Exploration 30.6:  RC Time Constant

battery = V      capacitor = μF        resistor = Ω          

Please wait for the animation to completely load.

In this animation you can close and open switches to see what happens to the voltage across the capacitor (red), the voltage across the resistor (green), and the total voltage across the capacitor plus resistor (blue).  Initially, the capacitor is charged.  After pushing "play," you should throw the switches (voltage is given in volts and time is given in seconds).   Restart. 

Set the switches so that you can get a good graph of the capacitor discharging and charging. 

  1. How much time (approximately) does it take the capacitor to charge and discharge? 
  2. Double the battery voltage.  How much time does it take to charge and discharge?
  3. Double the capacitance and measure the time to charge and discharge. 
  4. Double the resistance and measure the time to charge and discharge.

The value of RC (resistance times capacitance) is the RC time constant for the circuit and is a characteristic time.  Set the battery voltage to 1 V. 

  1. When the time equals R*C after throwing the switch, what is the capacitor voltage when it is discharging?  When it is charging?
  2. Compare your measurements to the values found from the equations for a charging or discharging capacitor:

Charging: V = V0 (1 - e -t/RC)           Discharging: V = V0 e -t/RC    

 

 

Exploration authored by Anne J. Cox.
Script authored by Wolfgang Christian and Anne J. Cox.
Instructor's Resource CD Edition: Do not post or distribute.
The complete version of Physlet Physics is available as a text with CD; Physlet Physics can be bundled with this Prentice Hall textbook.
© 2004 by Prentice-Hall, Inc. A Pearson Company