Open the editorEditor
Ready-to-teach lab

Ohm's law lab: measure it, then predict it

In this Ohm's law lab students read a real current on a multimeter, work out why it is that number, and then predict what a bigger resistor will do before they try it.

Ages 12 and up, first circuits unit. About 40 minutes. No code.

Open this labAssign to my class
The lesson

Objective and parts

By the end, students can

  • Read a current in milliamps and a voltage in volts from a multimeter.
  • Use V = IR on one resistor, with the voltage across that resistor and not the supply.
  • Predict a current before measuring it, and compare the two.

Parts, already on the bench

  • A 5 V supply (the 5 V pin of an Arduino Uno, with no program running)
  • One 220 ohm resistor
  • One red LED
  • A multimeter, wired in series and set to DC amps

Nothing to hand out or collect: the circuit opens built and wired.

Steps

What students do

Each step is short enough to read out. Students work in the simulator beside this page.

Open this lab
  1. Open the lab and press Run. The LED lights and the meter reads about 14 mA.
  2. Turn the meter dial to DC volts and move its probes across the resistor. Write down the reading, about 3.1 V.
  3. Measure across the LED the same way. It keeps about 1.9 V, whatever the resistor is.
  4. Work out 3.1 V divided by 220 ohms and compare it with the current the meter showed.
  5. Predict the current with a 1 k resistor. Click the resistor, change its value and measure.
  6. Do the same for 10 k, and note what happens to the brightness of the LED.
Questions

Questions for students

Hand these out as they are. The expected answers are under the list, for you.

  1. The supply is 5 V. Why is the current not 5 V divided by 220 ohms?
  2. What current did you predict for 1 k, and what did you measure?
  3. With 10 k the LED barely glows. What is the current now?
  4. Add the voltage across the resistor to the voltage across the LED. What do you get, and why?
Answer key for the teacher
  1. The LED keeps about 1.9 V for itself, so only about 3.1 V is across the resistor. 3.1 / 220 is about 14 mA; 5 / 220 would be about 23 mA.
  2. About 3 mA: roughly 3.1 V across 1000 ohms. The LED is clearly dimmer.
  3. About 0.3 mA. Ten times the resistance, one tenth of the current.
  4. About 5 V. The voltages around one loop add up to the supply voltage.
Go further

For students who finish early

  • Plot current against 1/R for five resistor values and look for the straight line.
  • Stop the simulation, turn the dial to ohms and measure the resistor on its own.
Your class

Give this lab to every student

Assign to my class puts this circuit on each student’s class page as a starter. They change their own copy and hand it in, and the work comes back to you in one list.

The class tools are free for 30 days for one class of up to 20 students, with no card. Opening and running the lab needs no account and stays free.

Assign to my classStart a free class
More labs

The other labs in the set

Series and parallelCircuits. 45 min.Voltage dividerCircuits. 40 min.LED and resistorCircuits. 30 min.RC circuitElectronics. 50 min.Transistor switchElectronics. 45 min.Logic gatesElectronics. 40 min.Arduino blinkArduino. 35 min.Button and LEDArduino. 40 min.Traffic lightArduino. 45 min.Ultrasonic distanceArduino. 45 min.Servo sweepArduino. 40 min.All labsThe whole set on one page.