Department of Electrical Engineering
Faculty of Engineering & Applied Sciences
Riphah International University, Lahore, Pakistan
Program: B.Sc. Electrical engineering Semester: I
Subject EEL-101 Engineering Workshop Date: …………….
Experiment 6: Measurement of DC Power in Circuit
OBJECTIVES:
• Determine the power in a variable resistor at various settings of resistance.
• Plot data for power as a function of resistance. From the plot, determine when maximum power is
delivered to the variable resistor.
Student Name: ……………. Student ID................
Lab Performance
No. Title Obtained Marks
1. Understanding and ability to conduct experiment (0-5)
2.
Implementation and Results (0-5)
Total
Lab Reports
No. Title Obtained Marks
1 Report Content (0-5)
2 Data Presentation (0-5)
Total
Remarks (if any): ………………………………….
Signature of faculty: …………………………
EXPERIMENT # 6
MEASUREMENT OF POWER IN DC CIRCUITS
Objective
• After performing this experiment, you will be able to:
• Determine the power in a variable resistor at various settings of resistance.
• Plot data for power as a function of resistance. From the plot, determine when maximum power is
delivered to the variable resistor.
Theory
When current flows through a resistor, electrical energy is converted into heat. Heat is then radiated from
the resistor. The rate that heat is dissipated is called power. Power is measured in units of joules per second
(J/s), which defines the unit called the watt (W). The power dissipated by a resistor is given by the power
law equation:
𝑃 = 𝑉𝐼
By applying Ohm’s law to the power law equation, two more useful equations for power can be found.
These are:
𝑃 = 𝐼2 𝑅
And
𝑉2
𝑃=
𝑅
The three power equations given above are also known as Watt’s law. In this experiment, you will determine
power using the last equation. Notice that if you measure the voltage in volts (V) and the resistance in kilo-
ohms (kΩ), the power will have units of milliwatts (mW). The physical size of a resistor is related to the
amount of heat it can dissipate. Therefore, larger resistors are rated for more power than smaller ones.
Carbon composition resistors are available with standard power ratings ranging from 1/8 W to 2 W. For
most typical low voltage applications (15 V or less and at least 1 kΩ of resistance), a 1/4 W resistor is
satisfactory.
Materials Needed
• One 12 kΩ resistor
• One potentiometer
Procedure
• Measure the resistance of R1. The color-code value is 12 k𝛀. 𝑹𝟏 = _______________
Construct the circuit shown in Figure 1. R2 is a potentiometer. Connect the center (variable) terminal to
one of the outside terminals. Use this and the remaining terminal as a variable resistor. Adjust the
potentiometer for 5 kΩ. (Always remove power when measuring resistance and make certain you are
measuring only the potentiometer’s resistance.)
Figure 1
• Measure the voltage across R1 and the voltage across R2. Enter the measured voltages in Table 1.
As a check, make sure that the sum of V1 and V2 is equal to 12.0 V. Then compute the power in
R2 using the equation:
𝑉2 2
𝑃2 =
𝑅2
Enter the computed power, in milliwatts, in Table 1.
• Disconnect the power supply and set R2 to the next value shown in Table 1.
Reconnect the power supply and repeat the measurements made in step 2. Continue in this manner
for each of the resistance settings shown in Table 1.
• Using the data in Table 1, graph the relationship of the power, P2, as a function of resistance R2
on Plot 1. Since resistance is the independent variable, plot it along the x-axis and plot power along
the y-axis. An implied data point can be plotted at the origin because there can be no power
dissipated in R2 without resistance. A smooth curve can then be drawn to the origin.
Variable Resistance 𝑽𝟏 (Measured) 𝑽𝟐 (Measured) Power in 𝑹𝟐
Setting 𝑹𝟐
𝑽𝟐 𝟐
𝑷𝟐 =
𝑹𝟐
5 k𝛀
10 k𝛀
12 k𝛀
15 k𝛀
20 k𝛀
25 k𝛀
30 k𝛀
35 k𝛀
40 k𝛀
Table 1
• Draw graph and attach with this
Observations and Conclusion
