Page 1 of 3
Name: ______________________________
ECET 231 – Circuit Analysis I
Lab 2
Ohm’s Law
Objective:
Students successfully completing this lab exercise will accomplish the following
objectives:
1. Gain understanding of the application of Ohm’s Law.
2. Learn to measure voltage and current using a Digital Multimeter (DMM).
3. Learn to perform basic power calculations.
Equipment:
Digital Multimeter (DMM), breadboard, 120 Ω, ½ W resistor, 1 kΩ, ¼ W resistor,
6.3 V light bulb.
Lab Report:
A combined formal lab report will be required for lab exercises 2, 3 and 4.
Reports will be due one week after lab 4 has been performed. All lab handouts
complete with tabulated data and calculations should be added as attachments
to your formal report.
Procedure:
Show all calculations in the steps below. Either attach calculations to the lab
report or include them in the report.
1.
Given a 120 Ω, ½ W resistor, calculate the current when the power dissipated in the
resistor is 0.5 W.
I120Ω (max) = _______________
Calculate the voltage applied to the resistor when it is carrying this current. Show your
calculation.
V120Ω (max) = _______________
2.
Given a 1 kΩ, ¼ W resistor, calculate the current when the power dissipated in the resistor
is 0.25 W. Show your calculation.
I1 kΩ (max) = _______________
Calculate the voltage applied to the resistor when it is carrying this current. Show your
calculation
V1 kΩ(max) = _______________
Note: Throughout this lab exercise, be sure not to exceed the values of voltage and
current calculated in steps 1 and 2 above. Exceeding these limits will pose potential
harm to lab equipment, components and yourself.
3.
Select two resistors: R1 = 120 Ω, ½ W, and R2 = 1 kΩ, ¼ W. Using the DMM, measure
and record the values of each resistor. If either of your resistors is out of tolerance, select
another that is within tolerance.
R1 = _______________ R2 = _______________
4.
Connect the circuit shown below. One DMM should be set to measure the voltage of the
power supply and another should be set to measure current through the resistor. Before
applying power, have the instructor verify that the connection is correct.
Page 2 of 3
Figure 1: A simple resistor circuit with voltmeter and ammeter
5.
Adjust the voltage across the 120 Ω resistor from 0 to 5 volts in 0.5-volt increments.
Measure the current and calculate the power dissipated in the resistor at each step.
Record your results in table 1. Slowly increase the voltage to V120Ω (max). Be careful not to
exceed this maximum voltage. Also, be aware that the resistor may be hot! Prevent burns
by turning off the power and allowing the resistor to cool before handling it.
Use Microsoft Excel or other graphing software to graph the following relationships:
•
Graph measured current through the resistor versus voltage. Plot current on the
vertical axis and voltage on the horizontal axis.
•
Graph calculated resistor power versus voltage. Plot power on the vertical axis and
voltage on the horizontal axis.
Table 1: Measured current and calculated power in a 120 Ω resistor
Resistor Voltage (V)
Measured Current (mA)
Calculated Power (W)
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
V120Ω(max)
6.
Replace the 120 Ω resistor with the 1 kΩ resistor. Adjust the voltage across the 1 kΩ
resistor from 0 to 12 volts in 2-volt increments. Measure the current and calculate the
power dissipated in the resistor at each step. Record your results in table 2. Slowly
increase the voltage to V1kΩ (max). Be careful not to exceed this maximum voltage. Also, be
aware that the resistor may be hot! Prevent burns by turning off the power and allowing
the resistor to cool before handling it.
Table 2: Measured current and calculated power in a 1 kΩ resistor
Resistor
Voltage (V)
0
2.0
4.0
6.0
8.0
10.0
12.0
V1 kΩ(max)
Measured
Current (mA)
Calculated
Power (W)
Page 3 of 3
Use Microsoft Excel or other graphing software to graph the following relationships:
7.
•
Graph measured current through the resistor versus voltage. Plot current on the
vertical axis and voltage on the horizontal axis.
•
Graph calculated resistor power versus voltage. Plot power on the vertical axis and
voltage on the horizontal axis.
Measure the cold resistance of a 6.3 V light bulb.
RCold (measured) = _______________
8.
Screw the 6.3 V light bulb into a socket. Connect the socket to the power supply in series
with the DMM. Set the DMM to measure current. Connect another DMM across the
terminals of the power supply. Set this DMM to measure voltage. Turn on the power
supply and increase the voltage to 6 V. Measure the current.
9.
Light bulb current = _______________
10.
Calculate the hot resistance of the light bulb.
RHot (calculated) = _______________
Questions for the Relevant Theory and Background Information Section of the Lab Report:
1.
What is Ohm’s Law?
2.
If you have voltage across and current through a resistor, how do you calculate the
resistance?
3.
If you apply a given voltage across a known resistance, how do you calculate expected
current?
4.
If you measure current flow into a known resistance, how do you calculate the voltage
applied across the resistance?
5.
Given a fixed voltage, how does the current vary with increased and decreased circuit
resistance?
6.
Given fixed circuit resistance, how does current vary with increased and decreased circuit
voltage?
Questions for the Experimental Data / Analysis Section of the Lab Report:
1.
With reference to your graphs of current vs. voltage, what is the significance of the resulting
straight line plot? Does it follow a pattern that is expected?
2.
With reference to your graphs of power vs. voltage: what happens to the value of power
when the voltage across the resistor is doubled?