Monthly Archives: May 2012

How Resistors and Capacitors Work

Now that we have started out with atoms and gone all the way to electronic band theory, which uses available energy states to explain why some materials are good at conducting electrons and others are not, we can start to discuss actual electronic devices! After all, fancy materials aren’t much good unless you have some way to use them.

Broadly speaking, we want devices that do something worthwhile, like light a room, make calculations, or power a motor. Most electrical devices work by manipulating a flow of electrons to extract some useful behavior. If we apply an electrical potential (which is also called voltage) to a device, then it will be energetically favorable for electrons to move through the device; this charge flow is called electrical current. The potential difference is often provided by something like a battery, where the differing chemical potential within the battery provides the voltage, and the device itself is connected to both terminals of the battery. Connecting a device to a battery forms a physical loop that electrons travel through, hence the name circuit. The battery itself is a circuit element, and so is the device that does something useful. There are quite a few interesting circuit elements but let’s start simple.

While the high electrical conduction of metals is extremely useful, sometimes it can be useful to have something that does not conduct electrons quite so well. Why? Because poor conductors offer the opportunity to convert electrical energy into other forms of energy, such as light or heat. This is the idea behind resistors, circuit elements that resist the flow of electrons without quite stopping it. Some resistor materials convert excess energy into heat, which can be the basis of electric heaters or electric stovetops. And the filament in an incandescent light bulb is acting as a resistor, one which heats up so much that it emits light (the reason for this is a whole other sack of beans). Resistors can be made by combining a conductive material with a non-conductive material, and are manufactured across an incredibly broad range of resistances. And independent of their heating or light-emitting properties, they are often used because the electrical current through them depends linearly on voltage.

And what happens if we push resistance to its limit, such that no electrons can actually pass through an insulating device? Applying a voltage drop would cause electrons to build up on one side of the device, attempting to pass through, until the repulsive force from the assembled electrons was enough to deter additional electrons from building up. The charge imbalance creates an electric field across the device, and this is what we call a capacitor. You can build a capacitor by bringing two parallel conducting plates close to each other and applying voltage. Since current can’t cross the gap between the plates, charge is stored on the capacitor plates, which can be discharged upon connection to a circuit. This is somewhat similar to a battery, although most batteries have stored chemical energy rather than electrical, and the speed of the chemical reaction which discharges a battery is usually much slower than the speed of electrons rushing to equilibrium when a capacitor is discharged. An older example of a capacitor is shown below; modern capacitors use thin films to create an insulating gap, and are considerably smaller than the capacitor pictured.

Resistors and capacitors are two of the most basic pieces that you can put into a circuit, and two of the most widely used. But some of the more complicated elements are interesting as well, and we’ll get into those in the next few posts!

Flame Challenge

The Flame Challenge recently asked people to submit an answer to the question ‘What is a flame?’ that 11-year-olds would find engaging, understandable, and fun. The final entries are currently up at the site linked above, and they are neat to see: some are written, some are illustrated, and some are videos. There are a lot of great ways to explain the same phenomenon!