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Today5/7 Questions? Circuits and Capacitors Course Evaluations

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Capacitors Capacitance tells me how many coulombs of charge are stored in a capacitor when it has 1 Volt across it. A 25 F capacitor will have 25 C of charge on it when it has 1 volt across it. Think of capacitance as “coulombs per volt”. Units:coulombs per volt or “Farads” Equation: C = q/V

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Capacitors and Energy How many Joules of energy are stored in the capacitor? 12 V 6F6F As the battery moves charge from one plate to another, the potential difference increases from zero to 12 V. The average is 6 V and the energy stored is q times 6 V (q V ave ). Energy = q V ave = 1 / 2 q V = 1 / 2 C V 2 C = q/ V

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Kirchhoff’s Rules, Loop The sum of all voltages around any closed loop is zero. (what goes up must come down) or V A,A = 0 !!must keep track of ups and downs!! (+/-)

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Kirchhoff’s Rules, Junction The sum of all currents at any junction is zero. (what goes in must come out) !!must keep track of ins and outs!! (+/-)

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Capacitors with Resistors Will current flow when the switch is closed? 12 V 6F6F 66 6F6F Yes, but only for an instant until the capacitor is charged. Yes, but it will take longer to charge the capacitor.

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Capacitors with Resistors Describe current and voltage long after the switch has been closed. 12 V 6F6F 66 6F6F No current, 12 V across the capacitor. No current, 12 V across the capacitor, zero V across the resistor. Loop rule still applies! V = IR for resistors still applies!

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Capacitors with Resistors Describe current and voltage long after the switch has been closed. 12 V 6F6F 22 2 Amps through the battery and both resistors. 4 V across 2 and 8 V across the capacitor and 4 Loop rule still applies! V = IR for resistors still applies! 44

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