Ohm's Law is the foundation of electronics. Once you understand it, circuits stop looking like magic and start making sense.
What Is Ohm's Law?
Ohm's Law states that the current flowing through a conductor is directly proportional to the voltage across it, as long as the temperature stays constant.
V = I × R
- V = Voltage (volts, V): the "push" that moves electric charge
- I = Current (amperes, A): the flow of charge
- R = Resistance (ohms, Ω): how much the material resists that flow
Rearranged, you can find any one value from the other two:
- I = V ÷ R
- R = V ÷ I
Tip: Picture water in a pipe. Voltage is the water pressure, current is the flow rate, and resistance is how narrow the pipe is.
Example 1: Finding Current
A 12 V battery is connected to a 4 Ω resistor. What is the current?
I = V ÷ R = 12 ÷ 4 = 3 A
Example 2: Finding Voltage
A current of 0.5 A flows through a 10 Ω resistor. What is the voltage across it?
V = I × R = 0.5 × 10 = 5 V
Example 3: Finding Resistance
A device runs on 9 V and draws 0.03 A. What is its resistance?
R = V ÷ I = 9 ÷ 0.03 = 300 Ω
Example 4: A Real-Life LED Circuit
You want to power an LED (2 V, 20 mA) from a 5 V supply. The resistor must drop the extra 3 V.
- Voltage across resistor = 5 − 2 = 3 V
- Current = 20 mA = 0.02 A
- R = 3 ÷ 0.02 = 150 Ω
Without this resistor, the LED would draw too much current and burn out.
Try It Yourself: Ohm's Law Calculator
Choose what you want to find, enter the other two values, and the answer appears instantly. Try the examples above to check your understanding.
Quick Reference
| Find | Formula |
|---|---|
| Voltage | V = I × R |
| Current | I = V ÷ R |
| Resistance | R = V ÷ I |
Common Mistakes to Avoid
- Forgetting to convert units (mA to A, kΩ to Ω)
- Mixing up which value goes on top in the formula
- Assuming Ohm's Law applies to every component (it doesn't work for diodes or transistors in the same simple way)
Final Thoughts
Ohm's Law is simple, but it's the key to understanding everything from phone chargers to household wiring. Practice with a few numbers of your own and it'll soon feel natural.
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