AIR UNIVERSITY
DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
EXPERIMENT NO 2
VERIFCATION OF OHM’S LAW
Lab Title:
Student Name:
Reg. No:
Objective:
LAB ASSESSMENT:
Attributes
Excellent
(5)
Good
(4)
Average
(3)
Satisfactory
(2)
Unsatisfactory
(1)
Satisfactory
(2)
Unsatisfactory
(1)
Ability to Conduct
Experiment
Ability to assimilate the
results
Effective use of lab
equipment and follows the
lab safety rules
Total Marks:
Obtained Marks:
LAB REPORT ASSESSMENT:
Attributes
Excellent
(5)
Good
(4)
Average
(3)
Data presentation
Experimental results
Conclusion
Total Marks:
Date:
Obtained Marks:
Signature:
LABORATORY
EXPERIMENT
NO. 02
VERIFICATION OF OHM’S LAW
Objectives:
1. To verify Ohm’s Law by calculating, and then by measuring voltage, current and resistance, and
then comparing the calculated and measured results.
2. To verify Ohm’s Law relationships by increasing one quantity while holding the second quantity
constant, then measuring and calculating the effect on the third quantity.
3. To demonstrate the three power formulae in DC circuits.
DISCUSSION
Ohm’s Law:
Ohm’s Law is the basis of many electrical circuit calculations and is one of the most important
theories you will learn: V=IR. The purpose of this experiment is to verify Ohm’s Law, which in
words, simply says that the current through a resistor is proportional to the voltage across it. The
way in which we accomplish this is to measure the voltage across and current through a known
resistor for several different pair of values. We can then plot the data on a graph, and if the
relationship is truly linear it should yield a straight line.
When graphing data such as those obtained in this experiment, either the x or y-axis can be
chosen to display the voltage or current values. When the y-axis is chosen as the voltage axis,
and the x-axis as the current axis, we say that we are plotting V versus I. the slope of the
lineΔV/ΔI should be equal to the resistance R of the resistor.
If on the other hand, current is plotted on the y-axis, and voltage along the x-axis, then slope of
the line ΔI/ΔV is equal to the conductance G of the resistor.
In this experiment you will plot I in mA versus V in volts, and therefore the slope will be the
conductance of R. When plotting a straight line on a graph such as this, it is important that you
draw the best straight line that you can through the data points.
Ohmic and Non-Ohmic Devices:
For some materials, the resistance is constant no matter how much voltage is applied across it.
These materials are said to obey Ohm's Law and are called ohmic devices. Since the resistance
(R) is constant, a plot of voltage (V) vs. current (I) yields a straight line as shown. Notice that
resistance is always the ratio of voltage across a device to the current through the device. But the
resistance is constant only for those materials that obey Ohm's Law. The three forms of Ohm’s
Law are:
I=V / R, V=IR and R= V / I
However, there are devices which do not obey Ohms' law i.e., they are non-ohmic. An example
of a non ohmic device is a filament bulb.
Let us apply a certain potential difference across the terminals of a filament lamp and measure
the resulting current passing through it. If we repeat the measurement for different values of
potential difference and draw a graph of voltage V versus current I, it will be seen that the graph
is not a straight line. It means that a filament is a non ohmic device. This deviation of I- V graph
from straight line is due to the increase in the resistance of the filament with temperature. When
the temperature of the conductor increases, the collisions between electrons and ions increase.
Thus, as a substance heats up because of electricity flowing through it (or by any heating
process), the resistance will increase. As the current passing through the filament is increased
from zero, the graph is a straight line in the initial stage because the change in the resistance of
the filament with temperature due to small current is not appreciable. As the current is further
increased, the resistance of the filament continues to increase due to rise in its temperature.
Power in DC Circuits
Electronic devices and circuits require energy to operate. Power is a measure (in watts) of the
energy (in joules) consumed by a given device in one second. For a resistor, three equations will
yield the power dissipated: P=IV, P=V²/R and P=I²R. In this experiment, you will verify these
formulae and plot graphs of the power versus the current, and then power versus the voltage. The
resulting curves are parabolas, and the equations of the curves are called quadratic. For Example;
By plotting two curves for both 1 Ωk and 390 Ω resistors on the same scales and axes, you will
be able to see which resistance consumes more power for a given current (or voltage).
Lab Task:
1. Perform the following steps to verify that
I=V/R
a. Measure the resistance of the 6.8 kΩ (color-coded value) resistor with the DMM.
R1measured =
,
b. Connect the circuit as in the figure below.
c. Adjust the power supply voltage to 15.0 V using the DMM set on its DC voltage mode.
d. Measure and record the voltage across the R1 resistor) VR1=
Make sure that the voltage across R1 is still 15V.
e. Measure and record the current through R1. (as shown in Figure).
IR1 =
P =VI =
f. Using the measured values of voltage and resistance, calculate the current through R1
using Ohm’s Law. Show calculations in the space below.
I = V/R =
IR1 =
g. What conclusion can you make from these procedures?
2. Perform the following steps to verify that
R =V / I
a. Connect the same circuit as in task 1, except change the value of resistance to 3.9 kΩ (colorcoded value).
b. Measure and record the voltage across R1 and the current flow through R1.
VR1 =
IR1 =
c. Calculate the resistance of R1 using the measured values of VR1 and IR1 with Ohm’s Law.
Show calculation. R1 =
d. Remove R1 from the circuit. Measure and record the resistance of R1 using the multimeter.
R1 =
P=I2R=
e. What conclusion can you make from these procedures?
3. Perform the following procedures to verify that
V = I ×R
a. Connect the circuit as in task 1 and 2, except change the resistor to 10 kΩ. (color-coded value)
b. Measure the resistance of this resistor making sure that the resistor is removed from the
circuit.
R1 =
c. Reinsert the resistor into the circuit and measure the current through the resistor.
IR1 =
d. Calculate, using Ohm’s law, the resistor voltage using the measured values of resistance and
current. Show calculation below.
V=I×R=
mA ×
Ω=
V
e. Measure the resistor voltage. Record
VR1 =
P= V2 / R=
f. What conclusion can you make from these procedures?
4. Perform the following procedures to verify that
IF THE RESISTANCE IS HELD CONSTANT, INCREASING THE VOLTAGE WILL
INCREASE THE CURRENT.
a. Connect the circuit as in task 1,2 and 3 using a 4.7 kΩ resistor.
b. Set the voltage source to 2.0 V.
c. Measure the current through the resistor. Record this value in the following table.
d. Repeat this procedure from 2.0V to 30.0V. And note down the required values in the table
given below.
IR1
PR1
R1
VR1
0V
2V
4V
6V
8V
10V
12V
14V
16V
18V
20V
22V
24V
25V
26V
28V
30V
5. Plot the following.
a) Draw relation between “V” & “I” on graph paper.
b) Draw relation between “P” & “V” on graph paper.
c) Draw relation between “P” & “I” on graph paper.
Conclusion:
4.7kΩ