 # RC Circuits PH 203 Professor Lee Carkner Lecture 14.

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RC Circuits PH 203 Professor Lee Carkner Lecture 14

Capacitance  Remember that a capacitor stores charge: Q = C  V  C =  0 A/d  The potential across the capacitor can change with time    The voltage across an uncharged capacitor is zero   The voltage across a charged capacitor is equal to the applied potential  V C = 

Resistors and Capacitors  If you connect a uncharged capacitor to a resistor and a battery the capacitor will go from 0 to maximum charge over some time   The current in the circuit will start at a maximum and them go to zero as the capacitor charges   A capacitor C paired with a resistor R has a charging time related to a time constant (  )  = RC  This is the time to charge a capacitor to about 63% of the final value

Charging a Capacitor

Time Curve

Charge Over Time  If we charge a capacitor by connecting it to a battery of voltage , the charge and voltage on the capacitor is: V C =  [1-e (-t/  ) ]  Capacitor charges rapidly at first and then the rate of charge separation slows   At about t = 4  the capacitor is nearly fully charged

Discharging a Capacitor   Energy in the capacitor is dissipated through the resistor  Q C = CV C V C = V 0 e (-t/  )  where V 0 is the initial voltage on the capacitor

Using an Ammeter   Must be placed in series   Has low resistance  Otherwise would decrease the current

Using a Voltmeter   Must be placed in parallel   Has high resistance  So only very little current flows through it

Next Time  Read 28.1-28.6  Problems: Ch 27, P: 47, 49, 56, Ch 28, P: 2, 12

Consider two identical light bulbs in series. If a third identical light bulb is added in series, what happens to the current through the battery and the power that flows from the battery? A)increase, increase B)increase, decrease C)decrease, decrease D)decrease, increase E)Both stay the same

Consider two identical light bulbs in parallel. If a third identical light bulb is added in parallel, what happens to the current through the battery and the power that flows from the battery? A)increase, increase B)increase, decrease C)decrease, decrease D)decrease, increase E)Both stay the same