p31220 Ohms Law
p31220 Ohms Law
p31220 Ohms Law
Purpose: Students will become familiar with DC potentiometers circuits and Ohm’s Law.
Introduction:
Ohm’s Law for electrical resistance, V = IR, states the relationship between current, voltage,
and electrical resistance. If R is constant, V is proportional to I. However, the resistance of a
device can’t always be assumed to be a constant. If you did the “Properties of Resistors” lab,
you might recall that electrical resistance varies with temperature. Diodes are designed to
conduct electricity in only one direction, and thermistors are designed to be especially sensitive
to temperature. Batteries have an internal resistance that is a consequence of the internal
chemistry of the battery. Chemical reactions in the battery cause the internal resistance to
increase. Batteries go “dead” not because they lose voltage, but because their internal resistance
increases to the point where current can no longer flow.
In this lab, you will observe how Ohm’s Law works, you’ll learn about voltage dividers, and
you’ll learn how to measure the internal resistance of a battery.
About Voltmeters, Ammeters, and Ohmmeters:
The table below summarizes the important characteristics of meters and how to connect them so
that the meter has a minimal effect on the circuit. The “resistor” in the table could be any circuit
element with resistance, such as an actual resistor, a light bulb, a motor, a diode, etc. The DMM
can be used as an ammeter, voltmeter, or ohmmeter. It is important to understand that Ammeters
have very small resistance, and Voltmeters have very large resistance. Ohmmeters basically are
voltmeters that use a known voltage from a battery inside the meter, and therefore should not be
used when other voltages are present.
Meter’s Connected to
Meter Measures own circuit Circuit Diagram Circuit is
resistance element
V
Voltage
Voltmeter “across”
resistor
very big In Parallel ON
Current A
Ammeter “through” very small In Series ON
resistor
In Parallel Ω
Resistance Big. Uses
OFF and
Ohmmeter ”of” its own Element is disconnected
resistor battery disconnected
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P31220 Lab
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P31220 Lab
240Ω
9V A
battery
Record your measurements and answer the Analysis Questions on the data sheet.
Experiment 2: Voltage Drops Across Series Resistors (15 minutes)
In Experiment 1, you might have observed that the measured voltage across the 240Ω resistor
was not exactly 9V. In this part of the lab, you will find out why. You have a plastic block with
a wire in it. Points a and e represent the two ends of the wire. Using a DMM as an ohmmeter,
measure the resistance between point a and the other four points on the wire, as well as the
actual resistance of the 27Ω resistor. Then construct the following circuit. Use the 6V power
supply instead of the battery. Measure the voltages between point a and the other four points
along the wire. In addition, measure the voltage across the 27Ω resistor and the voltage across
the power supply. Complete the data table and answer the Analysis Questions for Experiment 2.
6V 27Ω a b c d e
Power Long Wire
Supply
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P31220 Lab
Fig 3a: Potentiometer. Fig. 3b: Internal construction Fig. 3c: Circuit symbol
Connect the circuit shown in Fig. 4. Use the power supply, not the battery. Points a and b are the
outside terminals of the potentiometer as shown in Fig. 3b above. Point w is the middle (wiper)
terminal of the potentiometer. If you have connected everything properly, your DMM should
read 0V when the knob is turned all the way counterclockwise, and some maximum voltage
when the knob is turned all the way clockwise. If this is “backwards”, simply switch the wires
connected to terminals a and b.
V
w
a b
25Ω potentiometer
6V
Observe that you now have a variable voltage between points a and w. We can now use this
circuit as a variable voltage source for the rest of the lab.
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P31220 Lab
V
X
A
w
a b
25Ω potentiometer
6V
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P31220 Lab
General instructions:
Record your data in the data table at the end of the lab. You do not have to fill in all of the
boxes. Extra rows are provided for your convenience, and you may add more if you like.
Make sure that you plot enough points to see the data curves clearly.
Graph each circuit element before moving on to the next. You may need to fill in more data
points after you see what your graph looks like.
Plot both positive and negative values. The origin will be near the middle of your graph.
It’s OK if points are out of order in the data tables. However, when graphing, the data points
must be in ascending order. Graphical Analysis can sort them for you using “Data → Sort
Data Set”.
Include 2% error bars. If the error bars are covered by the point protectors, keep the point
protectors and write a note on the printouts.
Print the graphs and turn them in with your lab. Make sure that the curves are a dark color
before printing.
Clean-Up:
Disconnect all wires.
Turn off the DMMs.
REPORT any damaged equipment to the TA’s immediately so that they can fix it
before the next class.
Leave your table neat and tidy. Place all trash and recyclables in the proper containers.
For more information and practice:
There is an excellent Java applet that lets you build DC circuits, observe how they work, and
measure voltages, resistances, and currents. The web address is:
http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc
Modern electronic devices such as cell phones and computers are made out of diodes, transistors,
and other semiconductor devices. One common use for diodes is to convert alternating current
(like what comes out of the wall) to direct current (like what comes out of a battery). Solar cells
are basically semiconductor diodes. To learn more about how semiconductor diodes work, try
the following links:
http://electronics.howstuffworks.com/diode.htm
or
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/pnjun.html
A LED (light-emitting diode) is a semiconductor diode that gives off light when it is in the
conducting state. Single LEDs are commonly used in indicator lights and in battery-operated
flashlights. Often many LEDs are bunched together, as in automobile tail lights, traffic lights,
and replacements for incandescent light bulbs. This link explains how LEDs work:
http://electronics.howstuffworks.com/led.htm
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P31220 Lab Name: ________________________________
2. Although a battery is really one device, the circuit analysis is simplified if you treat it as a
pure voltage source in series with a pure resistance, called the “internal resistance.” The new
circuit is shown below. Everything inside the dashed line is inside the battery.
Recall that the voltmeter has nearly infinite resistance
V compared to the other resistors in the circuit, and that the
ammeter’s resistance is so small that you can ignore it.
The resistance of this circuit is therefore (240Ω + r).
Calculate r, using Ohm’s Law (V=IR) and V = 9V.
r 240Ω
9V A
Answer: _______________
Fig. 5: Circuit for measuring
the internal resistance of a
battery.
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P31220 Lab Name: ________________________________
3. (circle one) Using the circuit in the previous question, would you expect the measured
voltage across the 240Ω resistor to be LARGER, SMALLER, or the SAME as 9V? Explain
your thinking:
b. Assume that r = 60Ω. What is the new current in the circuit? Answer: __________
5. Why won’t measuring battery terminals with a voltmeter always tell you whether a battery is
weak? If weak and dead batteries are available, you might try testing them with your DMM.
Report what you did and what you found out below. MAKE SURE THAT THE DMM IS
ON VOLTS!
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P31220 Lab Name: ________________________________
a to b
a to c
a to d
a to e
6. Examine your data. Compare the measured voltage across the power supply with the sum of
the measured voltages across the resistor and the entire length of the wire (points a to e).
What can you conclude?
7. Consider your data for the wire only. Graph the voltage vs. length of the wire. Include 2%
error bars for the voltages, and 2 mm error bars for the lengths. Do a linear fit to your data.
What can you conclude about the relationships between the measured resistances, the length
of the wire, and the measured voltages along the wire?
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P31220 Lab Name: ________________________________
8. Predict, then test: What voltage would you expect to measure from point b to point e?
Prediction: ______________
Reason for this prediction: (do NOT change this if it was wrong!)
Test: ____________________
Correction to your thinking, if your prediction was wrong:
9. Repeat question 8, this time measuring the voltage from point c to point e.
Prediction: ______________
Reason for this prediction: (do NOT change this if it was wrong!)
Test: ____________________
Correction to your thinking, if your prediction was wrong:
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P31220 Lab Name: ________________________________
Hints: Take more data points where the graphs are changing rapidly. The most interesting
part of the light bulb’s graph happens when the bulb is not yet glowing. There IS current,
but not enough to make a glow. You’ll also want to take data points more often where the
diode’s graph is changing rapidly. Feel free to add pages, use the margins, or use the back
of the page if there aren’t enough blanks in this table.
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P31220 Lab Name: ________________________________
11. Examine the data and graph for the 27Ω carbon resistor. What is the data telling you about
the behavior of the resistance of this device? Explain in as much detail as you can.
12. You measured the actual resistance of the 27Ω resistor with an ohmmeter in Experiment 1.
Compare this number to the slope of the graph. Do these two numbers agree within error?
(circle one) YES NO Please discuss.
13. Examine the data and graph for the light bulb. What is the data telling you about the
behavior of the resistance of this device? Explain in as much detail as you can.
14. Incandescent light bulbs glow when the current heats the wire filament to a high temperature.
The brighter the glow, the hotter the filament. What does your graph tell you about how the
electrical resistance of the filament changes with temperature?
15. Examine the data and graph for the diode. What is the data telling you about the behavior of
the resistance of this device? Explain in as much detail as you can.
16. What are your final thoughts and impressions about this lab?
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