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Ohm's law
The relationship that links them, and how to use it.
The relationship that links them, and how to use it.
For a given resistance, the current is proportional to the voltage across it. Push twice as hard and twice as much flows. Make the path twice as resistive and half as much flows.
I = E ÷ RThe same relationship rearranged gives E = I × R and R = E ÷ I. Put E on top of a triangle with I and R below; cover the one you want and what is left is the formula.
In a resistor the current rises in step with the voltage, so the graph is a straight line through zero. A bigger resistance gives a shallower line: the same voltage drives less current. The resistance is the ratio E ÷ I, and for a resistor it stays the same whatever the voltage.
That is not true of every part. A lamp filament's resistance climbs as it heats, and a diode or LED passes almost nothing until the voltage reaches about its forward voltage, then conducts hard. Ohm's law still describes any single moment, but only for resistors is the ratio constant.
Always convert to base units first: volts, amperes, ohms.
| To find | Formula | Example |
|---|---|---|
| Current | I = E ÷ R | 13.8 V across 27 Ω: 13.8 ÷ 27 = 0.511 A (511 mA) |
| Voltage | E = I × R | 0.25 A through 47 Ω: 0.25 × 47 = 11.75 V |
| Resistance | R = E ÷ I | 5 V and 20 mA (0.02 A): 5 ÷ 0.02 = 250 Ω |
A real job: an LED resistor. An LED wants about 20 mA and drops roughly 2 V (typical for a red one). On a 13.8 V supply the resistor must drop the other 11.8 V, so R = 11.8 ÷ 0.02 = 590 Ω. Choose the next standard value up, 620 Ω, and the current is 11.8 ÷ 620 = 19 mA: slightly dim, never overloaded. Rounding up is the safe direction.
Combine this with power and you can answer nearly any DC circuit question: how much current, how much heat, how much battery.