Laboratory # 3:

OHM’S LAW
 Theory:
The rate of flow of electricity in a given circuit is called current, denoted as I, the potential
difference between the starting and the ending points of the circuit is known as the voltage
denoted as V and the opposition to the flow of current is called resistance, denoted as R.
According to Ohms law the ratio, V and I is always a constant factor for a particular conductor
when the temperature, length, and conductor material is kept constant. This constant is called the
Resistance that is characteristic of the conductor used. This is denoted by the formula
Voltag V
 cons tan t 
e ; R I
Curren
t

The unit of EMF is Volts, the unit of Current is Amperes and the unit of Resistance is Ohms.
Electricity is measured in these units. Ohms law has wide applications in electrical circuits
obeying Kirchhoff’s Laws, heat generation, Chemical analysis deposits, and most importantly for
deriving Light energy.

Ohms Law for DC circuits is

R since the current flow is a steady one, but for AC, the
V
I
current flow is a fluctuating one with some frequency, in such case the frequency is also taken
into consideration, here the Resistance of the circuit is called the Impedance represented by Z.
Ohms Law applies to the conductors whose resistance is independent of voltage. When an I,V
graph is drawn, it is seen the Ohms law is obeyed in the linear portion for ohmic resistors, for
non-ohmic resistance substances the I,V graph is bent and curved, showing negative resistance
properties like incandescent lamps where more voltage is applied, more heat is generated and the
resistance rises.
 Objectives:
 To study ohm’s law and prove experimentally that current is proportional to the voltage
across a dc circuit and to show that the proportionality constant is equal to the
resistance of the circuit.
 To study the relation between current and resistance in a dc voltages across the circuit.

 Materials and Equipment:

1. Digital Voltmeters dc and Ac
2. DC Power supply
3. Resistors variable in size

1
4. Circuit construction board
5. Connecting wires

2
 Procedure:
1. Adjust the power supply to deliver 1V (or as close as possible) use separate dc voltmeters for
this adjustment.

Vs R V

Figure a

2. Construct the dc circuit shown in diagram Figure a, and connect it to the 1V power supply.
3. Measure R a n d the voltage drop across R (VR) and record in data table 3.1 in the
appropriate column.
4. Calculate IT and record in the appropriate column in data table 3.1. Show your calculation.
5. Repeat step 3 and 4 by increasing the power supply in steps of one volt
6. Substitute the values of R in data table 3.2 and calculate the corresponding circuit currents
& record this information in the appropriate column in data table 3.42by fixing the voltage
source for all resistors

V
R = 22kΩ I = V = IR
R

 Data Table 3.1:

Source voltage VR IT (calc.)
1 1v 0.045 mA
2 2v 0.091 mA
3 3v 0.136 mA
4 4v 0.182 mA
5 5v 0.227 mA
6 6v 0.273 mA
7 7v 0.318 mA
8 8v 0.364 mA
9 9v 0.409 mA
10 10v 0.455 mA

3
  Data Table 3.2:
R1 Vs(fix) IT (calc.) VR

22k 5v 0.227 mA 5v

100k 5v 0.05 mA 5v

2.3M 5v 0.0023 mA 5v

80.6k 5v 0.062 mA 5v

81.2k 5v 0.0616 mA 5v

Open and short Circuit

 Procedure:
1. Adjust the power supply to deliver 6V.

Figure b
2. Construct the dc circuit shown in diagram Figure b, and connect it to the 6V power supply.
3. Measure R and the voltage drop across R (VR) and record in data table 3.3 in the appropriate
column.
4. Make short circuit on R4 then measure voltage drop across R3 and R4
5. Make open circuit by removing R4 then measure voltage drop across R3

 Data Table 3.3:

No. R V
1. 22k 1.573V
2. 100k 4.419V
3. 81.2k 2.212V
4. 80.6k 2.196V

4
 o Short Circuit

VR3 = 4.019V
VR4 = 0v

o Open Circuit

VR3 = 0v

 Review Question:
1. Draw the graph of IT Vs VRT (calc.) on a millimeter paper.

5
 0.5
0.455
0.45 0.409
0.4 0.364
0.35 0.318
0.3 0.273
0.25 0.227
IT

0.2 0.182
0.136
0.15
0.091
0.1
0.045
0.05
0
0 2 4 6 8 10 12
Vs(calc.)

2. Draw the graph of IT Vs VRT (meas) on millimeter paper.

 The graph is the same as to the top graph because V measured and V calculated is equal for all
attempts.
0.5 0.455
0.409
0.4 0.364
0.318
0.273
0.3 0.227
0.182
IT

0.2 0.136
0.091
0.1 0.045
0
0 2 4 6 8 10 12
Vs(meas)

3. What do you understand from your graph?

The IT vs. V measured graph shows the relationship between the current (I) and the voltage (V) that is
measured across a device.
The IT vs. V calculated graph shows the relationship between the current and the voltage that is
calculated using Ohm's law. The two graphs should be similar, but there may be some differences due to
measurement errors or other factors.
Here are some of the things that can be understood from a graph of IT vs. V measured and IT vs. V (calc.)
 The resistance of the device.
 The presence of any measurement errors.
 The presence of any other factors that are affecting the results.
4. Does the graph linear or nonlinear?
Yes, the graph of IT vs. V (measured) and IT vs. V(calculated) is linear.
5. If the graph is linear, what is the corresponding parameter in IT Vs VRT..
6
 If the graph of IT vs. V measured is linear, then the corresponding parameter in IT vs. VRT is the
resistance of the device.
6. Conclude any other relevant points based on this experiment.

Based on the experiment conducted on Ohm's Law, it can be concluded that there is a linear relationship
between the voltage applied across resister and the current flowing through it, provided the temperature
and other environmental factors remain constant. This relationship is represented by the equation V = IR,
where V is the voltage, I is the current, and R is the resistance of the conductor.

Through the experiment, it was observed that as the voltage increased, the current flowing through the
conductor also increased proportionally. This demonstrates Ohm's Law, which states that the current
passing through a conductor is directly proportional to the voltage applied across it, given a constant
resistance.

The experiment conducted on an open circuit demonstrated that when a circuit is open, meaning there is a
break in the path of the current flow, no current flows through the circuit. This is due to the absence of a
complete conductive path for the electrons to travel.
The experiment carried out on a short circuit showed that when a circuit experiences a short circuit, there is
a sudden and excessive flow of current through an unintended low-resistance path. This path offers
minimal resistance compared to the resister or components in the circuit.