 Site Navigation                            Physics Graphs: Charging a Capacitor

The following graph of charge vs. time represents the growth of charge (Q) on a capacitor while it is attached to a voltage source. The dotted asymptote represents the capacitor's final, or maximum allowed, charge, Qf which equals the product of the capacitor's capacitance (C) and the applied voltage ( ). When a capacitor is placed in a circuit with a resistor, it is call an RC circuit. For every RC circuit there is a specific value known at its "RC time constant" which is equivalent to the product of the resistance in the circuit (R) the capacitance of the capacitor (C).

When a tangent line is drawn at the beginning of a capacitor's charging curve, it will intercept the asymptote representing the capacitor's final charge at t = RC.

Some units that you need to know are:

• charge (Q) is measured in coulombs
• resistance (R) is measured in ohms
• emf ( ) is measured in volts
• capacitance (C) is measured in farads
• time (t) is measured in seconds

General Questions
 If the voltage source, , equals 120 volts and the capacitor, C, equals 100 microfarads, what is the maximum charge this capacitor can hold once the switch is closed? 1

 If the resistor present in our circuit is 500 kilo-ohms, what is circuit's RC time constant?
 2

 The formula used to calculate the amount of charge present on a charging capacitor at any time t, Q(t) is: (a) Use this formula to determine how much charge will be on the capacitor after the switch has been closed for 1 RC time constant. Express your answer to 4 significant digits. (b) What percent of the final maximum charge does this represent?
 3

 (a) How much charge will be on the capacitor after the switch has been closed for 5 minutes? Express your answer to 4 significant digits. (b) What percent of the final maximum charge does this represent?
 4

 (a) How much charge will be on the capacitor after the switch has been closed for 10 minutes? Express your answer to 4 significant digits. (b) What percent of the final maximum charge does this represent?
 5

 Use your previous three answers to describe the behavior of the graph as the time spent charging the capacitor continues to increase.
 6

C Colwell

Show Related AlgebraLab Documents AlgebraLAB Project Manager    Catharine H. Colwell Application Programmers    Jeremy R. Blawn    Mark Acton Copyright © 2003-2021 All rights reserved.