is the push, in volts.
- Current is the flow, in amps.
- Resistance holds the flow back, in ohms.
- V = I × R. Cover what you want in the triangle.
Read the video transcript
- Electricity is hard because you can't see it. So let's turn it into water.
- Picture water flowing round a pipe.
- The pump pushes the water. That push is voltage, measured in volts.
- How much water flows past each second is current, measured in amps.
- And a narrow bit of pipe holds the flow back. That's resistance, measured in ohms.
- Push harder, and more water flows. Make the pipe narrower, and less flows.
- That's the whole rule: voltage equals current times resistance. V equals I R.
- Try it. A 12 volt battery and a 4 ohm resistor. Current is 12 divided by 4: 3 amps.
- Here's the trick: the V I R triangle. Cover what you want to find. Cover I, and you're left with V over R.
- Save this for your next test.
- Now you: 10 volts across a 2 ohm resistor. How many amps? Tell me in the comments.
- Want science to click like this? Book a free trial lesson at bloomeducationonline.com.
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