Ohm's Law
V = I × R and how to use it to predict and verify circuit behaviour.
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What you will learn
- Apply V = I × R to simple automotive circuits
- Calculate expected current from voltage and resistance
- Use the law to spot a fault that measurement reveals
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Ohm's law links the three quantities: V = I × R. If a 12 V supply feeds a 6 Ω load, the current is 2 A. If two loads share the circuit, the voltage divides in proportion to their resistance — which is exactly why a small unwanted resistance in a ground steals voltage from the load.
I = V ÷ R
V = 12 V, R = 3 Ω → I = 4 A
Use it to predict the current a component should draw, then compare with the measured value.
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Key points
- Predict the value, then measure to confirm.
- Double the resistance, halve the current.
- Unwanted resistance is the hidden cause of many faults.
Key points
- V = I × R, I = V / R, R = V / I
- A 12 V supply across 6 Ω draws 2 A
- Voltage drop across a load is proportional to its resistance
All values shown are training examples. Real thresholds depend on the manufacturer, engine, temperature and system — always confirm with the vehicle service documentation.
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