01/02/2013
OHM'S LAW-1
Purpose
To study Ohm's law as applied to a "linear" DC circuit.
Theory
The resistance R of a conductor depends upon a number of factors including its nature,
dimensions and even temperature. Most conductors have a constant resistance at constant
temperature, so that the current I produced through the conductor is directly proportional to
the voltage V across it. The relationship called Ohm's law states that
I = V / R (R constant).
(1)
The resistance R of the conductor is measured in Ohm (  ), thus
1  = 1 Volt / 1 Ampere.
The equivalent resistance Rs of two resistances R1 and R2 connected in series, as shown in
Figure 1-a, is given by
Rs = R1 + R2.
(2)
In the parallel configuration, shown in Figure 1-b, the equivalent resistance Rp is given by
1/Rp = 1/R1 + 1/R2,
or
Rp = (R1 R2) / (R1 + R2).
(3)
For some conductors the current I is not proportional to V. For example, for metallic
conductors, if I is very large it will cause heating of the conductor so that it will be no longer
proportional to V. In that case, the conductor is called a non-ohmic one since it no longer
obeys Ohm's law.
© KFUPM – PHYSICS
revised 01/02/2013
1
Department of Physics
Dhahran 31261
01/02/2013
Experimental Set-up
The basic circuit used in this experiment comprises a d.c. power supply, an ammeter, a
voltmeter and the resistance(s) to be studied, as shown in Figure 2.
The ammeter (here a multimeter is used as an ammeter) will measure the current I flowing
through resistor R. It is therefore connected in series with the resistor. The voltmeter, which
will measure the potential difference V across the resistor, is connected in parallel with the
resistor.
The polarities of the power supply, the ammeter and the voltmeter are clearly indicated in
Figure 2. They must be respected or one would run the risk of damaging either the ammeter
or the voltmeter, or both.
Procedure
A.
Measurement of Single Resistance
1.
Connect the circuit as shown in Figure 2. R1 represents one of the two unknown
resistors provided.
2.
Switch on the power supply and vary the input voltage in regular steps between 0 and
5 volts. Record the voltage across R1 and the current I.
Note: Do not use the dial on the power supply to read the voltage, use the voltmeter.
3.
Plot a graph of I versus V and verify that a straight line is obtained. Find the value of
R1 from the slope of the graph.
4.
Replace R1 by R2. Measure the current I for, say, three values of the voltage V.
Calculate the average value of R2.
© KFUPM – PHYSICS
revised 01/02/2013
2
Department of Physics
Dhahran 31261
01/02/2013
B.
Measurement of Resistances in Series and Parallel
5.
Connect R1 and R2 in series (see Figure 1-a) and, using the circuit shown in
Figure 2, measure the current I for, say, three values of the voltage V.
Calculate the average value of the equivalent resistance Rs.
6.
Connect R1 and R2 in parallel (see Figure 1-b) and, using the circuit shown in
Figure 2, measure the current I for, say, three values of the voltage V.
Calculate the average value of the equivalent resistance Rp.
7.
Using the values of R1 and R2 obtained in part A, calculate the equivalent
series and parallel resistances, Rs and Rp using equations 2 & 3. Compare
these results with those found in steps 5 and 6, respectively.
8.
Calculate the % difference between the calculated values and the experimental
values.
Exercise
Assume that the percentage error in each of R1 and R2 is 5%,
R1 / R1 (100) = R2 / R2 (100) = 5% , use your results for R1 and R2 to calculate:
i.e.
a.
The maximum possible error in Rs, i.e., Rs when the two resistances are connected
in series.
b.
The maximum possible error in Rp, i.e. Rp when the two resistances are connected
in parallel. (Note that when calculating Rp from Rp = R1R2/(R1+R2), you have to use:
R p 
R p
R1
R1 
R p
R 2
R 2 . You can’t use the shortcut: Z=A/B and relationship
Z/Z=A/A+ B/B, can you explain why? ).
© KFUPM – PHYSICS
revised 01/02/2013
3
Department of Physics
Dhahran 31261