Lesson 4 of 8beginner

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Ohm's Law

The most fundamental relationship in electronics

Theory

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Ohm's Law — The Foundation

Ohm's Law states that the current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. It is the single most important equation in electronics.

Key Concept

V = I × R — This is the equation you'll use more than any other in electronics. Know it by heart.

Three Forms of Ohm's Law

Ohm's Law can be rearranged to solve for any of the three quantities:

•To find Voltage: V = I × R (multiply current by resistance)
•To find Current: I = V / R (divide voltage by resistance)
•To find Resistance: R = V / I (divide voltage by current)

Tip

Use the Ohm's Law Triangle: write V at the top, I at the bottom-left, and R at the bottom-right. Cover the quantity you want to find — the remaining two show you the formula. Cover V: you see I×R. Cover I: you see V/R. Cover R: you see V/I.

Practical Example

Consider a 9V battery connected to a 450Ω resistor. Using Ohm's Law, we can find the current: I = V/R = 9/450 = 0.02A = 20mA. This means 20 milliamperes of current flows through the circuit.

Important Notes

•Ohm's Law applies to 'ohmic' components — those with a constant resistance regardless of voltage. As you learned in Lesson 2, carbon-film, metal-film, and wire-wound resistors are all ohmic.
•Non-ohmic components (like diodes and LEDs) have a resistance that changes with voltage — Ohm's Law doesn't apply to them directly. You met these in Lesson 2.
•Ohm's Law is valid for both DC and AC circuits (with impedance replacing resistance in AC).

Formulas

Interactive Diagram

Interactive Circuit Diagram

Voltage9.0V

1V24V

Resistance450Ω

10Ω10000Ω

Calculator

V = I \times R

Enter any 2 values to calculate the rest

Voltage (V)

Current (A)

Resistance (Ω)

Circuit Challenges

Challenge 1 of 3

Simple Circuit — Find Current

A 9V battery powers a single 450Ω resistor. Calculate the current flowing through the circuit.

I = \frac{V}{R}

450Ω

? A

Calculate & fill in:

Current (I)A

Knowledge Check

Question 1 of 5