OHMLENS // EDA

Lesson 1.1 · Module 1: DC Physics & Network Theorems

Ohm's Law, Power & Why Resistors Get Hot

One resistor connected to a battery. It shows the most important rule in electronics, Ohm’s law, and why real parts get hot and can burn out.

Think of it like this: A resistor is like a narrow pipe. More pressure (voltage) pushes more water (current) through it, and the friction makes heat.

The live, interactive simulation for this lesson opens here when JavaScript is on.

Key ideas

  • Ohm’s law: current = voltage ÷ resistance. Double the voltage, double the current.
  • The resistor turns electrical power into heat. Power = voltage × current, so it grows very fast.
  • Tiny surface-mount resistors can only handle a little heat. Every part has a rating.

Try this in the simulation

  • Try the preset “5 V across 100 Ω in 0402 (smoke!)” and watch the part overheat and burn open.
  • Then press Replace and pick a bigger package, or a larger resistor, until it stays cool.
  • Hover any wire or part in the circuit to see its live voltage and current. Click to pin the readout.

How does Ohm’s law work, and why do resistors get hot?

Ohm’s law says that the current through a resistor equals the voltage across it divided by its resistance: I = U ÷ R. The resistor also turns electrical power into heat. Power is voltage times current, which is the same as I² × R, so doubling the current makes four times the heat. That is why a tiny surface-mount resistor can overheat and burn out when you push it past its power rating.

What you’ll learn

  • How to use Ohm’s law to find the current, voltage or resistance when you know the other two.
  • How to calculate the power a resistor wastes as heat.
  • What a power rating is, and why a bigger package can carry more.
  • What happens to a part that is pushed past its rating.

The formula, letter by letter

I=UR=5.00 V470 Ω=10.6 mAP=I2R=53.2 mW  /  100 mW (0603)\begin{aligned}\htmlClass{mv mv-i_r}{\htmlStyle{font-size:1.24em}{\htmlData{sym=I}{I}}} &= \frac{\htmlClass{mv mv-v_r}{\htmlStyle{font-size:0.94em}{\htmlData{sym=U}{U}}}}{\htmlClass{mv mv-r}{\htmlStyle{font-size:1.02em}{\htmlData{sym=R}{R}}}} = \frac{\htmlData{funit=V,v=5}{\text{5.00 V}}}{\htmlData{funit=Ω,v=470}{\text{470 Ω}}} = \htmlData{funit=mA,v=0.010638297872340425}{\text{10.6 mA}}\\ \htmlData{sym=P}{P} &= \htmlData{sym=I}{I}^2 \htmlData{sym=R}{R} = \htmlData{funit=mW,v=0.05319148936170213}{\text{53.2 mW}} \;/\; \htmlData{funit=mW,v=0.1}{\text{100 mW}}\ (0603)\end{aligned}

Shown with this lesson’s starting values. In the simulation the numbers change as you move the sliders.

II
Current (A · ampere) : How much charge flows each second.
UU
Voltage (V · volt) : The electrical push between two points.
RR
Resistance (Ω · ohm) : How strongly something opposes current.
PP
Power (W · watt) : How fast energy is used or turned into heat.

Common questions

What does the power rating of a resistor mean?

It is the most heat, in watts, that the resistor can get rid of for long without damage. A small 0402 surface-mount resistor is rated for about 0.06 W, while a larger 1206 one handles about 0.25 W.

How do I choose the wattage of a resistor?

Work out the power with P = I² × R (or U² ÷ R), then pick a part rated for at least twice that. The spare margin keeps it cool and makes it last.

Does Ohm’s law work for every part?

No. It describes resistors, where current is proportional to voltage. Diodes and LEDs do not follow it, because their current rises far faster than their voltage.

Keep going

Words used in this lesson

Parts in this lesson

All lessons · How components work · Glossary