Ohm's Law
How voltage, current and resistance relate, in series and in parallel.
Lesson progress
- Battery V
- 9 V
- Total current I
- 19.1 mA
- Equivalent resistance Req
- 470 Ω
- Total power P
- 172 mW
| Resistor | Voltage | Current | Power | Load vs ¼ W rating |
|---|---|---|---|---|
| R1 470 Ω | 9 V | 19.1 mA | 172 mW | Warm 69% |
- Caution: R1 is above half its power rating and will run hot. Designers usually keep resistors below 50% of their rating.
Ohm's law triangle
Tap a letter to solve for it
I = V ÷ R
9 V ÷ 470 Ω
= 19.1 mA
Cover the letter you want. If the other two sit side by side, multiply them. If one is above the other, divide.
- Voltage
- V · volt (V)
- Current
- I · ampere (A)
- Resistance
- R · ohm (Ω)
- Power
- P · watt (W)
1 mA = 0.001 A · 1 kΩ = 1000 Ω · 1 mW = 0.001 W
I–V characteristic
A steeper line means less resistanceVoltage pushes, resistance resists, current flows
Voltage (V) is the electrical “pressure” a battery provides. Current (I) is how much charge flows past a point each second. Resistance (R) is how strongly a part opposes that flow. In 1827 Georg Ohm found they are linked by one simple rule:
V = I × R
Double the voltage and the current doubles. Double the resistance and the current halves. On the I–V chart, every resistor is a straight line through zero, which is why resistors are called linear or “ohmic” components.
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