RC CIRCUIT


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OBJECTIVES:

1) Observe the charge up and decay of the voltage on a capacitor.
2) Measure the time constant for the decay, t = RC.
3) Observe that the sum of the voltage on the resistor and the capacitor is always equal
to the applied voltage.

APPARATUS:

Signal Generator
Oscilloscope
Resistors and Capacitors
Circuit Breadboard and Cable

INTRODUCTION

The Capacitor
A capacitor is a device that can store electrical charge. The simplest kind is a "parallel plate"
capacitor that consists of two metal plates that are separated by an insulating material such as dry
air, plastic or ceramic. Such a device is shown schematically below.


What's going on in this RC Circuit

1) Initially, the switch is open, and no current is flowing.
2) The switch is closed as in Figure 2. (a). The capacitor will charge up, its voltage will increase.
During this time, a current will flow, producing a voltage across the resistor according to Ohm’s
Law, VR = IR. As the capacitor is being charged up, the current will be decreasing, with a
certain time constant t, due the stored charge producing a voltage across the capacitor.


Kirchoff’s voltage law for AC circuits
In the Ohm’s Law, Part B, experiment, we saw that ‘voltage drops’ across the circuit elements
added up to the voltage applied to the circuit. If you choose a particular path to loop around the
circuit and count the voltage as positive if it increases in that direction and negative if it decreases,
this would be the same as saying that the sum of these voltages is zero around a loop. This same
law applies instantaneously, at each individual time, in a time-varying circuit such as the RC circuit.
Looking at figure 6, you can see that at each instant, Vc + VR = Vapp . We will also demonstrate
this in our circuit by measuring Vc ,VR and Vapp at various times.