What Is Ohm's Law?
Ohm's law states that the current (I) through a conductor is directly proportional to the voltage (V) across it and inversely proportional to its resistance (R): V = I × R. Knowing any two of these values lets you calculate the third. It is the single most fundamental relationship in circuit design.
Ohm's law is the most fundamental rule in electronics, defining the relationship between voltage (V), current (I) and resistance (R) in an electrical circuit. It's summed up in a single formula: V = I × R. That is, the voltage applied to a component equals the current through it times its resistance.
Ohm's Law Formula: V = I × R
The three quantities in the formula and their units are:
- V – Voltage, in volts (V)
- I – Current, in amperes (A)
- R – Resistance, in ohms (Ω)
If you know two of these three, you can always calculate the third:
| What you want | Formula | Example |
|---|---|---|
| Voltage (V) | V = I × R | 0.1 A × 220 Ω = 22 V |
| Current (I) | I = V / R | 12 V / 100 Ω = 0.12 A |
| Resistance (R) | R = V / I | 5 V / 0.01 A = 500 Ω |
Easy Recall with the Ohm Triangle
Instead of memorizing the formula, you can use the "Ohm triangle": V on top, I and R side by side below. Cover the quantity you're looking for; the position of the rest tells you the operation. Cover V and you see I × R; cover I and you see V / R.
Worked Example: Choosing a Resistor for an LED
Suppose you have a 5 V source, the LED drops 2 V, and you want 10 mA (0.01 A) to safely flow through it. The voltage across the resistor is 5 V − 2 V = 3 V. From there:
R = V / I = 3 V / 0.01 A = 300 Ω
So a 300 Ω resistor (nearest standard value 330 Ω) runs the LED safely. This shows how Ohm's law is useful in a real circuit.
Where Does It Come Up in Practice?
Ohm's law is hidden in almost every circuit: current-limiting resistors, voltage dividers set up to read a sensor, power and heating calculations… You use this law to determine how much current a component will receive or which value resistor to choose.
Common Mistakes
- Unit confusion: 10 mA and 10 A, or 1 kΩ and 1 Ω, are not the same. Convert everything to base units (A, V, Ω) before calculating.
- Applying it directly to AC circuits: in AC circuits with coils and capacitors, impedance comes into play instead of resistance; pure Ohm's law is valid for resistive (linear) loads.
- Forgetting power: as important as the resistor's value is the power it will dissipate (P = V × I); otherwise the resistor can overheat and burn.
Once you've grasped Ohm's law, the next step is to get to know the three quantities closely: What Are Current, Voltage and Resistance?
Frequently Asked Questions
What is the formula for Ohm's law?+
What are the units in Ohm's law?+
Does every component obey Ohm's law?+
How do you find power with Ohm's law?+
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