Rochester WeatherResistance versus Length in a Physics Teacher
Objectives:
To find the resistance of a physics teacher with respect to a tensile stress applied to the object.
Theory:
A tensile force will be applied to a physics teacher. As the force increases, the physics teacher will stretch. This will cause the amount of distance though which current must pass to increase, and the cross sectional area of the path through which the current passes to decrease. The combination of these will cause the resistance of the physics teacher to change.
In this lab, you will need to find the formulas for the cross sectional area of the physics teacher with respect to the length of it, the change in length with respect to force applied, and the resistance with respect to both length and cross sectional area. It would be extremely easy just to connect an ohm-meter to either end of the physics teacher and measure the resistance, therefore the resistance of the physics teacher will not be found using this method for this lab. Instead, you will connect a 60kV power supply to either end of the physics teacher. You will be supplied with a meter movement for this lab, but will not be given the full scale deflection current or coil resistance ratings. You will need to find these yourself, and choose a resistor appropriate to go in parallel with the meter movement to cause it to read the current properly. If you choose a resistor value too low, your meter may be damaged. You will have to pay for it. If you choose a resistor value too high, you will get inaccurate readings.
Although we could have listed formulas for calculating the proper value of resistor to use, we won't because it would decrease the profits we make on these lab manuals. We could increase the price, but then it would seem unreasonable and it might affect the tendency of future students to come here. So, if you want these formulas (Which you will need), buy the course book for course Electrical Engineering, DC circuits course. (This way, the book store makes more profit because you need two books too.) This book refundable.*
When writing up your lab, remember to include the table of contents, index, word count (broken down for each individual word), letter frequency. It must be handed in in a yellow lab folder, available from the campus store. Everything must be typed, including data on the yellow folder. The campus VAX or any other form of computer, word processor, or electric type writer may not be used. This must be done on a mechanical typewriter, and the graphs must be done by hand, in ink. It is suggested that you use various color ink to simplify the reading of the graph. Use of white-out is not acceptable.
Note: While any organic entity could be used in this experiment, a physics teacher will be used. This is mainly due to the abundance of physics teachers, and the low opinions held of these items. Also, they are annoying.
Equipment required:
- Physics teacher
- Hydraulic piston w/ compressor
- Meter movement
- Resistor (You will need to get these at Radio Shack before coming to class. It is recommended you get a number of different value resistors, since you do not know the resistance of the meter movement until you arrive at class.)
If you damage any of the RIT-owned equipment, you will have to pay for both the unit damaged and the cost to purchase a new one.
Procedure:
Connect the physics teacher to circuit as shown in figure 1. Remember to connect the hydraulic piston to both ends of the physics teacher also. Measure the current though the physics teacher for a normal length. Turn on the compressor to the hydraulic piston, and lengthen the specimen.
Note: You should engage the compressor in short bursts, and let the length of the physics teacher "coast" to the desired measurement length.
You should take measurements of the voltage and length of the physics teacher at about 1-foot intervals until the physics teacher breaks in half. At this point, the current should be zero. If it is not, your meter is misalligned. Try again with another specimen. If your meter is still misalligned, use a screwdriver to turn the small screw at the base of the meter needle. Turn this screw until the meter is adjusted to point to zero.
Calculations:
Plot the graphs V vs. I, I vs. L, V vs. L, R vs V, R vs I, R vs r, I vs r, r vs pi, r vs V, r vs I, and r vs rnd, where rnd is random numbers generated by Vax BASIC's random number generator. Remember to draw three curves for each graph (the best and two worst cases.) These graphs must be drawn on the white and green paper available from the campus store, and each piece must be stamped that it was bought there. Do each of these graphs on linear, semi-log, and log-log graph paper.
Calculate R, P (power), U (P.E.), K (K.E.), and the number of kilowatt hours used. You will be sent a bill for the power used in this experiment. (If you calculate too low, you will be charged the proper amount plus the amount you calculated as a service charge. If you calculate too high, you will be charged the amount you calculated.)
Discussion:
Evaluate and discuss your results. Remember to include specific values in the discussion. How could this experiment be improved? Discuss uncertainty values, and how they could be decreased.
Questions:
1. Is the L-R relationship directly or indirect? Is it proportional? If it is not, is it logarithmic? Explain your answer.
2. Did you hear the physics teachers bones being disjointed? If so, explain what it sounded like. If not, explain what you did hear. Did this cause constructive or destructive interference with other noises?
3. What would have happened if the leads were connected to the sides of the physics teacher instead of the ends? What if the physics teacher was expanded on the sides while leads were connected to the ends? Draw the graphs noted above that you would expect from each of these cases. (You need not include the worse case lines since it is only thoretical.)
4. What was the %error of the graph of r vs rnd? Was this more or less than you would expect? Explain. If you had over 2% uncertainty, explain how this could be decreased.
Figure 1:
Hydraulic Piston
[----[][][][]-----------]
+----||-----/\/\/\/----(m)----]physics_teacher_goes_here[--+
| | R1 + - |
| + PS1 - |
| |
| |
_____ _____
___ ___
_ _
--
* A 95% restocking charge will be applied to all books
refunded.
