Electricity is scary because it is invisible. You can’t see the power flowing through your wall charger, but if you touch a live wire, you will certainly feel it.
Because we can’t see electricity, learning about it feels abstract and difficult. But it doesn’t have to be. There is one simple mathematical relationship that connects every electrical device in the universe, from your toaster to the smartphone in your hand.
It is called Ohm’s Law.
Discovered by Georg Ohm in 1827, this formula is the “Golden Rule” of electronics. Whether you are trying to fix a broken lamp, install a car stereo, or design a circuit board, you cannot proceed without it.
The Magic Triangle (V-I-R)
Ohm’s Law describes the relationship between three things: Voltage, Current, and Resistance.
The standard equation is V = I × R.
To make it easier to remember, students around the world use the “Ohm’s Law Triangle.” You cover the letter you are trying to find, and the triangle tells you the math to do.
- V (Voltage): Measured in Volts (V).
- I (Current): Measured in Amps (A).
- R (Resistance): Measured in Ohms (Ω).
Solve Your Circuit
Need to find the missing resistor size? Don’t do the algebra manually. Use our free tool to solve for any variable.
⚡ Use Ohm’s Law CalculatorThe Best Analogy: “Water in a Pipe”
The easiest way to understand electricity is to stop thinking about electrons and start thinking about plumbing.
Imagine a large water tank connected to a garden hose.
1. Voltage is Water Pressure
Voltage is the “push.” If the water tank is very high off the ground, gravity creates high pressure. In electricity, a 9V battery has more “pushing force” than a 1.5V AA battery.
2. Current is the Flow Rate
Current (Amps) is the actual amount of water coming out of the hose. If you have high pressure (Voltage), you generally get more water flow (Current).
3. Resistance is the Hose Size
Resistance is anything that slows the flow down. Think of a thin hose vs. a wide firehose. Or, think of a kink in the hose. If you squeeze the hose (Resistance), less water comes out (Current drops), even though the pressure at the tap is the same.
Practical Example: The Exploding LED
Why do you need to calculate this? Let’s look at a common scenario.
You have a 9-Volt battery and a small red LED light.
- Battery: 9 Volts
- LED Limit: Can only handle 2 Volts and 0.02 Amps.
If you connect the 9V battery directly to the LED, the massive “Water Pressure” (Voltage) will blast through the tiny LED and it will pop instantly.
You need a Resistor. But what size?
Using Ohm’s Law, you can calculate exactly how much resistance you need to absorb the extra 7 Volts safely so your light glows perfectly without burning out. (Hint: Use our calculator to find out!).
Don’t Forget Power (Watts)
There is often a fourth letter involved in electronics: P (Power).
Power is the rate at which energy is used, and it creates heat. It is measured in Watts.
- Formula: P = V × I
This explains why high-power appliances (like a hair dryer) use so many Amps. They need massive flow (Current) combined with high pressure (Voltage) to create heat. This relationship is why your phone gets hot when it charges quickly—moving electrons creates friction, which is released as Watts (Heat).
Conclusion: The Safety Rule
Ohm’s Law isn’t just for passing physics class; it’s for safety. Knowing how many Amps your wires can handle prevents electrical fires.
Before you build your next DIY project, plug your numbers into the tool below to ensure your math is safe.