# Today5/7  Questions?  Circuits and Capacitors  Course Evaluations.

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Today5/7  Questions?  Circuits and Capacitors  Course Evaluations

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

Capacitors and Energy How many Joules of energy are stored in the capacitor? 12 V 6F6F 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

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!! (+/-)

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!! (+/-)

Capacitors with Resistors Will current flow when the switch is closed? 12 V 6F6F 66 6F6F Yes, but only for an instant until the capacitor is charged. Yes, but it will take longer to charge the capacitor.

Capacitors with Resistors Describe current and voltage long after the switch has been closed. 12 V 6F6F 66 6F6F 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!

Capacitors with Resistors Describe current and voltage long after the switch has been closed. 12 V 6F6F 22 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! 44

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