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V0.6
LAB EXERCISE COTR – L-E3T
Charge to Mass Ratio for Electrons
Lab format : This lab is performed using the Remote Web-based Science Laboratory (RWSL)
Relationship to theory : This lab allows students to determine the charge to mass ratio of an electron for themselves. This was a critical experiment during the development of electromagnetic theory.
OBJECTIVES

Measure the charge to mass ratio of an electron.
EQUIPMENT LIST

RWSL e/m Lab*
Student Supplied:

For RWSL Access: o Computer: PC running Windows Vista or later o Browser: Internet Explorer 8 or later o Internet: 5 Mb/s or faster Internet connection
* The equipment used is Nada Scientific’s Nakamura e/m Apparatus (N99-B10-7350). You can view the manual for this equipment at the company’s website: http://nadascientific.com/media/PDF/N99-B10-
7350_Manual.pdf
Figure 01: Nakamura e/m Apparatus (N99-B10-7350)
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INTRODUCTION
This e/m lab exercise deals with the fundamental characteristics of the electron: its charge to mass ratio and the charge on an electron. From these we can deduce the effective mass of the electron. These two constants are used throughout physics including quantum mechanics and spectroscopy.
WARNINGS

There are no special warnings associated with the lab exercise.
THEORY
(This part was originally written by David DeForge for the Web-based Associate of Science Project, was picked up by the NANSLO Project, and has been adapted for this lab exercise.)
The e/m apparatus is a gas discharge tube set within a Helmholtz coil, as illustrated in Figure D1, to examine the motion of electrons in a magnetic field. (A Helmholtz coil is made up of 2 parallel coils as shown below that are arranged on the same axis such that the distance between two coils is equal to their radius.) Helmholtz coils produce a nearly uniform magnetic field over their central region where the entire gas discharge tube is placed. When excited by collisions with electrons, the gas helium in the discharge tube emits light so we can see the path of the electrons.
Figure 02: Helmholtz Coils with Gas Discharge Tube showing the Electron Path Figure 03: The e/m Gas Discharge Tube
When a charged particle, of charge q, mass m and velocity v , enters a uniform magnetic field B , a force F
B
on the charged particle is created by the magnetic field interacting with the moving electron
(see Figure 4 below). This force is given by:
F
B
 q

EQ L-E3T.01
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The force direction is perpendicular to both the charged particle velocity and the magnetic field. r v
F
B
B
Figure 04: Force on a moving charged particle in a magnetic field. (Note that the velocity is up, the magnetic field is into the page, and the force is to the right as defined by the ‘right hand rule’.)
The magnitude of the force is
F
B
 qvB sin

EQ L-E3T.02 where

is the angle between velocity and the magnetic field.
In the case of an electron, charge simplifies to
 e , moving perpendicular to the magnetic field (
  
), this
F
B
 evB EQ L-E3T.03
The sign of the force is ignored here because we are only interested in the magnitude, but it should be noted that the direction will be opposite to what it would be for a proton.
We will assume that all other forces on the electron are negligible. An object under a net force that is perpendicular to its velocity will undergo circular motion with centripetal acceleration a c v
2
 r
EQ L-E3T.04
In this case, the net force is F
B
 ma c
so, substituting terms, evB
 m v
2 r
EQ L-E3T.05
In this experiment, electrons pass through an apparatus similar to the parallel plates covered in the
Electric Fields portion of the course as they enter the gas discharge tube. The potential difference between the plates is V. Assuming kinetic energy of electrons before entering the plates is negligible, the kinetic energy of the electrons after passing through the plates ( K after
) is equal to the potential energy they experienced before passing through the plates ( P before
) or:
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P before

K after
EQ L-E3T.06a
Remember that the potential energy available for a charge moving between 2 parallel plates is given by:
P before
 eV EQ L-E3T.06b
Kinetic energy is given by:
K after

1
2 mv 2 so from equation (EQ L-E3T.06a) we have:
1
2 mv
2  eV
We can rearrange equation (EQ L-E3T.05) to isolate the velocity v
 erB m and substitute this into the kinetic energy equation (EQ L-E3T.07) to get
EQ L-E3T.06c
EQ L-E3T.07
EQ L-E3T.08
1
2 m

 erB m
 eV or, with a bit of work, e

2 V
2 2 m r B or r 2 
2 Vm eB
2
EQ L-E3T.09
EQ L-E3T.10a
EQ L-E3T.10b
This shows that we can measure the ratio of the electron charge to the electron mass if we know the potential difference used to accelerate the electrons, the magnetic field strength, and the radius of the circular motion of the electron.
Depending on the magnetic field strength B and the accelerating potential V, the electrons can travel anywhere within the discharge tube. The Helmholtz coils produce a magnetic field that is constant to within 1%, and in the direction of the line joining the centres of the two coils, over the entire discharge tube. The magnitude of the Helmholtz coil magnetic field within the discharge tube is:
B
4
3/ 2 
0
NI
R
EQ L-E3T.11
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LAB x where

0
is the permeability of free space, I is the coil current, N is the number of windings in the coils, and R is the radius of the coils. (See (EQ L-E3T.14a) below for a more general equation.)
We can now combine equations (EQ L-E3T.10a) and (EQ L-E3T.11) for an expression for e/m in terms of items that are relatively easy to measure (accelerating potential V, coil current I, coil radius R, and radius of electron beam r). e m 
125 R
2
32 N 2

2
0



V
EQ L-E3T.12
In equation (EQ L-E3T.12), the terms inside of the brackets are constant.
For the RWSL e/m Lab apparatus these values are:
R

0.150
m
N

130 turns
EQ L-E3T.13a
EQ L-E3T.13b

0

4
 
10

7
EQ L-E3T.13c
A
THE HELMHOLTZ COILS (This section originally written by Rick Nowell and adapted for RWSL delivery by
Ron Evans):
When the distance between two coils is equal to their radius, you have a Helmholtz coil, which produces a uniform magnetic field over their middle two thirds. These are named after Hermann von Helmholtz a
German physicist (1821-1894) who worked on electrodynamics and æther theories.
You can view the manual for this equipment at the company’s website: http://nadascientific.com/media/PDF/N99-B10-7350_Manual.pdf
The general equation for the magnetic-field strength at a distance x along the centre axis through the middle of a Helmholtz pair is:
B x

1
2

0
NIR 2



R 2  x 2
 
3/ 2 

2 R 2  x 2 
2 Rx
 
3/ 2

EQ L-E3T.14a
Midway between the coils,
B
4
3/ 2 
0
NI
R x

R / 2 , and this equation reduces to (EQ L-E3T.11):
EQ L-E3T.14b
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Where N is the number of turns per coil, I is the current in the coils in amperes, μ o
is permeability of free space at 4π

10 -7
Webers/(A
 m) where A is ampere-turns and R is the radius of the coil in meters.
Magnetic Field Units: Recall that webers {m²kg/(s²A)} are measures of magnetic flux "φ", where 1 Wb = 10 8 field lines.
Teslas measure magnetic flux density, the "B" field, given in
webers per square metre. (If your equipment measures in more sensitive CGS gauss units then it can be converted with
{10 4 G = 1 T}. Eg. 7.5G = 0.75mT. MKS uses a square metre instead of a square centimetre of area: since 1 field line per square centimetre is one gauss and there are 100x100 cm² in a
m², this is where the 10 4 or 10,000 conversion number comes from.)
Figure 05: Magnetic Flux Lines
PROCEDURE
Measuring the Value of e/m
1.
Review Appendix 1: The RWSL e/m Apparatus
2.
Schedule a lab session on the RWSL e/m Lab. Your instructor will have the dates when it will be available as well as scheduling instructions.
3.
Make sure your computer system is certified for RWSL access as described in Appendix 1. If it can’t be, find a system at your local educational institution or somewhere else that can be certified.
4.
A few minutes before your scheduled lab session access the RWSL and, if you have lab partners contact them. You will need to coordinate who has actual control of the e/m apparatus when so make sure your communications backchannel is agreed upon and working for everyone in the lab group.
5.
At the appointed time of your session, one member of your lab group should request control of the RWSL e/m apparatus.
6.
Each member of the group should take a turn running through the e/m apparatus virtual instrument (VI) controls so everyone is familiar with them and can take data. See the “Using the
RWSL e/m Lab”, “Camera Controls”, and “Experimental Operation” sections of Appendix 1 for a review of these controls and instructions on acquiring your data.
7.
Follow the power up procedure described in Appendix 1.
8.
Give all members of your lab group a chance to control the e/m apparatus and collect your data.
(See Appendix 1 for instructions on how to do this.)
9.
For the following procedure, use anode voltages from 300V down to 200V in steps of 50V. At each voltage, adjust the current through the Helmholtz coils until the magnetic-field bends the circle of electrons to the 10cm diameter markers inside the tube. At each circle record path
diameter, anode voltage and coil current. a.
Set Anode voltage to 300V. b.
Adjust Magnetic Field Current So Electron Beam is Bent to 10cm Diameter: As described in Appendix 1, the markers on the glass rod are difficult to read in the video
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LAB x image, so set the blue slider to a diameter of 10.0 cm. Adjust the magnetic-field current until the beam is centred on the 10.0 cm marker for a given anode voltage. c.
Record the Helmholtz coil current and circle diameter for that anode voltage. Estimate the beam thickness for error analysis.
Note: avoid using results where the beam path is elliptical or helical since the equation we derived assumes a circular path. If this should occur, contact the RWSL tech on duty and ask him/her to adjust
the e/m tube so that the beam is again circular. d.
Change Anode Voltage: lower the anode voltage by 50V. Repeat steps 9b and 9c for
250 and then 200V.
10.
Test how the Circle Diameter depends on Magnetic Field Strength: Record Helmholtz current values for diameters of 8 cm, 6 cm, and 4 cm with an anode voltage fixed at 300V. Predict the outcome: would you expect to use twice as much, or half the magnetic field current to halve the circle diameter from 8 to 4 cm? Test & see.
11.
Test how the Circle Diameter depends on Anode voltage V: At lower anode voltages, the electrons will be travelling slower and will bend more in the same magnetic field. Record circle diameters for anode voltages of 300V and 150V. Keep the field current fixed at 3.00A.
12.
When we reduce the anode voltage by half (150V) does the beam bend to a smaller circle or go in a bigger circle? At 150V, would you expect the beam diameter to be twice as much as at
300V, half as much, or 0.7 as much?
NOTES:
Turn down coil current when not in use so the coils don't overheat.
Turn down the anode voltage when not in use to prolong the tube life.
13.
When everyone in your group has collected their data follow the power down procedure described in Appendix 1.
ANALYSIS AND/OR QUESTIONS
1.
Calculate B: Calculate B for each value of the coil current you used. Use equation (EQ L-E3T.11) and the constant values in equations (EQ L-E3T.13a, b, & c).
2.
Calculating e/m: For each run of the lab, calculate e/m from equation (EQ L-E3T.12), your measured values of V, I, r, and the constant values in equations (EQ L-E3T.13a, b, & c).
3.
Error Analysis: Estimate uncertainties for r and V. a.
r: From the estimated thickness of your electron beam, calculate the amount of uncertainty you'd get for r at the 10.0cm ring. b.
V: Are the higher or lower anode voltages more accurate?
4.
Examine your data and from the principles of error analysis, select your best e/m value (the value having the least uncertainty).
5.
Variation: Find the average of your 10.0 cm circle e/m values. Estimate the variation in your readings by finding the standard deviation in the average. Compare this e/m value to the
accepted value of (1.75890 ± 0.00004)

10 11 coul/kg.
6.
Using a value for "e" find the electron's effective mass "m".
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QUESTIONS:
1.
The electron gun shoots electrons at high speeds: using the accepted value for e/m: a.
How fast are the electrons going if the anode voltage is set to 300V? What percentage of the speed of light is this?
2.
The inner 2 cm circle marker cannot be reached for certain anode voltages and magnetic field strengths: a.
Would you have to increase or decrease the anode voltage to allow the inner circle to be reached? Why would that work? b.
Would you have to increase or decrease the magnetic field current to allow the inner circle to be reached?
3.
If your electron beam were 0.2cm thick, calculate the percentage of error you'd get for r at the
2.0 and 10 cm markers. Which circle radius has less error, the innermost or outermost ring?
4.
Would a higher anode voltage (300V) be more accurate than a lower voltage (200V) when measuring e/m?
5.
The Earth's magnetic-field is about 0.05 millitesla--what effect would the Earth's magnetic-field have on the experiment?
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Lab Exercise L-E3T Results
(Note: use these tables or create your own in your lab book).
e/m RESULTS
Anode
Voltage V
Beam Circle
Diameter 2r
(cm)
Helmholtz
Current
(A)
B Field
(Teslas)
calc
300 V
250 V
200 V
10.0
10.0
10.0
Testing circle radius as a function of B and V:
Anode
Voltage V
Beam Circle
Diameter 2r
(cm)
Helmholtz
Current
(A)
Average e/m
Stan Dev e/m
B Field
(Teslas)
calc
Name:
e/m value
Calculated
(coul/kg)
Ratio of
300 V
300 V
300 V
8.00
4.00
3.00
150 V 3.00
Error Check: Δr = the thickness of the electron beam = ____________cm.
Best value of e/m = _____________ ± ___________ coul/kg
% deviation from accepted e/m value of 1.75890

10 11 coul/kg = _____________ %
Best value for the effective mass of the electron:
Currents
Diameters
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REFERENCES
From Original Lab Exercise
1.
Halliday and Resnick, Fundamentals of Physics, 3 rd ed, #30.5
2.
Tipler, Physics for Scientists & Engineers, 3 rd ed, #24-2, #25-1
3.
3B Scientific: Instruction sheet, ELWE Fine Beam Tube, 05/07
Original Lab Manual by Rick Nowel, E. Tech, COTR
Adapted for Remote Delivery by Ron Evans, MSc
Under the Remote Science Labs for Second Year Physics Project funded by BCcampus
2012 - 2013
Public domain image in Figure 05 was imported from the original lab manual that was produced by COTR.
Images in Figures 02, 03, and 04 were taken from the CC licensed e/m lab produced by Dave Deforge under the WASc project led by NIC.
All other images were produced by Ron Evans and are covered by the CC license of this document.
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(This section has been adapted from the instructor training manual for the RWSL e/m apparatus lab that was produced under the NANSLO project.)
RWSL e/m Lab
The Remote Web-based Science Laboratory (RWSL) is a robotic and software interface designed to enable the student to access and use science lab equipment over the internet and collect authentic realworld data in real-time. The RWSL e/m Lab in particular, is used to measure the charge-to-mass ratio of an electron. This is accomplished by allowing the student to control the kinetic energy of an electron beam and the strength of a uniform magnetic field produced by a Helmholtz coil. Since the force on a charged particle moving in a magnetic field is equal to the charge on the particle times the cross product between the velocity and the magnetic field, the force the charged particle ‘feels’ will be perpendicular to both the velocity and the magnetic field of the particle. When the velocity is perpendicular to the magnetic field, the charged particle will travel along a circular path. The e/m apparatus utilizes this phenomenon to allow the charge-to-mass ratio of an electron to be measured. The apparatus provides the student with information about the energy imparted to the electron beam, the amount of current passing through the Helmholtz coils, and the diameter of the circular electron beam. This information allows students to calculate charge-to-mass ratio by using the equations provided in this lab exercise.
MAKING SURE YOUR COMPUTER SYSTEM CAN SUCCESSFULLY CONNECT TO RWSL
There will be a procedure in place for insuring your system will work well with the RWSL before you try accessing it. At the time of writing this process is changing, so ask your instructor for the current procedure that must be followed at the time you will be doing your RWSL e/m lab exercise. Your instructor may need to contact the RWSL techs to obtain this.
COMMUNICATING DURING RWSL MEDIATED LAB EXERCISES
It is important that you and the members of your lab group be able to communicate among yourselves during the lab session. It is also important that you be able to communicate with the RWSL tech on duty should unexpected issues arise. Your instructor may designate the back channel to be used based on his/her preferences or conversations with the RWSL tech who will be on duty for your lab sessions. If not then you will at least want to agree on this with your lab partners.
If you are choosing the back-channel yourself, please keep in mind your backchannel:
1) should allow multiple participants (or at least the number of members of your lab group)
2) must be agreed upon by everyone in your lab group ahead of time
3) should use minimal bandwidth so as not to degrade the connection to RWSL.*
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*Although video chatting might be desirable when talking to each other before and after the lab session, it’s important to remember that a streaming video connection requires a great deal of bandwidth. When connectivity is limited, this extra video connection could significantly degrade the connection to RWSL.
For this reason you should probably not use video communications with each other during the actual
RWSL connection. Audio is important as it is the most efficient way to convey information among lab group participants. An audio connection requires significantly less bandwidth than video, and while it may degrade the RWSL connection when bandwidth is marginal, this is not so likely. Typing into a chat window uses the least bandwidth of all. Consequently your communications set-up should include a text chat feature in case the audio fails or member of your lab group doesn’t have the necessary equipment to support audio communication.
You can choose your back-channel communications venue from a number of free commercial products including but not limited to:
1) Google+
2) Skype
3) MSN
4) telephone (if your lab group is 2 and you don’t live too far from each other)
5) Others
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USING THE RWSL e/m LAB
When you first access the RWSL e/m Lab your computer screen will display something like this:
Figure A1: Start screen
This screen is called a virtual instrument (VI) in general and in this case the e/m VI. As in most RWSL VIs, the right panel displays the video feed and the camera controls, and the left panel displays the experiment controls. At the bottom left of the screen you’ll see the “Reload Page” and “Refresh Video” buttons. If you have a problem with your connection to RWSL, the RWSL tech may instruct you to click on one of these buttons.
To gain control of the RWSL e/m Lab, right-click on the VI and request control. After you request control of the VI you may have to wait a few moments before you actually receive control. Be patient and occasionally try to drag the blue pointer under the image. When it moves you’ll know that you have control. If waiting for control seems to be taking too long, right-click on the VI and request control again. If it takes more than a couple of minutes, contact the RWSL tech on the communications ‘back channel’ or using the method your instructor specifies.
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CAMERA CONTROLS
Note: For a general discussion of camera controls see Appendix 1.1 below. This section only covers those controls that are specific to the RWSL e/m Lab. If you have never used the
RWSL then it is recommended that you review this appendix now before proceeding.
The camera controls for the e/m Lab are similar to those for other RWSL mediated lab exercises.
Figure A2: the camera controls
The variations specific to the e/m Lab camera controls are as follows:
Camera selection: The RWSL e/m Lab has two pan-tilt-zoom cameras available. You can use camera 1 to look around the lab, but camera 2 is peering down the tube at the actual e/m apparatus. It is recommended that you do not move camera 2 around because slight vibrations could compromise its alignment so that your measurements will be off.
Camera 1 pre-sets :
Preset 1: shows the outside of the apparatus. This is the default starting view.
Preset 2: shows the display on the Heater power supply.
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Preset 3: shows the display on the Helmholtz Coil power supply.
Preset 4: shows the display on the Anode power supply.
Presets 5 and 6 are not defined.
Preset 2 Preset 3 Preset 4
Figure A3: Heater Display Panel
Camera 2 pre-sets :
Figure A4: Helmholtz Coil Display
Panel
Figure A5: Anode Display Panel
Preset 1: will show the entire e/m tube, but you won’t see anything until the apparatus is turned on and glowing. This is the default starting view for this camera.
Preset 2: will show where the electron beam crosses the glass measuring rod within the e/m tube.
Presets 3, 4, 5, and 6 are not defined.
Preset 1 Preset 2
Figure A6: e/m tube
Figure A7: Glass Measuring Rod
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The PIP (Picture in Picture) control is unique to the e/m Lab:
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Figure A9: PIP button "on"
Figure A8: Picture-in-picture (PIP)
To use this feature, select camera 1 and set it to show the display you want to see while running the experiment. Once it is set, select camera 2. Clicking the PIP control button will generate a small picture of what camera 1 is looking at in the upper right corner of the camera 2 display. The purpose of this option will be described later.
“Inukshuk” control : The “Inukshuk” controls will function as usual for RWSL camera control. (See
Appendix 1.1) However, for this lab in particular you should not move Camera 2. Slight vibrations can compromise its alignment.
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RWSL E/M APPARATUS OPERATION
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Figure A10: e/m Apparatus controls
The left side of the e/m VI is dedicated to the control of the e/m Lab. (See Fig. 11)
First select Camera 2, preset 1 and you will see a dark image:
Figure A11: Camera 2 Preset 1
"on"
Figure A12: e/m Apparatus “off”
To start the e/m apparatus, turn on the various pieces in the following order:
1) Turn on the heater to warm up the e/m tube. Camera 2, preset 1 now shows:
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Figure 1: Heater button "on"
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Figure A14: e/m apparatus heater turned on
The heater heats up the cathode, which not only begins to glow but also begins to emit electrons in a random manner.
2) Turn on the anode to start the electron beam and adjust the voltage you want to apply to the anode. Voltage can be varied from 0 V to 500 V. In the example below, it is set close to 300V.
Figure 2: Anode button "on"
Figure 3: e/m apparatus anode is on
You can see that the value shown under the control knob (291.3, in this example) is very close to what the power supply reads. (To compare this you would go to Camera 1 preset 4 and read the value shown on the power supply display.)
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The anode accelerates the electrons in a single direction causing a beam to project downward.
It glows because there is a rarified helium atmosphere within the e/m tube. Some of the electrons in the beam interact with the helium gas which then glows. The amount of voltage applied to the anode allows you to calculate the average kinetic energy of the electrons in the beam. (See the Theory section above.)
3) Turn on the Helmholtz Coil to apply the magnetic field.
Figure 4: Helmholtz coil button "on"
Figure 5: Helmholtz coil is on
You can see the effect of the magnetic field on the electrons as they are now traveling in a circle instead of a straight line. (The fainter circle outside the electron beam arc is a reflection off the e/m tube itself.) You can also see the electron beam illuminating the glass measuring rod inside the e/m tube. To see this more clearly, switch camera 2 to preset 2 and you’ll see:
Figure A19: Glass measuring rod
As you can see, the glass measuring rod at the bottom of the image is very difficult to read. It is graduated in half-cm increments. In this image the electron beam appears to be intersecting
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LAB x the rod at about 9.5 cm. This is the diameter of the circular electron beam’s path around the e/m tube. To get a more accurate reading a scale has been added below the image and calibrated to match the less readable scale on the glass rod. The blue pointer is a slider that you can slide left or right until it lines up with the point where the electron beam intersects the glass rod as shown here:
Figure A71: Reading the measuring slider
Figure A60: Measuring slider
Now it is easy to see that the electron beam diameter is 9.55 ± 0.02 cm (In the opinion of this author, the thickness of both the beam and the blue measuring pointer makes the measurement more uncertain than ± 0.01 cm would indicate. Decide for yourself.) It is important for each member of your lab group to make multiple measurements at each setting so that random error can be reduced.
By varying the Helmholtz coil voltage and current the strength of the magnetic field is controlled. These controls affect each other to some extent and sometimes the numbers displayed under the dials are not as accurate as they might be. To get accurate readings of these two values it is important to see the readout from the Helmholtz coil power supply while watching the behavior of the electron beam. Set Camera 1 on preset 3, switch to camera 2 (still on preset 2) and click on the PIP control. Then you will see:
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Figure A82: Helmholtz coil controls
Figure A93: PIP on, Helmholtz coil display
Compare the readouts under the control knobs in the sub-VI to the readout from the actual display panel and you will note that although the 8.45 V under the voltage control knob is close to the 8.46 V displayed on the actual Helmholtz coil power supply, the 2.00 A under the Current control knob bears no resemblance to the 1.45 A displayed on the actual power supply. The 2.00
A under the VI Current control knob is a maximum, while the value shown on the power supply is the actual current flowing through the Helmholtz coil and the value that should be used in calculations. As you know from Ohms Law, electric potential (Volts) is closely related to the current (Amps) flowing through a circuit, so you can’t change one without changing the other.
Increased magnetic field strength is produced by increasing the current flow through the
Helmholtz coil. This increases the magnetic field strength causing the electrons to feel a stronger pull toward the centre of their circular path and the electron beam moves in a tighter circle. Reducing the current setting will have the opposite effect. These images show how progressively increasing the electric potential changes the current to the Helmholtz coil. Notice the smaller electron beam arc in each case:
Figure A104: Increasing the voltage (1)
Figure A115: Increased magnetic field (1)
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Figure A126: Increasing the voltage (2)
Figure A148: Increasing the voltage (3)
Figure A137: Increased magnetic field (2)
Figure A29: Increased magnetic field (3)
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LAB x
Decreasing the maximum current allowed will have the opposite effect:
Figure 1530: Decreasing the current
Figure A161: Decreased magnetic field
If we hold the current to the Helmholtz coil constant (in this case at the maximum) and vary the anode voltage we see that as the anode voltage is raised, the energy imparted to the electrons increases so the diameter of the electron beam circular path will increase as shown below:
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Figure A172: Anode voltage (1)
Figure A183: Magnetic field maximum (1)
Figure A194: Anode voltage (2)
Figure A205: Magnetic field maximum (2)
Figure A216: Anode voltage (3)
Figure A227: Magnetic field maximum (3)
Also notice that more energy is imparted to the rarified helium atmosphere inside the e/m tube and it glows more brightly with increased anode voltage.
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Wrapping up the experimental session
When your turn is over, or the lab session is wrapping up, shut down the various pieces of equipment in the reverse order from the way you turned them on. First turn off the Helmholtz coil, then the Anode, and finally the Heater.
Finally, make sure you right-click on the VI and select “Release the VI”. Other group participants will not be able to get control of the spectrometer until the VI is released.
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PHYSICS COURSE NAME
LAB x
APPENDIX 1.1: GENERAL RWSL CAMERA CONTROLS
With only a couple of exceptions, the camera controls for all RWSL-mediated labs will always appear on the right side of the LabView Virtual Instrument (VI) screen. In Fig. B1 (below) you can see the camera image as it appears for the spectrometer set-up (the image will be different when using other lab apparatus).
As you experiment with the various camera controls, the image above the Camera Controls will change in some way depending on which camera control you are using.
Camera Selection:
RWSL supports up to 4 different cameras. These are selected by clicking on the button that corresponds to each camera. You can confirm that this image currently has camera 1 selected because the left most button is lit up and appears bright green. If camera 2 were selected then the second button from the left will appear bright green and the first button will be unlit and appear dark. Cameras 3 and 4 are selected using the 3 rd from the left and the
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LAB x right most buttons respectively. Not all RWSL lab exercises utilize all four cameras. If a lab does not use all the available cameras, buttons which do not map to a camera will simply not respond.
Each camera can be preset with up to 6 different positions as indicated by the rectangular buttons in the Camera Controls. After selecting the camera you want to use, you can select the pre-set positions for that camera. If the camera has been configured with fewer than 6 presets, the rectangular button corresponding to an undefined preset position will simply not respond.
“Inukshuk” Controls (Pan, Tilt, and Zoom):
The “Inukshuk” control on the left side of the Camera Control area allows you to pan
(move left and right), tilt (move up and down), and zoom (as with a hand-held digital camera) the selected camera. The selected camera must have these functions built in for these controls to work. Please refer to the camera instructions specific to the lab you are working with for these details. Because the “Inukshuk” controls are so familiar, you will probably find them relatively intuitive to use. Play with them and you will see what each of the oval buttons does.
Camera On/off Button:
The circular green button in the middle of the 4 directional control buttons is a toggle that will turn the selected camera off and on.
When you are done exploring the camera controls, you can restore the initial camera view for the lab apparatus by selecting camera 1 (the left most camera button) and pressing preset 1.
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