E04-1
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
Department of Physics
8.02
Experiment 4: RC Circuits
OBJECTIVES
1.
To explore the time dependent behavior of RC Circuits
2.
To understand how to measure the time constant of such circuits
PRE-LAB READING
INTRODUCTION
In this lab we will continue our investigation of DC circuits, now including, along with
our “battery” and resistors, capacitors (RC circuits). We will measure the relationship
between current and voltage in a capacitor, and study the time dependent behavior of RC
circuits.
The Details:
Capacitors
Capacitors store charge, and develop a voltage drop
V
across them proportional to the
amount of charge
Q
that they have stored:
V
=
Q/C
.
The constant of proportionality
C
is
the capacitance (in Farads = Coulombs/Volt), and determines how easily the capacitor
can store charge.
Typical circuit capacitors range from picofarads (1 pF = 10
-12
F) to
millifarads (1 mF = 10
-3
F).
In this lab we will use microfarad capacitors (1
μ
F = 10
-6
F).
RC Circuits
Consider the circuit shown in Figure 1. The capacitor (initially uncharged) is connected
to a voltage source of constant emf
E
. At
t
= 0, the switch
S
is closed.
Figure 1
(a)
RC
circuit
(b) Circuit diagram for
t
> 0
In class we derived expressions for the time-dependent charge on, voltage across, and
current through the capacitor, but even without solving differential equations a little
(a)
(b)

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