Electricity basics

What Is Resistance?

Grades 3-8
15 minute activity
NGSS 4-PS3-2

Quick answer

Resistance is how much a material pushes back against electricity flowing through it. The more resistance there is, the less electricity flows. Conductors like copper have low resistance and insulators like rubber have very high resistance. Resistance is measured in ohms, written with the symbol Ω.

Short thick dough makes an LED bright while a long thin dough snake makes it dim — Tap to enlarge. A short, thick piece of dough lets lots of electricity through, so the LED is bright. A long, thin snake of dough resists more, so the LED is dim.

Tap to enlarge. A short, thick piece of dough lets lots of electricity through, so the LED is bright. A long, thin snake of dough resists more, so the LED is dim.

Think of a water slide

Imagine a crowd of kids going down a water slide. A wide, short slide lets lots of kids through quickly. A long, narrow slide slows everyone down, so fewer kids get through each minute.

Electricity works the same way. Resistance is anything that makes it harder for electric charges to get through. It does not stop the flow completely (unless the material is an insulator), but it slows it down.

What changes resistance?

Four things decide how much resistance something has:

Material: some materials let charges move easily (copper, aluminum, salty dough) and some hold them tight (plastic, rubber, air).
Length: a longer path has more resistance. Doubling the length of a dough snake roughly doubles its resistance.
Thickness: a thicker path has less resistance, because there is more room for the charges to flow side by side.
Temperature: most metals resist more when they get hot.

Scale of materials from low to high resistance: copper, aluminum foil, salty dough, tap water, sugar dough, plastic and rubber, air — Tap to enlarge. Materials from lowest to highest resistance, for objects the same size and shape. Conductors are on the left and insulators are on the right.

Why is resistance useful?

Resistance can sound like a problem, but circuits use it on purpose all the time.

Protecting parts: tiny parts called resistors limit how much current flows, so an LED does not get too much and burn out. In a dough circuit, the dough itself adds some resistance.
Making heat: toasters, hair dryers and space heaters push current through high-resistance wire on purpose, so it gets hot.
Making light: an old-style light bulb has a thin wire with so much resistance that it glows white hot.
Controlling things: a volume knob or a dimmer switch changes resistance to turn the sound or the light up and down.

Try it with dough

The dough snake test

Find out how length and thickness change resistance. This is a fair test: change only one thing at a time and keep everything else the same.

You need

Battery holder with 4 AA batteries
Conductive (salty) play dough
1 LED
Ruler

Steps

Roll a dough snake about as thick as your finger and 5 cm (2 inches) long. Set a small extra lump of dough next to one end, with a gap between them.
Push the red battery wire into the far end of the snake and the black wire into the extra lump.
Bridge the gap with the LED: long leg in the snake, short leg in the lump. Switch on and notice the brightness.
Switch off. Roll a new snake the same thickness but 30 cm (12 inches) long, curve it to fit, and test it the same way.
Now roll a 30 cm snake that is very thin, about as thick as a pencil, and test it again.
Put the three results in order from brightest to dimmest.

What happens

The short, thick snake makes the LED brightest. The long snake is dimmer, and the long, thin snake is dimmest. Longer and thinner dough has more resistance, so less current reaches the LED.

Not working?

Make sure the snake does not curl back and touch itself or the lump, or the electricity will take the shortcut. Fresh, moist dough works best. Want to go further? Ohm's Law Explained shows how to predict the change with math.

Key words

Resistance: How much a material pushes back against the flow of electricity.
Ohm (Ω): The unit used to measure resistance.
Resistor: A small part made to add a set amount of resistance to a circuit.
Current: How much electricity flows through a circuit.
Conductor: A material with low resistance, like copper or salty dough.
Insulator: A material with very high resistance, like rubber, plastic or sugar dough.

For teachers

Resistance depends on the material, the length and the cross-sectional area: R = ρL/A, where ρ (rho) is the resistivity of the material. Doubling the length doubles the resistance, and doubling the cross-sectional area halves it. Conductive dough has a much higher resistivity than copper, which is what makes length effects easy to see with an LED.

The dough snake test is a good introduction to fair tests and variables. Students change one variable (length or thickness), keep the others the same and observe the LED. For older students, our free Resistivity Lab (PDF) goes further: students use one or two multimeters to measure current and voltage across dough snakes, calculate resistance with Ohm's law, then work out the resistivity of the dough and show that it stays about the same while the resistance changes with shape. Find free lesson plans on our education page.

Resistance questions

What is resistance in simple words?

Resistance is how hard it is for electricity to flow through something. Low resistance lets lots of electricity through. High resistance lets only a little through.

What is resistance measured in?

Resistance is measured in ohms, written with the Greek letter omega (Ω). It is named after Georg Ohm, the German scientist who worked out how voltage, current and resistance are connected.

Does a longer wire have more resistance?

Yes. A longer wire or dough snake has more resistance, because the charges have farther to travel. A thicker wire has less resistance, because there is more room for the charges to flow.

What is the difference between resistance and an insulator?

Every material has some resistance. An insulator is a material with such high resistance that almost no electricity can flow through it at normal voltages. Conductors have low resistance.

What does a resistor do?

A resistor limits how much current flows in a circuit. Resistors protect parts like LEDs from getting too much current, set the brightness of lights and divide up voltages in electronics.

Does play dough have resistance?

Yes. Salty play dough conducts electricity, but it has much more resistance than a metal wire. That is why a long, thin dough snake makes an LED dimmer, and why you can see resistance happen in a dough circuit.

Advanced lab: measure the resistivity of dough with a multimeter (PDF)

Next, try a project

Make a conductor tester Test everyday objects to find out which ones conduct.