Download Lab 2 Ohms Law

Lab 2
Ohm’s Law
Background
The relationship between the electrical quantities current, voltage, and resistance is known as Ohm’s
law. Essentially, voltage is proportional to current. The ratio between the voltage and current is the
resistance. In this experiment, we will verify this relationship. Note that for some components (such
as resistors), the resistance does not change regardless of the voltage. This is not true for all
components. Therefore, it is necessary to measure the resistance over a range of voltages.
Experimental Setup
We will be using:
1)
2)
3)
4)
5)
6)
7)
8)
Power Supply
Logger Pro
Voltage Probe
Current Probe
Prototype Board
2 Resistors
1 Light bulb
Various cables, as needed. Three should be sufficient.
The setup to measure a resistor is shown below.
+
-
Current
probe
Resistor
I
Red
Black
Voltage
probe
I.
Resistor Measurement
1. Connect the experiment as shown. Do not turn on the power for the power supply.
2. Have the instructor or lab supervisor verify your experimental setup. Note the orientation of the
resistor – the text should not be upside down.
3. Turn on the power supply and set the current dial straight up.
4. Set the voltage to approximately one volt.
5. Connect the voltage probe to channel one of the logger pro.
6. Connect the current probe to channel two of the logger pro.
7. Open the logger pro file.
8. Open the Excel spreadsheet. You should only fill out the Excel cells that are white. Do not type
anything into the yellow cells. Never type a “%” sign and never put the units in the same cell as
the number. Do not add any rows, columns, or cells to the spreadsheet.
9. Verify that the logger pro is measuring current and voltage. Press the “Collect” button and verify
that the voltage matches approximately the voltage on the power supply. Vary the power supply
voltage from 1 to 5 volts and verify that the logger pro display tracks the power supply voltage.
10. Take 11 points of data between 0 and 6 volts. To record a data point, you must hit the “Keep”
button in logger pro. The exact voltages are not important but they should be equally spaced over
the 0 to 6 volt range and should not exceed 6 volts.
11. Hit the “Stop” button in Logger Pro.
12. Copy the voltages and currents to the spreadsheet.
13. In Excel, plot the current and voltage (with current on the X-axis and voltage on the Y-axis).
14. Add a linear trend line and include the equation.
15. Edit the equation to display reasonable variables (replacing the x and y).
16. Add a plot title and axes titles.
17. Determine and record the slope and intercept on the Excel spreadsheet.
18. Determine the measured resistance (this is very easy).
19. Repeat for the second resistor (which should have a different resistor value from the first).
20. Answer the questions at the end of the section.
II.
1.
2.
3.
4.
5.
Light Bulb
Replace the resistor with a light bulb.
Repeat the measurements in Part I.
Don’t forget to hit the “Stop” button in Logger Pro.
In Logger Pro, do a linear fit of the first 3 points and (approximately) the last 10 points.
This time, copy and paste the logger pro plot into the Excel spreadsheet. If you have trouble with
that, it’s easier to see someone else do it rather than trying to explain with text. So ask a
neighboring group if possible. If you still need help, let me know.
6. From the logger pro plot, determine the slope of the first 3 points and last 10 points.
7. Answer the questions at the end of the section.
III.
Reverse Current
1. Replace the light bulb with the first resistor, but this time reverse the orientation of the resistor
(flip it around).
2. Repeat the measurements in Part I.
3. In Logger Pro, do a linear fit of all of the data.
4. Copy the plot (with the linear fit line) to the Excel Spreadsheet.
5. From the logger pro plot, determine the slope and intercept.
6. Answer the questions at the end of the section.
7. Turn off the power supply.
IV.
Conclusion
1. Save your data file in dropbox (either in the morning or afternoon folder). The file name should
be S[Section]S[Station] Lab 2 Ohms Law.xls, where [Section] is 1 for the morning and 2 for the
afternoon and [Station] is your station number. Make sure that the names of all of the members
of your group are listed in case 1.
2. Please note that dropbox does not always work. Verify that the instructor has your lab report
before returning your equipment.
3. If you are in the morning class, log off your computer and turn off your monitor. If you are in the
afternoon class, shut down your computer and turn off your monitor.
4. Leave.
V.
Troubleshooting
Logger Pro not detected: Make sure that the USB cable is connected to the Logger Pro and the logger
pro is plugged in. There should be a green LED lit up on the Logger Pro and you should have heard it
beep several times when turning on. If it is still not detected, contact the instructor or the lab
supervisor.
Sensors not detected: Make sure they are plugged in to the correct side of the Logger Pro (NOT the
Dig/Sonic side). The voltage probe should be on channel 1 and the current probe on channel 2.
Plot does not show data: Hit the “Autoscale” button on Logger Pro. You may have to do this
repeatedly as you take more data, so it might be a good idea to get the highest and lowest voltage
measurements done first and then do the ones in between.
Plot still does not show data: Have you been hitting the “Keep” button? If you haven’t, start doing it
and pretend this never happened. If you have, let the instructor or lab supervisor know.
Data goes up in a line for a while and then flattens out: This probably means you went above 6 volts.
Hit “Stop” and restart the measurements.
Logger Pro will not show linear fit line: You have to hit “Stop” first. Let’s pretend this never
happened, too.