## Hands On Physics

### Background: The role of the capacitor.

You will use an electronic component called a capacitor to make the timer. Since a capacitor is an important electronic component, this is as good a time as any to get to know it. This activity will give you an idea of what a capacitor does.

A capacitor stores electricity. If you pump electrical current into it, the capacitor develops a voltage that exerts a force that resists additional current. In this experiment, you will use a hand crank that pumps current into very large capacitors. These capacitors are constructed such that for each Coulomb of charge you pump in it will push back with one volt. These capacitors are a lot like rechargable batteries. The big difference between batteries and capacitors is that a battery supplies a nearly constant voltage, whereas a capacitor works at all voltages up to its maximum safe rating. As you charge the capaictors with the hand cranked generator you can actually feel the capacitor "filling up."

Figure P6
Capacitors as Tanks

A capacitor is like a closed water tank with two inlets separated by a rubber membrane. The more water (charge) you pump in one inlet (wire), the more back pressure (voltage) will build up. The pressure (voltage) opposes the addition of more water (charge). So, the more water (charge) you add the harder it gets to add more. Also note that whatever water (charge) flows in one inlet (wire), the same amount of water (charge) comes out the other. But because of the membrane, no steady water flow (current) can be maintained through the tank (capacitor). A large tank (capacitor) can absorb a lot of water (charge) before generating a large back pressure (voltage). We could measure the capacity of a tank (capacitor) as the ratio of water volume to pressure (charge to voltage, which is the definition of a farad).

Figure P7
Capacitor Construction

A capacitor consists of two conducting sheets separated by an insulator. The sheets are usually wound up into a spiral. You get more capacity by increasing the area of the sheets and decreasing the thickness of the insulation.
You can make a good capacitor with aluminum foil separated by plastic wrap. Try interleaving the layers as shown below.

Figure P8
Making a Capacitor

Capacitors come in many different forms. Here are what different ones look like:

Figure P9
Kinds of Caps

The physical size of the capacitor is not a good guide to its electrical capacity. There are two things to remember:

• Breakdown voltage. All capacitors will fail if you try to put too much voltage across them. This is like the rubber membrane breaking if you put too much pressure across it. To get a larger breakdown voltage the physical size of the capacitor must be larger.

• Electrolytic capacitors. One way to make large capacitors is to chemically create a very thin insulator on a metal. This is what is done inside so-called electrolytic capacitors. The disadvantage of this approach is that the thin insulation will disolve if you put the wrong sign of voltage on it. These capacitors must always have one electrode more positive than the other. Their leads are marked with a "+" and "­p;". Be sure to never reverse these.

Figure P10
Capacitor Symbols

These are the electrical symbols for capacitors. The plus sign indicates an electrolytic capacitor. There is no difference between the ones with two straight plates and those with one curved plate.

Every capacitor leaks a little bit. Put charge in it and the charge will slowly leak away. Expensive capacitors, especially ones with fancy plastics between their electrodes, are the best. The kind with the least leakage uses Mylar. For our timer circuit, we need one of these low-leakage capacitors.

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