Voltage, current and resistance
Ohm's Law Explained
- Grades 5-8
- 20 minute activity
Quick answer
Ohm's law says that the current flowing through a circuit equals the voltage divided by the resistance: I = V ÷ R. More voltage pushes more current through. More resistance lets less current through. You can see it in dough: stretch a dough snake longer and the LED gets dimmer.
Voltage, current and resistance
Ohm's law connects three things that happen in every circuit:
- Voltage (V) is the push. It is measured in volts. A battery holder with 4 AA batteries pushes with about 6 volts.
- Current (I) is the flow, or how much electricity moves through the circuit each second. It is measured in amps (A).
- Resistance (R) is the squeeze, or how hard it is for the electricity to get through. It is measured in ohms (Ω).
Think of a garden hose. Voltage is how hard the water is pushed. Current is how much water comes out. Resistance is how much you pinch the hose.
The Ohm's law triangle
The triangle is an easy way to remember the three versions of the formula. Put your finger over the letter you want to find and look at what is left:
- Cover V and you see I next to R: V = I × R.
- Cover I and you see V over R: I = V ÷ R.
- Cover R and you see V over I: R = V ÷ I.
Try the math
Example 1: a 6 volt battery holder is connected to a 300 ohm resistor. How much current flows? I = V ÷ R = 6 ÷ 300 = 0.02 amps, which is 20 milliamps (mA).
Example 2: double the resistance to 600 ohms. Now I = 6 ÷ 600 = 0.01 amps, or 10 mA. Twice the resistance means half the current.
Example 3: a toy motor draws 0.5 amps from a 3 volt battery. What is its resistance? R = V ÷ I = 3 ÷ 0.5 = 6 ohms.
This is exactly what happens with a dough snake. Make the snake twice as long and its resistance roughly doubles, so less current reaches the LED and it glows dimmer.
Try it with dough
See Ohm's law with a dough snake
Keep the voltage the same and change the resistance by changing the length of the dough. Watch what happens to the current by watching the LED.
You need
- Battery holder with 4 AA batteries
- Conductive (salty) play dough
- 1 LED (red works best)
- Ruler
Steps
- Roll a dough snake about as thick as your finger and 10 cm (4 inches) long. Put a small lump of dough next to one end, with a gap.
- Push the red battery wire into the far end of the snake and the black wire into the lump.
- Bridge the gap with the LED: long leg in the snake, short leg in the lump. Switch on and notice how bright it is.
- Switch off. Add dough to make the snake 20 cm (8 inches) long, keeping it the same thickness, and test again.
- Repeat at 30 cm (12 inches). Zigzag the snake if you run out of room, but do not let it touch itself.
- Record the brightness for each length as bright, medium or dim.
What happens
The LED gets dimmer as the snake gets longer. The voltage stayed the same, but the resistance went up, so the current went down, just as Ohm's law predicts.
Not working?
Keep the snake the same thickness every time, or you are changing two things at once. If the LED is dim even with the short snake, try fresh batteries and fresh dough. See What Is Resistance? for more on what changes resistance.
Key words
- Ohm's law
- The rule that current equals voltage divided by resistance: I = V ÷ R.
- Voltage (V)
- The push that moves electricity, measured in volts.
- Current (I)
- How much electricity flows each second, measured in amps (A).
- Resistance (R)
- How hard it is for electricity to flow, measured in ohms (Ω).
- Milliamp (mA)
- One thousandth of an amp. Small LEDs use about 10 to 20 mA.
- Variable
- Something you can change or measure in an experiment.
For teachers
Georg Ohm published the relationship in 1827. Why the letter I for current? It comes from the French phrase intensité du courant, or current intensity.
An LED is not a simple resistor, so Ohm's law does not describe it on its own. An LED needs a set voltage to turn on, about 2 volts for red and about 3 volts for blue or white. The rest of the battery's voltage falls across the dough, and the dough's resistance decides how much current flows. That is why the length of the snake changes the brightness so clearly.
For a math extension, give students the voltage and the current and ask them to find the resistance, or graph LED brightness against snake length. For a hands-on version with real measurements, try our free Resistivity Lab (PDF), where students measure current and voltage with multimeters and calculate resistance. Find free lesson plans on our education page.
Ohm's law questions
What is Ohm's law in simple terms?
Ohm's law says that the more you push (voltage), the more electricity flows (current), and the more you squeeze (resistance), the less electricity flows. As a formula: current equals voltage divided by resistance.
What is the formula for Ohm's law?
V = I × R, where V is voltage in volts, I is current in amps and R is resistance in ohms. You can rearrange it to I = V ÷ R or R = V ÷ I.
Why is current written as I?
The letter I comes from the French phrase intensité du courant, meaning current intensity. It was used by the French scientist André-Marie Ampère, who the amp is named after.
What happens to current when resistance increases?
If the voltage stays the same, the current goes down. Double the resistance and the current is cut in half. That is why a longer dough snake makes an LED dimmer.
Who discovered Ohm's law?
German scientist Georg Simon Ohm. He published it in 1827 after experiments with wires of different lengths and thicknesses. The unit of resistance, the ohm, is named after him.
Does Ohm's law work for LEDs?
Not on its own. An LED needs a minimum voltage before it turns on, and after that its current rises very quickly. That is why LEDs are paired with something that adds resistance, like a resistor or, in a dough circuit, the dough itself.
Next, try a project
Build the Circuit Project Squiggle the dough to see resistance in action, with printable instructions.