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# What’s a Capacitor?

Capacitors are used in almost every single electronic device on the planet. Ask Science explores what they do and how they work.

By
Lee Falin, PhD
Episode #37

If you were to take apart pretty much any electronic device in your home, mixed in with the various bits and bobs of circuitry you would find that one device appears over and over again. Capacitors come in a variety of shapes and sizes, but they all have the same job: to store up electricity. Let’s look at some of the science behind how they work.

## A Long and Noble History

Way back in the 1700s, people knew about electricity, but still didn’t understand exactly what it was. For some time, scientists had been able to produce static electricity easily, but couldn’t really do anything useful with it. Many scientists believed that electricity was some kind of fluid, so they thought it might be possible to store it in a jar, just like any other fluid.

So one day, at the University of Leiden in the Netherlands, some scientists invented just such a jar, which came to be called a Leiden jar (though usually spelled with a “y” as Leyden jar). These special jars consisted of a glass jar with most of the inside lined with a thin metal foil. The outside and bottom of the jar were also lined with foil. The jar was sealed with some kind of insulating lid, such as wood or cork, and a brass rod was inserted inside, with a chain leading from the rod to the inner foil. Finally, the outside of the jar was “grounded,” which means it was connected to the earth by some form of conductor.

## All Charged Up

When connected to a source of static electricity, electrons flow into the jar through the metal rod, travel through the inner foil, and try to find a way out. However, since glass is an insulator, the walls of the jar keep the electrons from escaping and they just build up inside.

Meanwhile, all of those negatively charged electrons on the inner foil cause the electrons on the outer foil to get a bit edgy. As we learned in my previous episode on static electricity, like-charges repel. So the electrons on the outside of the jar want to get away from the excess inner electrons. Since the outside of the jar is grounded, those extra electrons escape to the earth.

Now we have an excess of electrons on the inside of the jar, and a deficiency of electrons on the outside of the jar. As you might also remember from the episode on static electricity, when we have a large difference in electrons between two surfaces, and those surfaces are connected in some way, the excess electrons flee rapidly in an electrostatic discharge. (The Florida State University Magnet Lab has a great animation that demonstrates how a Leyden jar works.)

## Pages

Please note that archive episodes of this podcast may include references to Ask Science. Rights of Albert Einstein are used with permission of The Hebrew University of Jerusalem. Represented exclusively by Greenlight.