Capacitance and Dielectrics



When you set an old-fashioned spring mousetrap or pull back the string of an archer’s bow, you are storing mechanical energy as elastic potential energy. A capacitor is a device that stores electric potential energy and electric charge. To make a capacitor, just insulate two conductors from each other. To store energy in this device, transfer charge from one conductorto the other so that one has a negative charge and the other has an amount of positive charge. Work must be done to move the charges gh the resulting potential difference between the conductors, and done is stored as electric potential energy. Capacitors have a tremendous number of applications. In electronic used by photographers or in energy-storage units for pulsed the energy and charge stored in a capacitor are recovered quickly. applications the energy is released more slowly. Springs in the of an automobile help smooth out the ride by absorbing the :from sudden jolts and releasing that energy gradually; in an anal- , a capacitor in an electronic circuit can protect sensitive  by smoothing out variations in voltage due to power surges. the presence of a spring in a mechanical system gives that  frequency at which it responds most strongly to an periodic force, the presence of a capacitor in an electric circuit a natural frequency for current oscillations.

This idea is circuits such as those in radio and television receivers, to broadcast signals at one particular frequency and other frequencies. also have practical effects that are not desirable. Adjacent  of a computer chip act like a capacitor, and the capacitors useful for smoothing out voltage the rate at which the potentials of the chip’s pins can limits the maximum speed at which the chip can an unpleasant effect that becomes more important as become smaller and are pushed to operate at faster  we’ll study the fundamental properties of capacitors. many of their most important applications in later chap-  Chapter 32, in which we’ll see the crucial role played  alternating-current circuits that pervade our   Rx any particular capacitor the ratio of the charge on  ID the potential difference between the conductors is a capacitance. The capacitance depends on the sizes conductors and on the insulating material (if any)  to the case in which there is only vacuum ,the capacitance increases when an insulating  is present This happens because a redistribution of , takes place within the insulating material .  tJObriiZ3lionwill give us added insight into the electrical.

Capacitors also give us a new way to think about electric potential energy. The energy stored in a charged capacitor is related to the electric field in the space between the conductors. We will see that electric potential energy can be regarded as being stored in the field itself. The idea that the electric field is itself a storehouse of energy is at the heart of the theory of electromagnetic waves and our modem understanding of the nature of light, to be discussed in Chapter 33

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