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        E / M
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>>eOVERm Lab manual PDF.<<

Charge to Mass Ratio of an Electron: Dimensional Analysis


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e/m Lab Experiment

Begin Q&A Forum for "E / M"

In the equation in the prelab, B=((4/5)^3/2 *(12.57E-7)*NI)/r, where does the 4/5^3/2 come from, and is this a constant that should be used in calculating B in the prelab quiz? I'm not sure if I'm not converting my answer to mT correctly or if I am using the wrong equation completley. Thanks!Anonymous
Sat. 04, Oct 2003, 12:25
Yes, (4/5)^(3/2) is part of the equation; that is why it is in there.
{BTW: the parentheses are critical here; yours are incorrect. See below.}
The 0.83/2 comes from the derivation of the B field equation. The derivation integrates the B field contribution from tiny segments around the whole double coil system.

To go from [Tesla] to [mT], simply move the decimal three places to the right.

BTW: The placement of parantheses matters a great deal.
(4/5)^3/2 = ((4/5)3)/2 = 0.256 (Wrong for the B field equation.)
4/5^3/2 = 4/(53)/2 = 0.016 (Wrong for the B field equation.)
(4/5)^(3/2) = 0.7155417527999328
The last one calls the operations in the correct order for the B field equation.
Sun. 05, Oct 2003, 05:54

I did the pre-lab for E/M twice and got question 11 wrong twice. I entered 1.76E11 as my answer and it keeps coming up wrong for the accepted value of E/M. I got this right out od the lab manual. There was also an error message within my quiz that said to e-mail the administrator to correct the problem. Can you help me w/ this last question?
Taylor, Kirk
Thu. 11, Mar 2004, 15:08
Oops, my mistake. Sorry about that. The quiz is now fixed and your score is 100%. Thanks for alerting me.
Thu. 11, Mar 2004, 15:34

The post-lab to e/m included a multiple choice question about changing the alignment of the apparatus with respect to the earths field lines. I answered as insufficient info nad didnt get any feed back after the quiz. This question does not suggest how the apparatus was oriented to the field lines before being rotated horizontally. The beam could have been enlarged or reduced slightly before being rotated or unaffected at all in the 2D plane measured in the globe, all depending on its orientation with respect to north.McCaskill, Douglas
Wed. 31, Mar 2004, 22:43
If you don't understand what I am saying below, please see me or your lab instructor for clarification.

Yes, the apparatus was on the table, it was horizontal before pointing the coils UP towards the dip field of the earth.
Yes, as you rotate the apparatus on the table (with the four feet always touching the table) the Earth's field lines will hit it and cause variations that depend on the rotation angle. But the only way to maximize this effect in Denver, CO is to do a three-dimensional rotation at which point you will see the maximum effects. To see this, the coil face would have to point right at or exactly against the Earth's Dip Field. When the coil sits on the table with it face parallel to the Earth's surface, the Earth's Magnetic Field lines are not parallel to the coil face, therefore, they don't have a full affect as they could if the coil face was pointed up or down.

The earth's magnetic field lines are parallel to the ground at the magnetic equator. At the magnetic north pole, the field lines are perpendicular to the ground. In Denver the field lines dip down at an angle between the two.
The question states (for you):

Pretend you tip the apparatus up at an angle so that the face of the Helmholtz coil is parallel to the direction of the earth's magnetic dip angle. The controls are now pointed down towards the table. What would the effect be on the beam of electrons? (Assume you have not changed V or I.)
Other students have "the controls are pointed up toward the ceiling..."
Your controls are to be pointing downward. But the key is also in the phrase about ...the coil's face is pointing parallel to the dipped magnetic field lines.

You know for certain which way the coil-generated field lines travel, from doing the experiment. Now, what happens if you introduce an external magnetic field (here, the Earth's Dip Field)?
Won't that change the net strength of B?
If B increases due to coil's and Earth's fields being parallel, then the Lorentz force increases. The Lorentz force supplies the radial forcse that causes the Magnetic field to be have a circular projection. What happens to the diameter of that circle, since B is stronger?
If B decreases due to coil's and Earth's fields being antiparallel, then the Lorentz force decreases. What happens to the diameter of that circle, since B is weaker?

Thu. 01, Apr 2004, 05:11

I tried to print the PostLab for e/m lab exercise, even though we don't begin that lab until next Wednesday. I think it has the date from last semester still.
Wed. 16, Jun 2004, 17:27
Most odd. I am so sorry. Thank you so much for letting me know!!!
It is fixed now. Silly me.
Wed. 16, Jun 2004, 19:19

On #3 of the "E/M" postlab, "You used a funtion of I and some other constants to find B. B, according to this function, was constant. Discuss this in light of your experimental results." Is it enough to say that B was constant because the value of I remained constant and these are proportional? The B had nothing to to with the voltage in the equation given in the manual.Anonymous
Mon. 28, Jun 2004, 09:58
Your instructor was supposed to cover this concept in class.
Please be fully aware of the questions before class so that you can ask during class.
Since this is such a critical question for the lab, I cannot answer it well via the forum.
But, I will give it a shot.

Your answer is not quite correct. So, consider the following "lead in" questions:
1) Is e/m constant?
2) If e/m is constant and B is constant for a given current, then V d-2 should be constant since e/m and B are constant.
See those relationships here: e/m = 8 B-2 * V d-2
3) But, is V d-2 really constant? You have a data table; look at the column for V d-2.
4) If V d-2 is not constant (hint, hint), then B has to change if e/m is to remain constant.
5) But if B has to change, which way does it change? As you go down the data table, does B have to get bigger or smaller? But that goes a little beyond the question asked.

So, you were right that the B has nothing to do with V in the manual. But, the V altered the diameter of the trajectory. And apparently, from your data, B is different at different places in the glass bulb; so B does vary with position in the bulb. Hmmm, but you can only say this right now from an empirical standpoint. Namely, your V d-2 column helps you refute the B field equation in the lab manual.

You should have also noticed that the compass needle would deflect by as much as 5° when you moved it from one side of the bulb to the other. This is an observational way to refute the constant B feild equation.

One more note of interest:
The average V d-2 is somewhere's around 7cm. This is approximately the center of the glass bulb with respect to the coils. Cool!
Mon. 28, Jun 2004, 19:32

For #4 of the E/M prelab, I kept getting the answer wrong. Unfortunately, I have no more tries. Could you please tell me what I did wrong? I used the equation: B= {(4/5)^(3/2)*(12.57E-7)*(130)*(1.4)}/o.15 = 1.09E-3[T]= 1.09E-6[mT]as my final answer. THANKS!!!Anonymous
Mon. 27, Jun 2005, 10:08
1.09E-3 [Tesla] = 1.09 [milliTesla]Douglas
Tue. 28, Jun 2005, 05:49

Hello Douglas!
Question 4, postlab:
When tilting the coils parallel to the earth's magnetic field, I thought the beam diameter decreased due to the additional magnetic field felt from the Earth's magnetic field. Why is this incorrect?
Thu. 14, Jul 2011, 18:58
If coil and earth are in agreement, then your thoughts are indeed correct.
I also think an important point to consider is which way the coil is facing. Is the coil facing up or down? This would certainly have an impact on addition or subtraction of coil and earth fields.
Fri. 15, Jul 2011, 05:05

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