Physics307L F08:People/Gibson/Notebook/070924
E/M Ratio Lab
DH 1
It might be best to use the section as a statement of your intentions, history etc. Instrumentation is better put afterwards or in a separate section.
DH 1.2
Try using footnotes and a citation to make a quote instead of stating your intention.
To begin this set up, we followed the procedure of connecting all the wires ect. as described in the lab manual from the previous semester. Here I will enter the paragraph listed from that. The equipment used is as follows: Two multimeters one used to measure the voltage from the high voltage source which is used to determine the average velocity of the electrons. The [math]\displaystyle{ 2nd }[/math] multimeter is used to read the current running through the Helmholtz coil.
The Helmholtz coils require us to warm up the electron gun for 2 minutes before we can view electrons in the helium tube.
Data
DH 2
What focus did you use? How did this affect the radius?
DH 3
You need to report your errors in your data
SJK 23:04, 17 October 2007 (CDT)
This was a great idea to take data in these three different ways...to possibly reveal systematic problems
The key to measuring the e/m ratio is to start taking the radius of the electron beam circle. This is done with a mirror scale behind the beam itself measured in cm. We continued to take 10 measurements first taking all measurements at constant voltage (making sure to be above 250V for precision of measurements), then took 10 at constant voltage then took 10 at both varying voltages and currents.
Measurements
1st 10 measurements are made at constant voltage, with varying current. 2nd 10 measurements are made at constant current, with varying voltage.
Set 1: Set 2:
Voltage: 270V Voltage: 270V
Current: 1.04 A Current: 1.01A
Radius: SOON cm (5.4 from right, 4.1 from left) Radius: 4.5 cm (5.5 from right, 4.1 from left)
Set 3: Set 4:
Voltage: 270V Voltage: 270V
Current: 0.95 A Current: 1.18 A
Radius: 4.6 cm (5.6 from right, 4.0 from left) Radius: 4.6 cm (5.0 from right, 4.0 from left)
Set 5: Set 6:
Voltage: 270 V Voltage: 270V
Current: 1.24 A Current: 0.99 A
Radius: 4.0 cm (4.8 from right, 3.9 from left) Radius of circle: 4.55 cm (5.5 from right, 4.1 from left)
Set 7: Set 8:
Voltage: 270V Voltage: 270 V
Current: 1.22 A Current: 1.13 A
Radius: 4.35 cm (4.9 from right, 3.9 from left) Radius: 4.20 cm (5.2 from right, 4.0 from left)
Set 9: Set 10
Voltage: 270V Voltage: 270 V
Current: 1.08 A Current:1.30 A
Radius: 4.3 cm (5.3 from right, 4.1 from left) Radius: cm (4.6 from right, 3.9 from left)
This group of data is under constant current of 1.20 ASJK 23:38, 17 October 2007 (CDT)
I uploaded a new Excel file to show you how to use the slope of the V versus r^2 graph to obtain the e/m value. See this file
Set 1: Set 2:
Voltage: 270 V Voltage: 289 V
Current: 1.35 A Current: 1.35 A
Radius of circle: 3.7 cm Radius: 3.9 cm
Set 3: Set 4:
Voltage: 298 V Voltage: 307 V
Current: 1.35 A Current: 1.35 A
Radius: 3.95 cm Radius: 4 cm
Set 5: Set 6:
Voltage: 313V Voltage: 320 V
Current: 1.35 A Current: 1.35 A
Radius: 4.1 cm Radius: 4.15 cm
Set 7: Set 8:
Voltage: 330V Voltage: 348V
Current: 1.35 A Current: 1.35 A
Radius: 4.25cm Radius: 4.35 cm
Set 9: Set 10:
Voltage: 360 V Voltage: 397V
Current: 1.35 A Current: 1.35 A
Radius: 4.45 cm (5.0 from right, 4.2 from left) Radius: 4.6 cm (5.1 from right, 4.3 from left)
TOTALLY RANDOM
Set 1: Set 2:
Voltage: 180 V Voltage: 189 V
Current: 1.21 A Current: 1.02 A
Radius: 3.7 cm (3.9 from right, 3.5 from left) Radius: 4.15 cm (4.7 from right, 3.6 from left)
Set 3: Set 4:
Voltage: 198 V Voltage: 207 V
Current: 1.09 A Current: 1.18 A
Radius: 4.15 cm (4.6 from right, 3.7 from left) Radius: 4.1 cm (4.4 from right, 3.8 from left)
Set 5: Set 6:
Voltage: 212V Voltage: 220 V
Current: 1.25 A Current: 1.30 A
Radius: 3.95 cm (4.2 from right, 3.7 from left) Radius: 3.85 cm (4.1 right, 3.6 from left)
Set 7: Set 8:
Voltage: 230V Voltage: 248V
Current: 1.33 A Current: 1.14 A
Radius: 3.85 cm (4.2 from right, 3.7 from left) Radius: 4.5 cm (5.0 from right, 4.0 from left)
Set 9: Set 10:
Voltage: 260 V Voltage: 288V
Current: 1.01 A Current: 1.33 A
Radius: 4.8 cm (5.5 from right, 4.1 from left) Radius: 4.3 cm (4.6 from right, 4.0 from left)
Interpreting the data (Finding [math]\displaystyle{ e/m }[/math])
DH 4
You're missing the motivation of analyzing the data in this way. Analyze you data after you've talked about what you're looking for. For example, a "theory" section following the introduction that outlines or has citations to the theory would help.
Here to determine e/m we used the following from the lab manual: for constant velocity, plotting r vs. I^-1 with a least squares plot gives a value for e/m. For constant current, the plot goes like r^2 vs V. As shown below:SJK 23:43, 17 October 2007 (CDT)
I don't think you correctly used the linear fit to deduce the e/m value. See my uploaded tweak of your excel file to see how to do this for the V versus r^2 plot.
Constant Velocity
Constant Current: Here is the excel file used to determine the various parts of the e/m lab. This also includes the graph of r^2 vs. V
Although I can't get my percent error to show using the formula actual-theoretical/theoretical *100 gives me an error of 92.3%. I realize this must be some experimental error. Other reports of error for data set 1 and the random set is included in the excel file above, each display 200+ % error and ~75% error. I honestly don't know how to compensate for this.
Conclusions
DH 5
Some of this could have been introduced earlier. Also you should cite where you got that number for the "actual" e/m ratio. Also, it would be good to have something about the B field of the coils
In our search for determining the ratio e/m, we used the formula [math]\displaystyle{ \frac{e} {m}= \frac{2V} {B*I^2r^2} }[/math]SJK 23:24, 17 October 2007 (CDT)
I don't think this formula is correct!
to determine the e/m ratio from the average of our 3 data sets. We then compared this to the actual value of e/m = 1.76E+11 C/Kg we saw quite a discrepancy. To account for this several things were considered and we first looked to another groups data to see if they were reporting answers that were in the ball park of what we were getting. This turned out to be the case that they too showed similar orders of magnitude for their e/m ratio tests (Lorenzo and Tomas).
The first using data involved a constant voltage, the 2nd constant current and the third were completely random values as can be seen above in the measurement tables.
1st Data set measurement
e/m = 6.28343E+11 +/- .725E+11 C/Kg (Koch says (6.3 +/- 0.7) * 10^11)
2nd Data set measurement
e/m=3.39E+11 +/- .0272E+11 C/Kg so, (3.39 +/- .03) E+11
3rd Data set measurement
e/m = 3.07111E+11 +/- .00840E+11 (3.07 +/- .01)*10^11
Questions: Why do we see a beam at all? -A current is supplied to a metal plate in the hydrogen tube, then when this plate gets hot it burns off electrons which then are accelerated between two plates then arced through in a circle due to the B field caused by the Helmholtz coils.
We ignored the earth's magnetic field in this experiment. How much error does this actually induce?
-http://www.ngdc.noaa.gov/seg/geomag/ has an awesome calculator for figuring this out, however I'll skip it for now and move on but it does pose an interesting error.SJK 23:49, 17 October 2007 (CDT)
Great link! You should put it on the lab manual page, which of course needs to be updated.
Suppose the beam emitted protons instead of electrons... how does this change? -Well if you keep the current in the same direction through the helmholtz coils now, you would see nothing, however if you reversed the current you could possibly see the beam again.
SJK 23:49, 17 October 2007 (CDT)
remember the mass of the proton is quite a bit higher..
