Try drinking a thick milkshake through a thin straw. You suck as hard as you can, but only a little comes up. The straw holds the milkshake back.

Electricity gets held back too, as it flows through wires, lamps and toasters. This holding back is called resistance. Let’s find out where it comes from, and why it makes things hot.

What holds electricity back?

Electricity in a wire is a flow of tiny particles called electrons. A push called voltage makes them drift along the wire, all together. This flow is the electric current.

But a wire is not an empty tube. It is packed with metal atoms, sitting in neat rows. The atoms are never still: they jiggle all the time, even in a cold wire.

As the electrons drift along, they keep bumping into the jiggling atoms. Every bump slows them down. How strongly something holds back the flow is called its resistance.

Electrons bumping into atoms inside a wire A close-up of the metal inside a wire. Atoms sit in neat rows and jiggle. An electron zigzags between them from left to right, bumping into atom after atom. Wavy arrows show heat rising from the wire. heat atom electron
Electrons zigzag through the metal and keep bumping into its atoms. Every bump slows them down and makes the atoms jiggle harder, which warms the wire.

Like a narrow pipe

Two identical pumps pushing water through a wide pipe and a narrow pipe Two identical pumps push water with the same push. The top one pushes into a wide pipe, and lots of water flows out of its end. The bottom one pushes into a narrow pipe, and only a trickle comes out. two identical pumps wide pipe: lots of water narrow pipe: only a trickle
The same push sends lots of water through a wide pipe, but only a trickle through a narrow one. A narrow pipe is like a path with a high resistance.

Electricity works the same way. With the same push, a lot of current flows through something with a low resistance. Only a little flows through something with a high resistance.

What makes resistance bigger or smaller?

Four things decide how much a wire holds electricity back:

  • The material. Copper and silver let electrons through very easily, so electric cables are made of copper. Other metals, such as iron, hold electrons back more. Plastic and rubber hold them back so much that almost nothing flows: they are insulators.
  • The length. In a long wire, the electrons have more atoms to get past. A wire twice as long has twice the resistance.
  • The thickness. A thick wire has more room, like a wide corridor where many people can walk side by side. The thicker the wire, the lower its resistance.
  • The temperature. When metal gets hotter, its atoms jiggle harder. The electrons bump into them more often, so the resistance goes up.
A short, thick wire and a long, thin wire At the top, a short, thick wire lets many electrons through side by side: low resistance. At the bottom, a long, thin wire winds back and forth, and only a few electrons squeeze through: high resistance. short and thick low resistance long and thin high resistance
A long, thin wire holds electrons back much more than a short, thick one.

How do we measure resistance?

Resistance is measured in ohms. The short sign for an ohm is Ω, the Greek letter omega. The unit is named after Georg Ohm, a German scientist. He found the rule that connects voltage, current and resistance: Ohm’s law.

WhatResistance
1 metre of copper wire in a house cableabout 0.01 Ω
The heating part of a 2,000-watt kettleabout 26 Ω
An old-style 60-watt light bulb, while it glowsabout 900 Ω
A piece of plastic the size of a sugar cubemore than a million million Ω

A small part whose only job is to have a certain resistance is called a resistor. Almost every electronic gadget has resistors inside.

Resistance makes heat

Every time an electron bumps into an atom, the atom jiggles a little harder. And faster jiggling atoms mean a hotter material. So resistance turns electrical energy into heat.

Sometimes that heat is exactly what we want. A toaster, a kettle and a hair dryer all contain wires with a high resistance. When current flows through them, they get hot — hot enough to toast bread or boil water.

An old-style light bulb works the same way. Its very thin wire, called a filament, gets hotter than 2,000 °C, so hot that it glows white. But only a tiny part of the energy becomes light. Almost all of it becomes heat, which is why these bulbs waste so much electricity.

At other times, the heat is a waste. Cables, chargers and laptops get warm while they work. To keep this waste small, cables are made of copper. Cables for things that use lots of electricity, such as an electric cooker, are extra thick.

How fast something uses up electrical energy is called its power.

Go deeper: The formula for a wire’s resistance for grown-ups and the extra curious

The resistance of a wire depends on its material, its length and its thickness:

R=ρ⋅LAR = \rho \cdot \frac{L}{A}
  • RR is the resistance, in ohms (Ω).
  • ρ\rho (the Greek letter rho) is the resistivity of the material: how strongly the material itself holds electrons back. Copper has about 1.7×10−8 Ω⋅m1.7 \times 10^{-8}\ \Omega \cdot \text{m} (ohm-metres). Nichrome, a mix of nickel and chromium used in heating elements, has about 65 times more.
  • LL is the length of the wire, in metres (m).
  • AA is the area of the wire’s cross-section (the size of its cut end), in square metres (m²).

For example, 1 metre of copper wire with a cross-section of 1.5 mm² (1.5×10−6 m21.5 \times 10^{-6}\ \text{m}^2), a common size in house cables, has a resistance of:

R=1.7×10−8 Ω⋅m⋅1 m1.5×10−6 m2≈0.011 ΩR = 1.7 \times 10^{-8}\ \Omega \cdot \text{m} \cdot \frac{1\ \text{m}}{1.5 \times 10^{-6}\ \text{m}^2} \approx 0.011\ \Omega

A wire twice as long has twice the resistance. A wire twice as wide has a cut end four times as big, so its resistance is only a quarter.

Go deeper: What really slows the electrons down for grown-ups and the extra curious
  • In a metal, every atom lets go of one or two of its outer electrons. These free electrons wander through the whole metal. The atoms, now called ions, stay in a regular pattern called a crystal.
  • Electrons bouncing off atoms like balls is a simplified picture. Quantum physics shows that a perfectly regular crystal whose atoms stood perfectly still would hardly hold electrons back at all. What scatters them are the vibrations of the atoms, which grow with temperature, and flaws in the crystal: missing atoms, atoms of other elements and the borders between tiny crystals.
  • That is why the resistance of copper rises by about 0.4 % for every degree Celsius, and why mixes of metals, such as nichrome, resist far more than pure metals.
  • Not every material behaves like a metal. In many thermistors (small parts used in electronic thermometers), the resistance goes down as they warm up.
Go deeper: Where the pipe picture goes wrong for grown-ups and the extra curious
  • Water rubs against the walls of a pipe, so it is held back mostly near the walls. In a wire, the whole metal holds electrons back, all the way across. The pipe picture gets the result right — longer and thinner means more resistance — but not the reason.
  • Resistance does not use up electrons. Every second, as many electrons flow out of a toaster’s heating wire as flow into it. What the electrons lose on the way is energy, and it becomes heat.

Check yourself

Which wire has the lowest resistance?

Show the answer

A short, thick copper wire — Copper holds electrons back less than iron, and a short, thick wire gives them the easiest path of all.