Physics307L F09:People/Dougherty/Eoverm

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e/m ratio measurement apparatus

The heating element connected to the anode gun is heated to the point of boiling off electrons which are then accelerated by a voltage across a circular plate. the electrons in motion then move in a "curled" path by a magnetic field caused by a current in the helmholtz coils. this magnetic field caused by the current constantly puts a force on the electrons making them turn in a circular "orbit." then the helium in the bulb causes collisions with the electrons causing a bluish radiation of visible light. we can measure the radius of the beam by a ruler behind the bulb. by measuring the voltage applied, the heat applied, and the current, along with the radius of the beam, we can come up with the e/m ratio.

accepted value = 1.76*10^11 coul/kg

raw calculations/averages

our values

SJK 00:39, 13 November 2007 (CST)
00:39, 13 November 2007 (CST)
It's good that you have errors and units, and enough information is there for reader to figure out what's going on. However, it would be more correct to have the units on the uncertainty too. Also easier to read with only the significant figures, and same exponent on both the value and the uncertainty. For example the first one you could write: (3.21 +/- 0.06) C/kg.
3.2116e+011 coul/kg +/- 6.3934e+009
3.2246e+011 coul/kg +/- 7.7978e+009
4.5411e+011 coul/kg +/- 1.2047e+010
2.7461e+011 coul/kg +/- 8.7489e+008

possible reasons for error

The lab manual discusses why the e/m ratio we came up with is higher than normal. it says that the voltage across the anode creates an un-uniform field which causes the Ve to slow down. also the collisions of the electrons with the helium also causes Ve to slow down. and since that is directly related to 1 / (R2), then it greatly effects the radius measured.

another few problems is the width of the beam, although the manual says the best way to measure is the outside of the beam. also it is hard to line up the beam with the ruler behind the bulb because of light and the length measured (mm). another couple problems is the bulb itself. the circular bulb creates a parallax of the light viewed and can distort your measurements. and also the bulb itself is very unstable. lining it up so its center is directly over the origin of the ruler is tough and also if the table is bumped or the apparatus is bumped it can move the bulb and skew measurements.

as of the voltmeter and ampmeter. as discussed in class. they are directly hooked up and directly measured so the probability of error is surely less then 1%.


overall i am happy with the outcome of the experiment. the values we came up with were fairly consistent so we can rule out a lot of random error. however the lab notebook and i informed you of the systematic error of the anode gun and the collisions of the electrons and the helium. this possibly accounts for the main reasons why our values are so much higher. but of course there is the error in reading the radius of the electron beam. the light bending off the bulb itself, the width of the beam, and also lining up of the beam on the ruler.

what i could do if i had more time

i think it would be possible with more time and more data to find the actual error of the anode gun and the helium in the tube. in the manual it gives you the helium pressure inside the bulb. with that you might be able to calculate hoe much bigger the radius would be with out the helium collisions. however, the anode of the gun would be much harder to calculate its error. because of its circular shape, that is why the electric field is un-uniform and why the beam itself is also effected.

Koch comments

Overall, great data and good analysis (despite the fact that it's impossible to obtain un-flawed data with this lab). Main things missing or needing correction:

  • Your "primary notebook" (the notebook for recording raw data and observations) still needs more information that would be critical if you or someone else wanted to repeat the data later. For example, manufacturer and model number of equipment.
  • More description of details of what you're doing, so that the notebook isn't just full of data.
  • More plots of data would be nice.