DC Circuits · #2 of 20
Ohm's Law + Power
V = I × R, P = V × I
Why it matters
Ohm’s Law is the foundation of all electronics. Without it, you’re guessing. Power tells you if components will overheat and fail.
The idea
Ohm's Law
The relationship between voltage (V), current (I), and resistance (R): V = I × R <div class=Demo
Adjust Voltage and Resistance to see how current and power change.
Watch for:
- High current → components heat up (red indicator)
- Safe operating zone → green indicator
- Power limit → what happens if you exceed component ratings
Key takeaways
- V = I × R — know any two, calculate the third
- P = V × I — power becomes heat
- Components have maximum power ratings — exceed them at your peril
- Always calculate current before connecting components
Going deeper
Ohm’s Law applies to linear (resistive) circuits. Diodes, transistors, and other non-linear components have more complex relationships. For DC circuits with resistors, Ohm’s Law is king.
Math details
Ohm's Law:
V = I × R
I = V / R
R = V / I
Power:
P = V × I
P = I² × R (substitute V = I×R)
P = V² / R (substitute I = V/R)
Example calculations:
Given: V = 5V, R = 1000Ω
I = 5V / 1000Ω = 0.005A = 5mA
P = 5V × 0.005A = 0.025W = 25mW
Component ratings:
Resistor: usually 0.25W (1/4W) or 0.5W (1/2W)
LED: typically 20mA max current, ~0.1W power
ESP32 GPIO: 40mA max source, 28mA max sink