Lecture%2011%202220%20Spring%202009 - Physics 2220...

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1 Physics 2220 Announcements: HW 7 is due tomorrow HW 8 is due Friday Exam 1 is next Monday starting at 4:15 p.m. Help room (Physics 117) hours Help room (Physics 117) hours: Monday 4-8 pm Tuesday and Wednesday 3-6:30 pm Thursday 4:30-8 pm Friday 2-6 pm Turn in a piece of paper with name and responses if you don’t have your clicker with you today, or if you haven’t given me your clicker ID yet Energy in a Capacitor – Overview • Consider the circuit to be a system • Before the switch is ld t h i td closed, the energy is stored as chemical energy in the battery • When the switch is closed, the energy is transformed from chemical to electric potential energy Energy Stored in a Capacitor • Assume the capacitor is being charged and, at some point, has a charge q on it The work needed to transfer a charge • The work needed to transfer a charge from one plate to the other is • The total work required is q dW Vdq dq C = 2 0 2 Q qQ Wd q CC == Energy, cont • The work done in charging the capacitor appears as electric potential energy U : 2 2 11 () Q UQ V C V = = Δ • This applies to a capacitor of any geometry • The energy stored increases as the charge increases and as the potential difference increases • In practice, there is a maximum voltage before discharge occurs between the plates 22 2 C Energy, final • The energy can be considered to be stored in the electric field • For a parallel-plate capacitor the energy For a parallel plate capacitor, the energy can be expressed in terms of the field as U = ½ ( ε o Ad ) E 2 • It can also be expressed in terms of the
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Lecture%2011%202220%20Spring%202009 - Physics 2220...

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