DC Circuits
Objective:
To learn how to build and run an electrical circuit.
To measure current and voltage
anywhere in the circuit. To determine the relationships between Current, Voltage and Resistance. To
learn Kirchoff's Rules
Apparatus:
SnapCircuits board, circuit elements, cables, DC power supply, multimeter (combination
voltmeter/ammeter/ohmmeter), light bulb (
6V limit – do not exceed for a single bulb in series
).
Introduction
So far our labs have focused on electrical fields that have macroscopic effects – a charged rod that
attracts bits of paper or foil, two charged copper spheres or scotch tape strips that attract or repel each
other.
Now we will examine the microscopic effects of electrical fields in wires that conduct
electricity.
You will build simple circuits and learn to measure Voltage, Current and Resistance (both
directly and from the slope of V vs. I.
Theory
The following summarizes what you have have already learned in lecture about circuits:
Electric Potential
The electric field E is related to the rate of
change of electric potential V. If the electric potential
changes by the amount
V
, the electric field in the direction of the displacement is:
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=−
V
s
Current
Current I is defined as the flow of charge Q per until time:
I
=
Q
t
Power
Power P is the rate of energy flow per unit time through a circuit or circuit element:
P
=
VI
=
I
2
R
Ohm's Law
For many a material, the current I through it is proportional to the potential difference across it, with
the resistance R being the constant of proportionality, as well as the slope of the V vs. I graph:
V = IR
Materials that behave this law are known as
ohmic
materials; those that do not are
nonohmic.
The power that is dissipated through a resistor also obeys
Kirchoff's Junction Rule
The current entering any point in a circuit must equal the current leaving that point. This ensures that
there is no charge buildup anywhere, which would violate conservation of charge and energy:
A
junction
is where roads or paths meet or
connect.
In the junction on the right, the current I
1
splits up into
I
2
and
I
3
.
So that there is no excess
buildup of charge or current in the circuit, this
condition must be fulfilled:
I
1
=
I
2
+ I
3
.
or
I
1
 I
2
 I
3
= 0
Kirchoff's Loop Rule
The algebraic sum of all potential differences around any closed loop in a circuit is zero:
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 Fall '08
 RABE
 Physics, Current, Resistance, Resistor, power supply, Electrical resistance

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