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A capacitor is a device consisting of two metal plates that
stores
electric charge. Between these two plates is an insulator (a material that does not conduct electricity) to prevent charges from moving between the two plates. One of these plates is a negative plate, it holds negative charges, the other, is a positive plate, it holds positive charges.
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Pictures of Capacitors. Notice that all these
capacitors
have two
wires coming out of them, one attached to each plate.
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Capacitors are used in many devices,
such as the flash in cameras. In a flash camera, a capacitor charges up and stores the electric charge (that's the whining sound you hear in some cameras) and releases the charge to a light bulb which creates a flash when the picture is taken.
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An old school camera and a new disposable camera. Notice how the flashbulbs have decreased in size.
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But what do we mean
by "positive" and "negative" plates? When we connect the ends of a battery to the ends of a capacitor, the battery will pull negative electric charges from one plate and move them to the other plate. This movement of charges is knows as a current, and it will exist for a short time until the capacitor is fully charged. The result is one plate with an excess of negative charges and the other plate with an excess of positive charges, or, a positive and negative plate.
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Here we connect a battery to the two plates of a capacitor.
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A short time later, negative charges begin moving from one plate to the other.
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In the end we are left with two charged plates
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Let's look at a really cool movie that depicts this. (click here)
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Capacitors are used in many other devices, like TV's. The capacitors found in TV's are very powerful, they can hold a lot of charges on their plates. They are very dangerous to touch, then, because if you touched them you would get shocked (the charges would move through YOUR body). Because of the large amount of charges, this can be as dangerous or more dangerous than sticking your finger in a wall outlet. Because of this, TV's have WARNINGS on them not to open them up.
Click here to see a demonstration of the charge being removed from a capacitor. You can imagine that you could get quite a shock but touching that yourself.
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Switching gears a little, we will now talk about the photoelectric effect. Einstein won the nobel prize for describing this effect. The photoelectric effect is how we can get electricity from light. The photoelectric effect can be seen when you shine light on a metal. When you do this, electrons come popping out of the metal, creating electricity.
 
There's even more to the story than that. Some light won't make electrons pop out of the metal at ALL. That's because the light has to have enough ENERGY to cause the electrons to come out of the metal. It just so happens that the energy of the light is related to it's color. Red light it's the lowest energy, violet is the highest, and all other colors (like in the rainbow) have energies between these two. In fact, the order of lowest to highest energy is the same as the energy of the rainbow:
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RED ORANGE YELLOWGREENBLUEINDIGOVIOLET
-----------------INCREASING Energy------------->
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