# Physics307L:People/Long/Eoverm

## E/M Ratio Lab summary

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The objective of this lab is to find the charge to mass ratio for an electron. Using an apparatus with an electron gun that fired electrons inside a helium gas bulb under the influence of magnetic field. The electrons excite a ring of Helium atoms, and we measured the radius of the ring, and knowing about the Lorentz force, we calculated e/m. primary notebook entry for this lab

## Results

Under constant current and varying voltage, my calculated value for e/m:

$\frac{e}{m}= 2.216\cdot 10^{11}\frac{C}{kg}$

$lower bound: 2.11\cdot 10^{11}\frac{C}{kg}$

$upper bound: 2.33\cdot 10^{11}\frac{C}{kg}$

The accepted value from wikipedia is:

$1.758820150(44)\cdot 10^{11}\frac{C}{kg}$

Under constant voltage and varying current, my calculated value for e/m:

$\frac{e}{m}=1.944\cdot 10^{11}\frac {C}{kg}$

$lower bound: 1.78\cdot 10^{11}\frac{C}{kg}$

$upper bound: 2.118\cdot 10^{11}\frac{C}{kg}$

## Conclusions

Given the level of systematic errors this experiment has, I'd say my values are satisfactory at best. For some reason the value from part two was closer to the accepted value. Otherwise my results in general were all slightly above expected. We took a great deal of data, and I feel like I should have used more of it, but I was unsure of how to incorporate all of it into my results. For the experiment, I am not too sure of any other ways of improving this experiment without designing a whole new experiment. As far as reducing human error in measuring the radius, there is not much else that can be done. Also it may have something to do with the purity of the helium in the bulb, and the age of the bulb. I'm not familiar enough with the properties of helium, but perhaps there is something influencing the results.

SJK 18:12, 15 December 2009 (EST)
18:12, 15 December 2009 (EST)
As previous students have written diatribes about, I think you're right that the best way to improve the experiment is to completely redesign it. I think it's OK, though, because you can learn a lot about systematic error through this experience.