The Capacitor

For good description a capacitor, go to the following web site:

http://www.techitoutuk.com/knowledge/electronics/components/capacitors/capac.html

For a good animation of a capacitor, go to this web site:

http://micro.magnet.fsu.edu/electromag/java/capacitor/

In the above diagram, when the switch is closed to the left, electrons from the top plate of the capacitor are attracted to the positive terminal of the battery.  Once at the positive terminal, they are in effect "pumped" through the battery.  The electrons then leave the negative terminal and are deposited on the bottom plate.   This process continues until the capacitor is charged.  This occurs when potential difference across the plates is the same as that of the battery.  Once the capacitor is charged, the switch is closed to the right.  Then the electrons travel from the bottom plate back to the top one, momentarily lighting the bulb in the process.

Capacitors are used to store energy, as in the case for sending current to the flash of a camera.  Keys on a keyboard use capacitors as does a defibrillator.  Radios are tuned using capacitors.  The applications are many.

Suppose that the battery in the above picture is a 6 B battery.  Question: Would more or less charge be deposited on the plates if the 6V battery were replaced by, for example, a 12 V battery.  The answer is more electrons would be deposited.  This is because electrons are deposited on the bottom plate until the potential difference across the plates is the same as that of the battery.  As the potential of the battery increases, so does the number of electrons that can be deposited.  Also, if the potential of the electrons decreases, the number of electrons that can be deposited on the plate also decreases.

When using a capacitor, you must pay attention to the maximum voltage which can be used. This is the "breakdown voltage." The breakdown voltage depends on the kind of capacitor being used. You must be especially careful with electrolytic capacitors because the breakdown voltage is comparatively low. The breakdown voltage of electrolytic capacitors is displayed as a Working Voltage.  (Note the 50 V breakdown voltage indicated on the blue capacitor below.)  The breakdown voltage is the voltage that when exceeded will cause the dielectric (insulator) inside the capacitor to break down and conduct. When this happens, the failure can be catastrophic.

Some Capacitors

It should now be clear that the stored charge Q on the plates of a capacitor is directly proportional to the voltage V of the battery. That is

Q = kV.

The constant in this equation is called the capacitance of the capacitor and the symbol C is used in place of the k.  So the equation becomes

Q = CV.

Solving for C, we get C = Q/V.  This suggest that unit for capacitance is the coulomb/volt, which is called a farad, in honor of Michael Faraday.  So,

1 F = 1 C/V.

Just like different springs obey Hooke's Law according to the formula F = kx, where k takes into account the "stiffness" of the spring - Different capacitors obey the equation Q = CV, where C takes into account the extent to which a capacitor can store charge.

In this class, and in your labs, you will work with a parallel plate capacitor, which is made from two metal plates or metal foils separated by an insulator called a dielectric material.  The dielectric materials can be made from ceramic, mica, polypropylene, polyester, electrolytic, tantalum and even air.  The larger capacitors look like tubes, this is because the metal foil plates are rolled up with an insulating dielectric material sandwiched in between.

There are two defining features of a given capacitor that distinguishes it from another one: The size of the plates, measured in square meters, and the distance between the plates.  Experiments have shown that the capacitance of a given capacitor is directly proportional to the area, A,  of the plates and the distance between the plates, d.  That is,

At this point, your instructor will work some examples in class.