Back to Courses

Ohm's Law & Calculations

What is Ohm's Law?

Ohm's Law is the most fundamental equation in electronics. It describes the relationship between voltage, current, and resistance in a circuit.

V = I × R

Voltage = Current × Resistance

This simple equation can be rearranged to solve for any variable if you know the other two:

V = I × R

Find Voltage

I = V ÷ R

Find Current

R = V ÷ I

Find Resistance

Practical Example: LED Circuit

Problem: You want to power a red LED with a 5V battery.

Given:

  • Battery voltage: 5V
  • LED forward voltage: 1.8V (red LED)
  • Desired LED current: 20mA (0.02A)

Step 1: Calculate voltage drop needed across resistor

V_R = 5V - 1.8V = 3.2V

Step 2: Use Ohm's Law to find resistor value

R = V_R / I = 3.2V / 0.02A = 160Ω

Result: Use a 160Ω resistor (or the nearest standard value like 150Ω or 180Ω)

Warning: Without a resistor, the LED would draw excessive current and burn out instantly! This is called a short circuit.
More Examples

Example 1: Find Current

A 220Ω resistor is connected to a 9V battery. What current flows?

I = V / R = 9V / 220Ω = 0.041A = 41mA

Example 2: Find Voltage

A 470Ω resistor has 15mA flowing through it. What's the voltage drop?

V = I × R = 0.015A × 470Ω = 7.05V

Example 3: Find Resistance

You have 12V and want to limit current to 50mA. What resistor do you need?

R = V / I = 12V / 0.05A = 240Ω

Power Calculations

You can also calculate power using Ohm's Law:

P = V × I

Power = Voltage × Current

P = I² × R

Power = Current² × Resistance

P = V² ÷ R

Power = Voltage² ÷ Resistance

What is Power?

Power (P) is measured in Watts (W). It represents the rate at which electrical energy is converted to other forms like light or heat.

Example: A circuit with 5V and 0.1A (100mA) dissipates 0.5W (500mW) of power.

Test Your Knowledge
Take this quiz to test your understanding. You need 80% or higher to pass and earn credit for this course.
5 questions80% required to pass
Next: Kirchhoff's Laws →