ScientistGrades 10–12⏱ 20 min
Ohm's law and circuits
Turn up the voltage, change the resistance, add resistors in series or parallel, and watch the current follow I = V / R.
Standards: HS-PS3-1
1Wonder
A phone charger, a toaster and a flashlight all run on electricity, but they use very different amounts of current. What decides how much current flows?
2Explore
This activity needs a bigger screen
Turn your phone sideways, or open this lesson on a tablet or computer. The explanation and quiz below work here.
Current I 2 A Total resistance 3 Ω Power P = V × I 12 W
Try this
- Double the voltage. What happens to the current? Look at the point on the graph.
- Keep the voltage the same and make R₁ bigger. What happens to the current?
- Add a second resistor in series. Is the total resistance bigger or smaller than R₁?
- Now connect it in parallel. Bigger or smaller? Why do you think that is?
3Explain
Step 1 of 5
Voltage, current and resistance
The battery’s voltage V pushes charges around the circuit. The flow of charge is the current I. The resistance R of a resistor slows that flow.
Ohm's law
In series: R = R₁ + R₂. In parallel: 1/R = 1/R₁ + 1/R₂.
4Practice
Question 1 of 5
A 12 V battery is connected to a 4 Ω resistor. What is the current?
5Recap
- Voltage pushes, current flows, resistance slows the flow.
- Ohm’s law: V = I × R. For a fixed resistor, I against V is a straight line.
- Series: resistances add, and the same current flows through each.
- Parallel: the total resistance is smaller than any branch, and each branch gets the full voltage.