Lesson_3.1_Printable_PPT

# Lesson_3.1_Printable_PPT - Capacitance As a conductor gets...

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Capacitance As a conductor gets charged, its potential increases. The amount of charge given to a conductor to increase its potential by 1 volt is called the capacitance of the conductor. We use C to represent capacitance. Since capacitance is Q charge per unit volt, C V = Since Q is measured in coulombs and V is measured in volts, C is measured in coulombs per volt ( C.V -1 ) and is called a farad ( F ). Since farad is a very large unit we very often use microfarad ( μ F = 10 -6 F ), nanofarad ( nF = 10 -9 F ) and picofarad ( pF = 10 -12 F ) which are the submultiples of a farad.

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The potential of a spherical conductor of radius R that carries a charge Q is: 0 4 Q V R πε = Capacitance of a spherical conductor is: Q Q 4 ε C V = 0 4 Q R = = 4 πε πε πε πε 0 R
Capacitor Consider a conductor carrying a charge Q so that its potential is V volts (a in the figure) + C=Q/V Q C = + - - - - - - + + + + + + + neutral conductor The capacitance of e conductor is: Now bring a neutral conductor near the charged conductor (b in the figure) The negative charges in the neutral conductor move so that they reside closer to the positively charged conductor making the other end of the neutral conductor positive. (a) V - + (b) the conductor is:

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+ C=Q/V + - - - - - - + + + + + + + neutral conductor The accumulation of negative charges near the positively charged object lowers its positive potential to V’ so that V’ < V. This decrease in potential of the charged conductor without changing the amount of charge on it causes an increase in its capacitance. (a) - + (b) In order to increase the potential back to V , more positive charges need to be given to the conductor. Thus the presence of a neutral conductor near the charged conductor raises the capacitance of the charged conductor.
conductor is grounded? The negative charges from the ground flows into the neutral conductor neutralizing the positive charges. This leaves the neutral conductor + - - - - - - - neutral conductor + + + + + + + now negatively charged. This lowers the positive potential of the charged conductor further, again increasing its capacitance. grounded conductor near it. + - - +

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## This note was uploaded on 05/01/2011 for the course PHY 2049 taught by Professor George during the Spring '11 term at Edison State College.

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Lesson_3.1_Printable_PPT - Capacitance As a conductor gets...

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