What is Ohm's Law?
Ohm's law is the fundamental relationship between voltage, current and resistance in an electrical circuit. It states that the current through a conductor is directly proportional to the voltage across it, for a fixed resistance.
Ohm's law states that voltage equals current times resistance: V = I × R, where V is in volts, I is in amperes, and R is in ohms (Ω).
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Step-by-step worked examples
A resistor of 20 Ω has a current of 0.5 A flowing through it. Find the voltage across it.
V = I × R V = 0.5 × 20 = 10 V
A 12 V battery is connected to a 4 Ω resistor. Find the current.
V = I × R → I = V / R I = 12 / 4 = 3 A
A lamp draws 0.25 A when connected to a 6 V source. Find its resistance.
V = I × R → R = V / I R = 6 / 0.25 = 24 Ω
Flashcards
Quick quiz
Q1.Ohm's law is written as:
Q2.A 3 A current flows through a 15 Ω resistor. What is the voltage?
Q3.If resistance doubles while voltage stays the same, current:
Q4.What is the SI unit of resistance?
Common mistakes
Mixing up which variable to solve for. — Correct: Rearrange carefully: V = IR, I = V/R, R = V/I — pick the form that isolates the unknown.
Assuming Ohm's law applies to every device. — Correct: It only applies to ohmic (linear) conductors like resistors at constant temperature; diodes and bulbs are non-ohmic.
Ignoring units when calculating. — Correct: Always use volts, amperes and ohms consistently before multiplying or dividing.
Thinking higher resistance means higher current at fixed voltage. — Correct: Higher resistance reduces current — resistance opposes current flow.
FAQ
What is Ohm's law?
Ohm's law describes the relationship between voltage, current and resistance: V = I × R.
What is the Ohm's law formula?
V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms.
What are examples of Ohm's law?
Calculating the current through a light bulb, sizing a resistor in a circuit, and finding voltage drop across a wire are common examples.
How do you calculate voltage, current or resistance using Ohm's law?
Use V = I × R for voltage, I = V/R for current, or R = V/I for resistance, depending on which two values you know.