Resistance and Ohm's Law
V = IR links voltage, current and resistance.
About this topic
Resistance is the opposition to current flow, measured in ohms. Ohm's law, V = IR, says the current through a conductor is proportional to the voltage across it at constant temperature. Resistance depends on the material, length and cross-sectional area: R = ρL/A.
A longer or thinner wire has more resistance, like a longer, narrower pipe resisting water flow. For example, a 12 V supply across a 4 ohm resistor drives a current of 3 A, and doubling the voltage doubles the current.
Components that follow Ohm's law are ohmic; filament lamps and diodes are not, since a lamp's filament heats up and its resistance rises, giving a curved graph of current against voltage. Resistors are used to control currents in circuits, set LED brightness and divide voltages in sensors.
Students sometimes think V = IR is true for every device; it is really a definition of resistance, and Ohm's law is the special case where R stays constant. Straight-line graphs mean ohmic behaviour, and the gradient of an I-V graph is 1/R.
Key ideas
- Resistors limit current
- Longer, thinner wires resist more
- Ohmic materials are linear
Quick check
Use cases: where Resistance and Ohm's Law helps
- Homework: review the key idea behind Resistance and Ohm's Law before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Resistance and Ohm's Law without notes.
- Visual learning: play with the interactive lab to see how Resistance and Ohm's Law changes when you adjust its inputs.
- Teaching a friend: explain Resistance and Ohm's Law in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Resistance and Ohm's Law?
V = IR links voltage, current and resistance.
What will I learn in Resistance and Ohm's Law?
Resistors limit current. Longer, thinner wires resist more. Ohmic materials are linear.
Is Resistance and Ohm's Law part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Resistance and Ohm's Law lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Resistance and Ohm's Law?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.