Lab 3: Simple DC Circuits
1
Introduction
This lab will allow you to acquire handson experience
with the basic principles of simple electric circuits.
These circuits consist of discrete resistors and light
bulbs that are connected to a DC power supply using
conducting wires. Discrete resistors are usually made
from poor electrical conductors such as carbon. Con
ducting wires have negligible resistance because they
are usually made from copper, which is an excellent
electric conductor.
In the first part of this lab, you will test Ohm’s law,
which describes the properties of resistors. In the next
part, you will verify Kirchhoff’s rules that apply to all
circuits that are in steady state, i.e.
circuits with a
constant current flow. You will discover that it is pos
sible to derive rules for how resistors combine in series
and parallel circuits using Kirchhoff’s rules. The ex
periments will allow you to test your derivations. You
will also learn how the resistance of a wire is related
to its length and area of cross section.
In, the last
part of the lab, you will perform some simple exper
iments with circuits consisting of light bulbs. These
experiments will test your understanding, and allow
you to figure out how these circuits are wired.
EXERCISES 1 AND 2 PERTAIN TO THE
BACKGROUND
CONCEPTS
AND
EXER
CISES
310
PERTAIN
TO
THE
EXPERI
MENTAL SECTIONS.
2
Background
When a (typical) resistor is connected to a power sup
ply, the current
I
flowing through the resistor is pro
portional to the electric potential difference
V
across
the terminals of the resistor.
This relationship is
called Ohm’s law, and is expressed by the following
equation,
I
=
V
R
(1)
Here, R is the resistance. The SI unit of resistance is
the ohm, Ω. Consider a closed circuit loop consisting
MT65
MT66
MT69
MT67
MT68
MT70
MT80MT111MT119MT101MT114MT32MT115MT117MT112MT112MT108MT121
MT82
MT49
MT82
MT50
MT82
MT51
Figure 1: A circuit containing three resistors
of a network of resistors connected to a DC power sup
ply or battery as shown in figure 1. When any closed
loop in this circuit (such as ABCDA or BEFCB) is
traversed, the algebraic sum of the changes in elec
tric potential is equal to zero. This is a statement of
Kirchhoff’s voltage rule
, and it follows from the
law of conservation of energy.
At any junction in the circuit (such as B), the current
flowing into the junction is the same as the sum of
currents flowing out of the junction. This is known as
Kirchhoff’s current rule
, and it is a consequence
of the fact that charge is conserved.
Using Kirchhoff’s rules and Ohm’s law it is possible
to derive rules for how resistors can be combined in
circuits. When a circuit is made up of resistors (
R
1
,
R
2
,
R
3
, etc.) connected in series, it can be shown that
the total or effective resistance
R
T
is just the sum of
the individual resistances,
R
T
=
R
1
+
R
2
+
R
3
(2)
When these resistors are connected in parallel, the
effective resistance is given by,
1
R
T
=
1
R
1
+
1
R
2
+
1
R
3
(3)
The resistance
R
of a cylindrical conductor, such as a
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 Spring '10
 Hor.
 Power, Light, Resistor, power supply, Electrical resistance, Series and parallel circuits

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