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Ralph Cueva Partners: Michael Bunger and Pavel Aprelev Physics 121H May 24, 2010
Circulating Charged Particles in a Magnetic Field Abstract
The purpose of the experiment was to determine the ratio of the electron’s
charge to its mass . We used the apparatus to measure the radius of an electron
beam. Our value for is . When we compare the
experimental value to the actual value, , we calculated a 11.1% error.
The experimental value did not fall within the uncertainties. Introduction/Theory
A charged particle moving in a magnetic field experiences a force that is:
(1)
Represents the charge of the particle, represents the particle’s velocity,
and is the magnetic field, if is perpendicular to , the trajectory is circular As shown in image 1. Therefore we can use the momentum principle. Since we can derive the formula for the radius of the circular path as follows:
Image 1 -The trajectory taken by particles as they experience a constant magnetic force. If the velocity is perpendicular with the magnetic force then the path is circular, as shown in image 1.
Magnetic Field with direction into the page
The Circular path taken by the particle
The magnitude of the magnetic force is given by , this is then equal to the
magnitude of the change of momentum . Since we know the motion is circular,
the magnitude of change in momentum equals (centripetal force) which then equals to the magnetic force given in formula (1). The angle is because . If <<c then gamma , then we can solve for R as follows:
(2)
Where is the mass of the charged particle and the charge of an electron is
e. If an electron, initially at rest, is accelerated through an electric potential difference , then its kinetic energy is equal to the change of the potential energy:
and because energy is conserved we know that:
(3)
From equations (2) and (3) we can eliminate , and substituting the
magnitude of the electron charge e for , we obtain the following:
(4)
and solve for as follows:
(5)
Formula (5) is key to our experiment. This is the equation that we used to
calculate the ratio. We know that the slope is for the function of . The
slope of this line equals the ratio.
To calculate the ratio we need to know the potential that accelerates
the electrons, the magnitude of the magnetic field and the radius of the circular
path of the electron beam . The magnetic field of the Helmholtz coils can be found using the following:
(6)
where the number of turns of the wire in the coil is , constant
, the current in the coils is , and the coil’s radius is .
Procedure
Figure
1 -‐shows our basic setup. We included Multimeters in order to read an accurate current and accurate acclerating voltage. The Helmholtz coils create the magnetic field that allowed for the circular path. The vaccumed bulb had the
Figure 1- the e/m apparatus and power supplies
Helmholtz Coils
Vacuumed Bulb
electron gun and helium gas that allowed for the visibility of the beam of electrons.
Figure 2 -‐Offers a close look at the vacuumed Bulb. We can see the data read out is a glass ruler. In order to measure the radius correctly we aligned the beam with its reflection, which is visible in the glass ruler. For better accuracy we took both right and left side radii.
The purpose of the apparatus is to allow for the measurement of the ratio
of the charge of an electron to its mass by observing the measurement of the radius of the circular path taken by the electron beam.
The circular path is due to the effects of a constant magnetic field. A pair of Helmholtz coils that surrounds the vacuum tube provides this magnetic field. The vacuum tube contains an electron gun and helium gas. A clear view of the path is produced when electrons hit this gas. Like the cyclotron at Harvard, this simple apparatus works under the same principle. This device contains a source of particles, the gun, an accelerator method, and a data read out, the glass ruler. The source of electrons comes from the heated filament in the gun.
We began the experiment by setting up the apparatus. We first set up all three power supplies. We used a DC Power Supply to produce a constant current through the Helmholtz coils. We used a second power supply, the PASCO SF-‐1985, with AC connections to heat up the cathode of the electron gun. We used the third power supply, PASCO SF-‐9585A, with DC connections to accelerate the electrons. We used a voltammeter to read out the accelerating voltage .
With the setup finished, we turned on the power supplies and set the current 1.46+-‐.01A. We used an ammeter to make sure the current was constant throughout the experiment. We set the heating voltage to 6V and the accelerating voltage to a starting 300V. After each reading we decreased the magnitude of by 10V until we it reached 100V. Under any circumstances we did not do the following: set the
Figure 2 A closer look at the Electron beam
amperage to anything higher than 2A, or set the Voltage to anything over 6.3V, if this
is done the filament of the gun will burn out, thus destroying the apparatus.
After the electron beam was clear we started recording data. By using the glass ruler in the apparatus we recorded the left and right radii of the circular path. Two of us gave readings while one of us recorded the data in an excel file. Data/Analysis The data shown below is the left and right radius of the electron beam at various Voltages. Our data shows that the data readout method, the glass ruler is efficient. The difference in both read outs was small.
Radius of the electron Beam
R(left) R(right) R(left) R(right) R(avg) 300 4.8 5 4.8 5.1 0.04925 0.002426
290 4.8 4.9 4.7 4.9 0.04825 0.002328
279 4.7 4.9 4.7 4.9 0.048 0.002304
270 4.6 4.8 4.6 4.8 0.047 0.002209
259 4.5 4.8 4.5 4.7 0.04625 0.002139
250 4.5 4.7 4.5 4.6 0.04575 0.002093
239 4.4 4.5 4.4 4.5 0.0445 0.00198
231 4.3 4.5 4.3 4.5 0.044 0.001936
221 4.2 4.4 4.2 4.4 0.043 0.001849
210 4 4.2 4.1 4.3 0.0415 0.001722
200 3.9 4.2 4 4.2 0.04075 0.001661
191 3.8 4.1 3.8 4.1 0.0395 0.00156
181 3.7 4 3.8 4.1 0.039 0.001521
171 3.6 3.9 3.6 4 0.03775 0.001425
160 3.5 3.8 3.5 3.8 0.0365 0.001332
150 3.3 3.7 3.4 3.8 0.0355 0.00126
140 3.2 3.6 3.2 3.5 0.03375 0.001139
131 3 3.5 3.1 3.4 0.0325 0.001056
120 3 3.3 3 3.3 0.0315 0.000992
111 2.9 3.2 2.8 3.1 0.03 0.0009
101 2.6 3 2.6 3 0.028 0.000784 Data table 1 shows the left and right radius dependence in the accelerating voltage
Figure 3 Voltage VS the Square Radius
Figure (3) shows the plot and fitted line to our data. The voltage was the independent variable, and was dependent on . Where the slope of the fitted
line is
The actual ratio can be found as follows
We calculated the magnetic field by using formula (5) as follows
After calculating the B field we then used the slope of the fitted line and formula (5) as follows
y = 8E-‐06x + 2E-‐06
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0 50 100 150 200 250 300 350
R 2 (m2)
Vacc (V)
By solving for m/e we got
By dividing our answer 1 we got the ratio as follows
Uncertainties for all measured Quantities Our measured Quantities are the magnetic field of the Helmholtz Coils, the
Current running thought the coils, the slope of the fitted line (this will include the uncertainties of the radiuses recorded and the accelerating voltages , and the
ratio of .
We calculated the uncertainty for the magnetic field as follows:
Our final result for the magnetic field is The uncertainty of the current comes from the accuracy of the ammeter, which is 1% thus
Our result for the current is We used excels regression to find the uncertainty for the radiuses and the accelerating voltage as follows
Uncertainty for the slope=
Our result for the slope of the line is
Finally, we used the uncertainties to calculate the uncertainty of the ratio as follows
Our result for the ratio is
Comparison of experimental value with actual value
We used the following equation to find the percent difference of the experimental ratio to the actual ratio
Discussion/Conclusion
The experiment’s purpose was to determine the ratio of the electron’s
charge to its mass . We used the apparatus to measure the radius of an electron
beam that had a circular path. This path was a direct effect of the magnetic force the Helmholtz coils have on the electrons. The magnetic field of the Helmholtz coils was so large (two order of magnitude larger than the earth’s magnetic field) that we did
not take into account earth’s magnetic field. Our value for is
. When we compared the experimental value to the actual
value, we got an 11.1% error. The experimental value did not fall within
the uncertainties. The error most likely occurred in the read out the left and right radii. Better accuracy in our readings for the accelerating voltage, current in the coils, and radius of the circular path can solve the problem. References http://www.physics.harvard.edu/~wilson/cyclotron/history.html