PC1222 Fundamentals of Physics II
Basic Circuits
1
Objective
•
Investigate the relationship among three variables (resistance, current and voltage) in
direct current circuits.
•
Investigate the circuit characteristics of resistors in series and parallel arrangements.
2
Equipment
•
PASCO Circuits Experiment Boards
•
Digital multimeter (DMM)
•
Dcell batteries
•
Resistors
•
Wire leads
3
Theory
3.1
Resistance and Ohm’s law
When a voltage or potential difference (
Δ
V
) is applied across a material, the current (
I
) through
the material is found to be proportional to the voltage,
I
∝ Δ
V
.
The resistance (
R
) of the
material is defined as the ratio of the applied voltage and the resulting current – that is
R
=
Δ
V
I
(definition of resistance)
(1)
For many materials, the resistance is constant, or at least approximately so, over a range of
voltages. A resistor that has constant resistance is said to obey Ohm’s law or to be “ohmic”.
From equation (1), it can be seen that the unit of resistance is volt/ampere (V/A). However,
the combined unit is called the ohm (
Ω
), in honor of Georg Ohm.
A plot of
Δ
V
versus
I
for an ohmic material is a straight line.
Materials that do not
obey Ohm’s law are said to be “nonohmic” and have a nonlinear voltagecurrent relation
ship. Semiconductors and transistors are nonohmic.
Physics level 1 laboratory
Page 1 of 8
Semester I, 2011/12
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PC1222: Basic Circuits
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In common practice,
Ohm’s law
is written as
Δ
V
=
IR
(Ohm’s law)
(2)
where it is understood that
R
is independent of
Δ
V
. Keep in mind that Ohm’s law is not a
fundamental law such as Newton’s law of gravitation. It is a special case, there being no law
that materials must have constant resistance.
In an electrical circuit, a voltage source (e.g., a battery or power supply) supplies the
voltage for charge flow and the magnitude of the current is determined by the resistance
R
in the circuit.
For a given voltage, the greater the resistance, the less current through the
resistance, as may be seen from Ohm’s law,
I
=
Δ
V
/
R
.
In an electrical circuit with two or more resistances and a single voltage source, Ohm’s
law may be applied to the entire circuit or to any portion of the circuit. When it is applied to
the entire circuit, the voltage is the terminal input voltage supplied by the voltage source and
the resistance is the total resistance of the circuit. When Ohm’s law is applied to a particular
portion of the circuit, the individual voltages, currents and resistances are used for that part of
the circuit.
Figure 1: Resistor colour code.
For resistors routinely used in electronic instrumentation, resistance is coded by a series
of coloured bands on the resistor. The key to the resistor colour coding system is given in
Figure 1. The four bands are placed with three equally spaced bands close to one end of the
resistor followed by a space and then a forth band. The first two bands are the first two digits
in the value of the resistor and the third band gives the exponent of the power of 10 to be
multiplied by the first two digits. Thus a resistor with its first three bands labelled Yellow
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 Spring '11
 Tan
 Direct Current, Gate, Parallel Circuit, Series Circuit, Volt, Resistor, Electrical resistance, Series and parallel circuits

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