The Three Ohm's Law Equations

Depending on which value is unknown, use one of these equations:

Example 1 – Calculate Current

A 12 V battery is connected to a 100 Ω resistor.

Find the current.

Solution

I = V ÷ R

I = 12 ÷ 100

I = 0.12 A = 120 mA

Example 2 – Calculate Voltage

A resistor carries 3 A of current and has a resistance of 8 Ω.

Find the voltage.

Solution

V = I × R

V = 3 × 8

V = 24 V

Example 3 – Calculate Resistance

A circuit operates from 9 V and draws 0.03 A.

Find the resistance.

Solution

R = V ÷ I

R = 9 ÷ 0.03

R = 300 Ω

Example 4 – LED Current Limiting Resistor

An LED has:

• Supply voltage = 12 V
• LED forward voltage = 2 V
• Required current = 20 mA

Voltage across the resistor:

12 − 2 = 10 V

Resistance:

R = 10 ÷ 0.02

R = 500 Ω

The nearest standard resistor value is 510 Ω.

Example 5 – Automotive Lamp

A 12 V automotive lamp draws 2 A.

Find its resistance.

R = V ÷ I

R = 12 ÷ 2

R = 6 Ω

Example 6 – USB Charger

A USB charger supplies 5 V to a device that has an equivalent resistance of 10 Ω.

Current:

I = 5 ÷ 10

I = 0.5 A

Example 7 – Power Resistor

A resistor of 50 Ω is connected across 100 V.

Find:

• Current
• Power

Current:

I = 100 ÷ 50 = 2 A

Power:

P = V × I

P = 100 × 2 = 200 W

Quick Practice Questions

1. Calculate the current through a 220 Ω resistor connected to 9 V.
2. A resistor carries 500 mA at 24 V. What is its resistance?
3. A 470 Ω resistor carries 15 mA. What is the voltage across it?
4. A 12 V supply feeds a 4 Ω resistor. Calculate the current.
5. A resistor has 330 Ω resistance and is connected to 3.3 V. Find the current.

Answers

Key Points

• Always identify which quantity is unknown before choosing a formula.
• Convert milliamperes to amperes before calculations.
• Check that the units are correct.
• Verify whether the answer is reasonable.
• Ohm's Law is one of the most frequently used tools in electronics.