What voltage, current and resistance really mean, how they connect, and how to use one short equation to size resistors, check loads and debug circuits.
Every simple circuit comes down to three things. The easiest way to feel how they interact is to picture water flowing through a pipe.
Raise the pressure and more water flows. Narrow the pipe and less flows. Ohm's Law turns that intuition into an exact equation.
In 1827 Georg Ohm found that for many materials the current is proportional to the voltage across them. The result is:
To avoid memorising three versions, use the triangle. Cover the quantity you want to find. If the other two sit side by side, multiply them. If one sits above the other, divide.
A 12 V battery is connected across a 4 Ω heating element. How much current flows?
I = V / R = 12 / 4 = 3 A
A sensor draws 0.5 A through a 220 Ω resistor. What is the voltage across the resistor?
V = I × R = 0.5 × 220 = 110 V
A 5 V supply drives an LED that needs 20 mA and drops 2 V. The resistor must absorb the remaining 3 V. Convert 20 mA to 0.02 A first.
R = V / I = 3 / 0.02 = 150 Ω
P = V × I = 3 × 0.02 = 0.06 W (a ¼ W resistor is plenty)
Power is the rate at which energy is used, and it follows from the same law. Because P = V × I, you can swap in V = I × R or I = V / R to get whichever form suits the values you already have.
| You know | Power formula |
|---|---|
| Voltage and current | P = V × I |
| Current and resistance | P = I² × R |
| Voltage and resistance | P = V² / R |
Always check the result against the part's power rating. A resistor can have exactly the right resistance and still burn out if it has to dissipate more heat than it was built for.
In a series circuit the same current flows through every component, and the voltages add up to the supply voltage. In a parallel circuit every branch sees the same voltage, and the branch currents add up. Apply Ohm's Law to each component separately, or to the whole circuit using its total resistance. The Series / Parallel R calculator finds that total for you.
Try each one before you open the answer.
I = 9 / 470 ≈ 0.0191 A, or about 19.1 mA.
R = 6 / 0.3 = 20 Ω.
P = I² × R = 0.2² × 100 = 4 W. A ¼ W resistor would fail quickly here.
It says that the current through a component depends on the voltage pushing it and the resistance opposing it: V = I × R. More voltage means more current; more resistance means less current.
Voltage is measured in volts (V), current in amperes or amps (A), and resistance in ohms (Ω).
Not directly. LEDs, diodes and transistors are non-ohmic, so their resistance changes with current. You size a series resistor around them using Ohm's Law, but the part itself follows a datasheet curve.
Power is P = V × I. Substituting V = I × R gives P = I² × R, and substituting I = V / R gives P = V² / R.
Skip the arithmetic. Enter any two of V, I, R or P and get the rest instantly.
Open the Ohm's Law Calculator