Ohm’s law on real problems
The equation takes a minute. Knowing which of the three numbers you are missing takes practice, so here are the three questions it actually gets asked on a bench, with the unknown moved to where it really sits.
Pick what you are missing, type the two you know, and load a real problem from the buttons underneath.
The three shapes a real question comes in
"What resistor do I need?" — you know the voltage the resistor has to absorb and the current you want. R = V ÷ I. The trap is the first number: for an LED it is not the supply voltage, it is what is left after the LED takes its share.
"Is this safe?" — you know the voltage and the resistance and you want the current before you connect it. I = V ÷ R. Do this one before powering up, not after.
"What is this thing drawing?" — you measured a voltage across a known resistor. I = V ÷ R again, and this is how a multimeter with no current range still tells you the current.
Watts, in one paragraph
P = V × I. A quarter-watt resistor is the common size and it is nearly always enough — 20 mA through a 100 Ω resistor is 0.04 W. It stops being enough the moment you put a resistor directly across a supply: 12 V through 100 Ω is 1.44 W, which is a resistor you can smell.
A worked one, end to end
A 5 V rail, a red LED that drops about 2 V, and you would like 15 mA.
- The resistor has to absorb 5 − 2 = 3 V.
- R = 3 V ÷ 0.015 A = 200 Ω.
- Your kit will not have 200 Ω, so you fit 220 Ω and get 13.6 mA. Fine.
- Heat: 3 V × 0.0136 A = 0.04 W. Any resistor in your kit will do.
Four lines, and it is the same four lines for a buzzer, a relay coil or a pull-up. The voltage divider is the only common circuit where the arithmetic is meaningfully different.
Edit this page — content/fundamentals/electricity/ohms-law-on-real-problems.mdx
Discuss this article
Ask about this page. The answer stays here, on the page it belongs to, for whoever hits the same wall next.