Ohm's Law: V = IR Explained
The foundational formula of all electrical engineering. Learn how voltage, current, and resistance interact, and use our interactive calculator to solve for any missing variable.
What is Ohm's Law?
Formulated by German physicist Georg Ohm in 1827, Ohm's Law describes the exact relationship between the three core components of electricity: voltage, current, and resistance.
The law states that the current flowing through a conductor between two points is directly proportional to the voltage across the two points. In simpler terms: if you increase the voltage (push), you increase the current. If you increase the resistance (friction), you decrease the current.
The Formula: V = I × R
V = I × R
Voltage equals Current multiplied by Resistance
- V (Voltage): Measured in Volts (V). The electrical pressure pushing the electrons.
- I (Current): Measured in Amperes (A). The rate of flow of electrons. (Often called 'Intensity').
- R (Resistance): Measured in Ohms (Ω). The opposition to electron flow.
Derived Formulas
Using basic algebra, you can rearrange Ohm's Law to solve for any single variable as long as you know the other two:
- To find Current: I = V ÷ R
- To find Resistance: R = V ÷ I
- To find Power (Watt's Law): P = V × I
Combining Ohm's Law and Watt's Law allows you to derive power even if you only have current and resistance (P = I² × R) or voltage and resistance (P = V² ÷ R).
Interactive Ohm's Law Calculator
Enter any TWO values below. The calculator will automatically solve for the remaining two.
Practical Examples
| Scenario | Known Values | Formula Used | Result |
|---|---|---|---|
| Finding Resistance of a Bulb | 120V, 0.5A | R = V ÷ I | 240 Ω |
| Current drawn by a Heater | 240V, 20Ω | I = V ÷ R | 12 A |
| Voltage drop across a wire | 15A, 0.2Ω | V = I × R | 3 V |
| Power of an LED strip | 12V, 2A | P = V × I | 24 W |
Frequently Asked Questions
The three main variations of Ohm's Law are: Voltage (V) = I × R, Current (I) = V ÷ R, and Resistance (R) = V ÷ I.
Ohm's Law states that current (I) is directly proportional to voltage (V) and inversely proportional to resistance (R). This means if you increase voltage, current goes up. If you increase resistance, current goes down.
Power (P), measured in Watts, can be calculated using Watt's Law, which works hand-in-hand with Ohm's Law. The basic power formula is P = V × I. By substituting Ohm's Law variations into it, you also get P = I² × R and P = V² ÷ R.