Concise Physical Chemistry

c18 JWBS043-Rogers September 13, 2010 11:29 Printer Name: Yet to Come

294 EXPERIMENTAL DETERMINATION OF MOLECULAR STRUCTURE

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Field

FIGURE 18.6 A charged parallel plate capacitor.

18.7 DIPOLE MOMENTS

If a parallel plate capacitor with a vacuum between its plates is brought up to a charge

ofQcoulombs by imposing a potential differenceVvolts between the plates, its

capacitanceisC 0 (Fig. 18.6):

C 0 =

Q

V

whereC 0 , as the name implies, is analogous to the capacity of a container. It tells

how much charge the capacitor can hold per volt of potential difference.

There are two ways of increasing the capacitance of a capacitor: (i) by making

the plates larger or (ii) by allowing the space between the plates to be filled by some

substance called adielectric(Fig. 18.7). All substances have two poles, a positive

pole and a negative electrical pole, when they are in an electrical field. Thesedipoles

align themselves in opposition to the field between the plates, so they reduce the field.

The potential difference between the plates is decreased, so more charge can be put

into the capacitor to reestablish the original potential. Since the capacitor can hold

more charge, its capacitanceCxwith substancexbetween the plates is always greater

than the capacitanceC 0 with the apparatus pumped out to a vacuum.

Field

Dipole

Vectors

FIGURE 18.7 A charged capacitor with a dielectric.