chapter26

# chapter26 - Charges positive negative conservation...

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Charges positive (+) negative (-) conservation Force (field) Potential (energy) 12 12 12 r r F 2 2 2 2 1 0 2 2 1 4 1 r q q r q q k e πε = = Force between point charges e q = F E Force on charge in the field 0 A E ε Φ q d E = = Connect field with its source: charge = - B A d V s E qV U = Connect field with energy What for? To understand the world Or to move on to capacitor, one of the three passive components in circuits

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Chapter 26 Capacitance and Dielectrics
Capacitors = that which have capacitance to hold = containers Capacitors are devices that store electric charge Any conductors can store electric charge, but Capacitors that specially designed devices to story a lot of charges Examples of where capacitors are used include: radio receivers filters in power supplies to eliminate sparking in automobile ignition systems energy-storing devices in electronic flashes How is my English?

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Capacitance The capacitance , C , is defined as the ratio of the amount of the charge Q on the conductor to the potential increase ΔV of the conductor because of the charge: This ratio is an indicator of the capability that the object can hold charges. It is a constant once the object is given, regardless there is charge on the object or not. This is like the capacitance of a mug which does not depend on there is water in it or not. The SI unit of capacitance is the farad (F) V Q C Q V C= 1V 1C F 1 =
More About Capacitance Capacitance will always be a positive quantity The capacitance of a given capacitor is constant The capacitance is a measure of the capacitor’s ability to store charge The farad is an extremely large unit, typically you will see microfarads ( μ F=10 -6 F), nanofarads (nF=10 -19 F), and picofarads (pF=10 -12 F)

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Capacitance of a one conductor system is small: for example, Isolated Sphere Assume a spherical charged conductor with radius R The sphere will have the same capacitance as it would if there were a conducting sphere of infinite radius, concentric with the original sphere Assume V = 0 for the infinitely large shell Even for R =1m, C = 0.1 nF Note, this is independent of the charge and the potential difference e e k R R Q k Q V Q C = =
How to Make a Capacitor? Requirements: Hold charges The potential increase does not appear outside of the device, hence no influence to other devices. Is there such a good thing? For this parallel plate capacitor, the capacitance is area surface the is , 0 0 A d A d A Ed A V Q C ε ε σ σ σ = = = σ 0 2 ε σ = E σ - 0 0 2 2 ε σ ε σ = - = E 0 = E 0 ε σ = E 0 = E d Ed V =

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How to Make a Capacitor? Requirements: Hold charges The potential increase does not appear outside of the device, hence no influence to other devices.
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