Consider the RC circuit in the figure below. The switch was at position a for a

ID: 2121576 • Letter: C

Question

Consider the RC circuit in the figure below. The switch was at position a for a long period of time and it is suddenly switched to position b at time t = 0.

http://loncapa2.physics.sc.edu/res/msu/physicslib/msuphysicslib/60_CircuitsTransient_RC/graphics/prob01a_rccircuit.gif

For each statement select True or False.

TrueFalse The current through the resistor equals the current across the capacitor at all times.
TrueFalse In the instant after the switch is thrown the current across the capacitor is zero.
TrueFalse In the instant after the switch is thrown, the voltage across the capacitor is zero.
TrueFalse In the instant after the switch is thrown the voltage across the resistor is zero.

What is Vc at time t = 2.00 ms?
Use the following data : R = 432.5 %u2126, C = 4.00 %u03BCF, Vb = 3.65 V.

What is the total energy dissipated by the resistor R after the switch is thrown?

Using the same data as before, consider the case when the switch is in position b for a long time, and then it is switched to position a at time t = 0. What is Vc at time t = 2.00 ms?

Explanation / Answer

1) In the instant after the switch is thrown the current across the capacitor is zero.

FALSE: The switch has been closed for a long period of time so there is a charge on the capacitor, and there is current flowing through the circuit as the capacitor discharges.

2) The current through the resistor equals the current across the capacitor at all times.

TRUE: Devices are in series and series circuit current is always constant

3) In the instant after the switch is thrown, the voltage across the capacitor is zero.

FALSE: Capacitor is charged, so when the switch is closed the capacitor begins to discharge, so there is a voltage change.

4) In the instant after the switch is thrown the voltage across the resistor is zero.

FALSE: There is a voltage change across the capacitor plates, so there will be a voltage change across the resistor.