Ohm’s law & power
Ohm’s law ties the three together: V = I × R. Know any two and you can find the third. Put 5 V across 1 kΩ and exactly 5 mA flows. Need 10 mA through an LED from a 5 V supply? The same equation tells you which resistor to use. Most everyday circuit math is some rearrangement of this one formula.
Power is the rate energy gets converted, in watts: P = V × I. Combined with Ohm’s law you also get P = I² × R and P = V² / R. Power becomes heat in most components, and heat is usually what you’re worried about. A resistor dropping 5 V at 100 mA dissipates half a watt, enough to cook a small part rated for a quarter watt.
A good habit: whenever current flows through a resistance, ask where the power goes. Run P = V × I on anything that drops real voltage at real current (regulators, power resistors, transistors) and you’ll catch parts that would run hot before the board is even built.
Key points
- V = I × R. Any two values give you the third.
- P = V × I gives power in watts; P = I² × R and P = V² / R are the same law rearranged.
- Every component has a power rating. Exceed it and the part overheats and fails.
- Doubling the current through a fixed resistance quadruples the heat, since power scales with I².
Practice
0 of 4 answered · Not startedWrong answers just let you try again, and hints are there if you want them. Answering every question first time, without hints, is what earns mastery.
- 1
A resistor drops 5 V while carrying 100 mA. How much power is it dissipating?
- 2
You put 5 V across a 1 kΩ resistor. What current flows?
- 3
What resistor value gives exactly 20 mA from a 5 V supply?
Ω - 4
A 3.3 V rail drives a 100 Ω load. How much power does the load dissipate, in watts?
W
