by Chris Woodford. Last updated: June 2, 2015.
When you first learn about electricity, you discover that materials fall into two basic categories called conductors and insulators. Conductors (such as metals) let electricity flow through them; insulators (such as plastics and wood) generally do not. But nothing's quite so simple, is it? Any substance will conduct electricity if you put a big enough voltage across it: even air, which is normally an insulator, suddenly becomes a conductor when a powerful voltage builds up in the clouds—and that's what makes lightning. Rather than talking about conductors and insulators, it's often clearer to talk about resistance: the ease with which something will let electricity flow through it. A conductor has low resistance, while an insulator has much higher resistance. Devices called resistors let us introduce precisely controlled amounts of resistance into electrical circuits. Let's take a closer look at what they are and how they work!
Photo: A typical resistor used in an electronic circuit. It works by converting electrical energy into heat, which is dissipated into the air.
What is resistance?
Electricity flows through a material carried by electrons, tiny charged particles inside atoms. Broadly speaking, materials that conduct electricity well are ones that allow electrons to flow freely through them. In metals, for example, the atoms are locked into a solid, crystalline structure (a bit like a metal climbing frame in a playground). Although most of the electrons inside these atoms are fixed in place, some can swarm through the structure carrying electricity with them. That's why metals are good conductors: a metal puts up relatively little resistance to electrons flowing through it. Plastics are entirely different. Although often solid, they don't have the same crystalline structure. Their molecules (which are typically very long, repetitive chains called polymers) are bonded together in such a way that the electrons inside the atoms are fully occupied. There are, in short, no free electrons that can move about in plastics to carry an electric current. Plastics are good insulators: they put up a high resistance to electrons flowing through them.
This is all a little vague for a subject like electronics, which requires precise control of electric currents. That's why we define resistance more precisely as the voltage in volts required to make a current of 1 amp flow through a circuit. If it takes 500 volts to make 1 amp flow, the resistance is 500 ohms (written 500 Ω). You might see this relationship written out as a mathematical equation:
V = I × R
This is known as Ohm's Law for German physicist Georg Simon Ohm (1789–1854).
Resistance is useless?
How many times have you heard bad guys say that in movies? It's often true in science as well. If a material has a high resistance, it means electricity will struggle to get through it. The more the electricity has to struggle, the more energy is wasted. That sounds like a bad idea, but sometimes resistance is far from "useless" and actually very helpful.
In an old-style light bulb, for example, electricity is made to flow through an extremely thin piece of wire called a filament. The wire is so thin that the electricity really has to fight to get through it. That makes the wire extremely hot—so much so, in fact, that it gives off light. Without resistance, light bulbs like this wouldn't function. Of course the drawback is that we have to waste a huge amount of energy heating up the filament. Old-style light bulbs like this make light by making heat and that's why they're called incandescent lamps; newer energy-efficient light bulbs make light without making much heat through the entirely different process of fluorescence.
Photo: The filament inside an old-style light bulb. It's a very thin wire with a reasonably high resistance. It's designed to get hot so it glows brightly and gives off light.
The heat that filaments make isn't always wasted energy. In appliances like electric kettles, electric radiators, electric showers, coffee makers, and toasters, there are bigger and more durable versions of filaments called heating elements. When an electric current flows through them, they get hot enough to boil your water or cook your bread. In heating elements, at least, resistance is far from useless.
Resistance is also useful in things like transistor radios and TV sets. Suppose you want to lower the volume on your TV. You turn the volume knob and the sound gets quieter—but how does that happen? The volume knob is actually part of an electronic component called a variable resistor. If you turn the volume down, you're actually turning up the resistance in an electrical circuit that drives the TV's loudspeaker. When you turn up the resistance, the electric current flowing through the circuit is reduced. With less current, there's less energy to power the loudspeaker—so it sounds much quieter.
Photo: This variable resistor is the volume control from a transistor radio.
How resistors work
People who make electric or electronic circuits to do particular jobs often need to introduce precise amounts of resistance. They can do that by adding tiny components called resistors. A resistor is a little package of resistance: wire it into a circuit and you reduce the current by a precise amount. From the outside, all resistors look more or less the same. As you can see in the top photo on this page, a resistor is a short, worm-like component with colored stripes on the side. It has two connections, one on either side, so you can hook it into a circuit.
What's going on inside a resistor? If you break one open, and scratch off the outer coating of insulating paint, you might see an insulating ceramic rod running through the middle with copper wire wrapped around the outside. A resistor like this is described as wire-wound. The number of copper turns controls the resistance very precisely: the more copper turns, and the thinner the copper, the higher the resistance. In smaller-value resistors, designed for lower-power circuits, the copper winding is replaced by a spiral pattern of carbon. Resistors like this are much cheaper to make and are called carbon-film. Generally, wire-wound resistors are more precise and more stable at higher operating temperatures.
Photo: Inside a wire-wound resistor. Break one in half, scratch away the paint, and you can clearly see the insulating ceramic core and the conducting copper wire wrapped around it.
How does the size of a resistor affect its resistance?
Suppose you're trying to force water through a pipe. Different sorts of pipes will be more or less obliging, so a fatter pipe will resist the water less than a thinner one and a shorter pipe will offer less resistance than a longer one. If you fill the pipe with, say, pebbles or sponge, water will still trickle through it but much more slowly. In other words, the length, cross-sectional area (the area you see looking into the pipe to see what's inside), and stuff inside the pipe all affect its resistance to water.
Electrical resistors are very similar—affected by the same three factors. If you make a wire thinner or longer, it's harder for electrons to wiggle through it. And, as we've already seen, it's harder for electricity to flow through some materials (insulators) than others (conductors). Although Georg Ohm is best known for relating voltage, current, and resistance, he also researched the relationship between resistance and the size and type of material from which a resistor is made. That led him to another important equation:
R = ρ × L / A
In simple words, the resistance (R) of a material increases as its length increases (so longer wires offer more resistance) and increases as its area decreases (thinner wires offer more resistance). The resistance is also related to the type of material from which a resistor is made, and that's indicated in this equation by the symbol ρ, which is called the resistivity, and measured in units of Ωm (ohm meters). Different materials have very different resistivities: conductors have much lower resistivity than insulators. At room temperature, aluminum comes in at about 2.8 x 10−8 Ωm, while copper (a better conductor) is significantly lower at 1.7 −8 Ωm. Silicon (a semiconductor) has a resistivity of about 1000 Ωm and glass (a good insulator) measures about 1012 Ωm. You can see from these figures how vastly different conductors and insulators are in their ability to carry electricity: silicon is about 100 billion times worse than copper and glass is about a billion times worse again!
Resistance and temperature
The resistance of a resistor isn't constant, even if it's a certain material of a fixed length and area: it steadily increases as the temperature increases. Why? The hotter a material, the more its atoms or ions jiggle about and the harder it is for electrons to wriggle through, which translates into higher electrical resistance. Broadly speaking, the resistivity of most materials increases linearly with temperature (so if you increase the temperature by 10 degrees, the resistivity increases by a certain amount, and if you increase it by another 10 degrees, the resistivity rises by the same amount again). If you cool a material, you lower its resistivity—and if you cool it to an extremely low temperature, you can sometimes make the resistivity disappear altogether, in a phenomenon known as superconductivity.
Chart: The resistance of a material increases with temperature. This chart shows how resistivity (basic resistance of a material, independent of its length or area) increases almost linearly as the temperature increases from absolute zero up to about 600K (327°C) for four common metals. Drawn using original data from "Electrical Resistivity of Selected Elements" by P. Desai et al, J. Phys. Chem. Ref. Data, Vol 13, No 4, 1984 and "Electrical Resistivity of Copper, Gold, Palladium, and Silver" by R. Matula, J. Phys. Chem. Ref. Data, Vol 8, No 4, 1979, courtesy of US National Institute of Standards and Technology Open Data.