--- title: "Ohm's Law: voltage, current, resistance" description: "Apply Ohm's Law (V = IR) to calculate current, voltage, or resistance in a simple circuit, and explain that resistance opposes the flow of current" canonical: https://lightmysky.com/learn/science/ohms-law-voltage-current-resistance-mt_H3ZDK0EYNV source: https://lightmysky.com/learn/science/ohms-law-voltage-current-resistance-mt_H3ZDK0EYNV.md retrieved: 2026-10-19 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Ohm's Law: voltage, current, resistance Apply Ohm's Law (V = IR) to calculate current, voltage, or resistance in a simple circuit, and explain that resistance opposes the flow of current Subject: Science · Area: Energy · Ages 12 to 13 Page: https://lightmysky.com/learn/science/ohms-law-voltage-current-resistance-mt_H3ZDK0EYNV ## Ready when they can - States Ohm's Law as V = IR and identifies the units for each quantity - Rearranges V = IR to find the unknown value given the other two - Explains that resistance opposes current flow and identifies factors that affect resistance (material, length, thickness) ## Lesson: Ohm's Law: V = IR You already know the two big ideas in a circuit: voltage is the push, and current is the flow of charge that the push creates. Resistance is the third idea. It measures how much something in the circuit fights against that flow. Ohm's Law joins all three in one short formula: V = I x R. V is voltage, measured in volts. I is current, measured in amps. R is resistance, measured in ohms, written with the symbol Ω. *(drawing: Same 10-volt battery, three different resistors. The bars show the current in amps: 5, then 2, then 1. As resistance climbs, current drops.)* The formula bends three ways, so if you know any two values you can always find the third. To find voltage, multiply: V = I x R. To find current, divide: I = V divided by R. To find resistance, divide the other way: R = V divided by I. Division also tells you something useful. Keep the battery the same and raise the resistance, and the current falls. Double the resistance and the current is cut in half. **Example.** The class torch runs on a 6-volt battery pack and its bulb has a resistance of 2 ohms. To find the current, use I = V divided by R: 6 divided by 2 is 3, so 3 amps flow. A toy motor pulls 3 amps from a 12-volt battery. To find its resistance, use R = V divided by I: 12 divided by 3 is 4, so the motor has 4 ohms. And if 2 amps flow through 6 ohms, then V = I x R gives 2 x 6 = 12 volts. What makes resistance big or small? Three things: the material, the length, and the thickness. Copper lets charge pass easily, so its resistance is low. Rubber fights hard against charge, so its resistance is very high. A long wire gives charge a longer path to struggle through, so resistance grows. A thick wire gives charge more room to move, so resistance shrinks. That is why a long, thin wire has more resistance than a short, thick wire made of the same material. **Tip.** Water in pipes is a handy picture. The battery is like a pump that pushes the water along, the current is the flowing water, and a resistor is a narrow stretch of pipe that slows everything down. Make the narrow part longer or tighter, and less water gets through each second, just like raising resistance cuts the current. **Recap.** V = I x R. Know any two of voltage, current, and resistance, and rearranging the formula gives you the third. More resistance, or a longer, thinner wire, means less current for the same battery. *Adapted from OpenStax College Physics (CC-BY 4.0), openstax.org* · [license](https://creativecommons.org/licenses/by/4.0/) ## Practice 14 questions on this page, each with its working shown. ## Needs first - [Current, voltage, and what they measure](https://lightmysky.com/learn/science/current-voltage-and-what-they-measure-mt_2bnXrfS4Iq) ## Opens up - [Series vs parallel circuits](https://lightmysky.com/learn/science/series-vs-parallel-circuits-mt_loEMaQ8kFA) - [Current-Voltage Graphs for Components](https://lightmysky.com/learn/science/current-voltage-graphs-for-components-mt_n_CiOa-05J)