CAPACITORS PES 1000 PHYSICS IN EVERYDAY LIFE WHAT IS A CAPACITOR? A capacitor can be formed from any ungrounded conductor or pair of conductors. A very common capacitor is made from two parallel plates of metal. Charging a capacitor consists of using a battery to move charge onto one of the conductors until the maximum potential of the battery is reached. The charge usually is transferred from the other conductor, leaving it with an opposite charge. The charged capacitor is now storing energy because the separated charges have the potential to rejoin. The energy is being stored within the electric field that exists between the conductors. A capacitor is different from a battery because it can discharge (releasing the stored energy) in a very short time interval. Batteries are designed not to do this, but to deliver their energy at a slow,

constant rate. CAPACITANCE The ability of a capacitor to store energy is characterized by its capacitance. The capacitance is defined as the amount of positive charge moved per unit voltage. The capacitance is entirely dependent on the shape and relative positions of the conductors, and not how much or which sign of charge is on them at any particular time. A battery with twice the voltage will move twice the charge, so the doubling cancels out. The variable used to measure capacitance is C (not to be confused with the abbreviation for the unit of charge, the Coulomb). Capacitance is measured in units of Farads (F). This name may sound familiar because it was named after Michael Faraday. 1 Farad = 1 Coulomb / 1 Volt Some common capacitor geometries: Parallel plates, Nested spheres, parallel strips rolled into a cylinder, etc. =/ DIELECTRIC

The closer the plates, the greater the capacitance, BUT the separated charge can accidentally rejoin if any part of one conductor touches the other. One way to keep them separated is to put any insulating material between them. Many materials can be used. The general name for this material is dielectric. Not only does the dielectric make the capacitor less likely to accidentally discharge, it actually increases the capacitance in two ways. It allows the conductors to be placed closer together, which increases capacitance. It weakens the electric field, which allows more charge to be transferred to the conductor. If the voltage between the conductors is strong enough or if there is a flaw in the dielectric, then the charge will jump between the conductors. This is called dielectric breakdown. LIGHTNING Dielectric breakdown occurs during a lightning storm. The friction between clouds and air transfers charge. One cloud could build up a large positive charge, and the ground below would build up a negative charge in response. The charge is separated by air (an insulator), so the charge remains separate until the voltage reaches enough to cause the dielectric breakdown of the air.

+++++++++++++ The electric charge jumps from cloud to ground in the form of a lightning bolt. The distance of air is less between tall structures and the cloud, so lightning tends to strike them first. Lightning rods protect the structures to which they are attached in two ways: If lightning strikes, the grounded rod transfers it to the ground without passing through the structure, thus avoiding fire or electrical damage. The rod lowers the chance of a strike to begin with. The charge from the ground tends to flow up to the tip of the rod, which is a sharp point. The charge then escapes from the rod onto nearby particles (coronal discharge), thus dissipating the charge, lowering the voltage, and reducing the chance of a strike. - - - - - - - - - - - - C2 C1

CAPACITORS IN PARALLEL AND SERIES The symbol for a capacitor in a circuit looks like two parallel plates. As with resistors, collections of capacitors in a circuit can be replaced with a single capacitance for the system. Pairs of capacitors can be combined using the series and parallel rules: CEq 1 1 1 = + 1 2 Series capacitance decreases C1

Parallel capacitance accumulates This is opposite of resistors: resistors in series collectively increase the overall resistance, but in parallel, the collective resistance drops. C2 CEq = 1+ 2 USES FOR CAPACITORS In any system that needs energy delivered rapidly, a capacitor can be used. The capacitor (or a bank of capacitors) is charged gradually over time from the power source. The capacitor or capacitors can then be discharged all at once, supplying more energy in a short interval than the power source could have on its own. Application: Flash for a camera A capacitor is charged from the battery. Then the energy is dumped quickly through the bulb, creating a flash of light. Application: High-power physics experiments A bank of capacitors is charged from

the city power lines. The energy is delivered quickly to the experiment. Banks of capacitors are used in an alternating current power delivery system. Capacitors are used to regulate the oscillation of the alternating current to compensate for other components in the system and deliver power most efficiently. Another use of capacitors is within sensors. CAPACITIVE SENSORS Q Since the capacitance of a pair of conductors depends on their geometry, then changes in their geometry can be detected by looking for a change in their capacitance. Imagine two parallel plates separated by a distance with a flexible dielectric. Using a battery, charge is transferred from one to the other, and the battery is disconnected. The voltage between the plates is the same as the batterys original voltage. Now imagine that the plates are moved closer together. This increases their capacitance. Since the amount of charge hasnt changed, the voltage between them must decrease.

So detecting a voltage drop means the plates have been pushed together. This is the principle behind computer keys, for instance. The key press causes a voltage drop which the computer detects. Other versions of capacitive sensors are used in touch screens, stud detectors, etc. Or CAPACITOR SIMULATION Link to simulation: Things to do: On the Intro tab: Check the Capacitance and Electric Field Lines boxes. Observe the capacitance change as the Separation

distance changes and as the Plate Area changes. On the Dielectric tab: Check the Capacitance and Electric Field Lines Observe the capacitance change as you slide the Dielectric between the plates. On the Multiple Capacitors Check the Total Capacitance box.

Observe the capacitance of different circuits. CONCLUSION A capacitor is composed of two conductors that can hold opposite charges, thereby storing electrical energy. Capacitance is the measure of the energy storage capacity. It is measured in Farads. Material placed between the conductors (a dielectric) increases both the structural strength and the capacitance. Capacitors can be connected in parallel and series, and have combination rules similar but distinct to the rules for resistors. Capacitors are useful whenever a quick delivery of power is needed or to modulate AC systems.

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