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lookang
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Posts: 1794 http://weelookang.blogspot.com  « Embed this message on: October 04, 2010, 02:04:38 pm » posted from:SINGAPORE,SINGAPORE,SINGAPORE Ejs Open Source Charge Particle in Magnetic Field B Java Applet in 3D
reference:
http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=1800.msg7327#msg7327 Created by prof Hwang Modified by Ahmed
http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=1450msg5484;topicseen#msg5484 Created by prof Hwang

Charge In B-Field
This 3D Ejs Charge particle In B-Field model allows the user to simulate a moving charged particle in two identical uniform magnetic fields separated by a zero magnetic field gap. A charge moving in a magnetic field experiences a magnetic force given by the Lorenz force law $\vec{F}=\vec{v}\times\vec{B}*q$ = v*B*q*sin(theta)
where theta specifies the angle between the velocity vector v and the magnetic field B. In this simulation, the velocity and B-field are perpendicular (theta = 90 degrees) and the force is maximum and perpendicular to both v and B as predicted by $\vec{v}\times\vec{B}$. You can adjust the magnitude of the magnetic field B, the mass m and charge q of the charged particles. The slider at the top controls the width of the field free region (it is a percentage of half the window width). The magnetic field is assumed to be uniform $Bz\hat{z}$ inside the magnet region. and the field is zero when outside the boundary.
You can change the location and velocity of the charged particle with mouse drag and drop or with sliders.
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• Please feel free to post your ideas about how to use the simulation for better teaching and learning.
• Post questions to be asked to help students to think, to explore.
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Let's work together. We can help more users understand physics conceptually and enjoy the fun of learning physics!  activities adapted from http://www.opensourcephysics.org/items/detail.cfm?ID=8984 Charge in Magnetic Field Model written by Fu-Kwun Hwang edited by Robert Mohr and Wolfgang Christian

1 When $Bz\hat{z}$ is positive and the charge particle is completely inside the $Bz\hat{z}$ field region, which way do positively charged particle circle (clockwise or counter-clockwise as view from the top looking down). Use the Fleming left hand (thumb Force, second finger B field and middle finger current i ) or right-hand cross product rule $\vec{F}=\vec{v}\times\vec{B}*q$ to determine if the field points into or out of the screen?
2 Explain why particle traveling in the region without B field, travel in a straight path.
3 Charged particle that remain in the uniform B magnetic field (orange color field vectors) experience uniform circle motion. Why? What provides the centripetal force?
4 design an experiment to investigate systematically, in a table the data and effects of varying the charge particle mass, m and charged q.
5 Do they experience the same force F?
6 What accounts for particle moving in circles of different radii (for the ones that stay in the uniform magnetic field)?
7 How can you change the different parameters to decrease the radius? Explain why each change results in a smaller radius.
8 challenging Optional: If you have EJS installed, modify this model to simulate a cyclotron. http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/cyclot.html It may be useful for you to know that Model ->Initialization page to see how the initial position and velocity of the particle(s) is(are) set as well as looking at the Model-> Custom page to see the equations of motion for a particle in the magnetic field as well as the gap region.
 « Last Edit: December 08, 2010, 01:34:54 pm by lookang » Logged
lookang
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Posts: 1794 http://weelookang.blogspot.com  « Embed this message Reply #1 on: October 04, 2010, 02:07:56 pm » posted from:SINGAPORE,SINGAPORE,SINGAPORE changes made

1. color scheme
2 magnet NS text and position
3 B field visual start at edge of magnet for ease of associating the influence of F = q.v^B where ^ is cross product.
4 rearrange the bottom panel
5 add z and vz into the evolution page and values display for http://link.aip.org/link/?AJP/65/726/1 journal article has this student learning challenge (Bagno & Eylon, 1997)

The question is the paper is:
The velocity of a charged particle moving in a magnetic field is always perpendicular to the direction of the field.

The responses:
37% think it is true,
The reasons and interviews analyzed indicated that the causes are:
a. Recitation of formula: v, B and F are always perpendicular according to left hand or right screw law 81
b. No reason 19
reference:
Bagno, E., & Eylon, B.-S. (1997). From problem solving to a knowledge structure: An example from the domain of electromagnetism. American Journal of Physics, 65( 8 ), 726-736. doi: 10.1119/1.18642

My thoughts:
but the answer is false. it could be a supposition of uniform velocity and circular motion, thus there is an angle =! 90o between v and B, much like a helix path

6 add Force display value F = q.v^B
7 activate the 3 axes coordinate system for ease of communicate and associating motion to x y z direction

other good resources:
http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=1431.0 Charged particle motion in static Electric/Magnetic field by Fu-Kwun Hwang
http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=36 Charged particle motion in E/B Field JDK version by Fu-Kwun Hwang
http://www.opensourcephysics.org/items/detail.cfm?ID=8984 Charge in Magnetic Field Model written by Fu-Kwun Hwang
edited by Robert Mohr and Wolfgang Christian
http://www.opensourcephysics.org/items/detail.cfm?ID=9997 Charge Trajectories in 3D Electrostatic Fields Model written by Andrew Duffy
http://www.opensourcephysics.org/items/detail.cfm?ID=8996 E x B Trajectory Model written by Anne Cox
http://www.compadre.org/osp/items/detail.cfm?ID=8984 Charge in Magnetic Field Model written by Fu-Kwun Hwang edited by Robert Mohr and Wolfgang Christian
 *** There are 1 more attached files. You need to login to acces it! « Last Edit: December 07, 2010, 04:35:28 pm by lookang » Logged
Fu-Kwun Hwang
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Posts: 3085   « Embed this message Reply #2 on: October 05, 2010, 10:08:36 am » posted from:Taipei,T'ai-pei,Taiwan What is the trajectory in your mind if there is an electron inside a vacuum with magnetic field?

Assume the energy of the electron is 1keV: try to calculate it's velocity.
Assume the magnetic field is 1kG(Gauss)=0.1T(Tesla) and velocity is perpendicular to the magnetic field.
: try to calculate it radius and frequency.

With the above calculated data: try to think of the motion of the electron.

If the velocity of the electron is not perpendicular: assume 1% of velocity is parallel to the magnetic field.
What do you think about the average motion of the electron will be look like? Logged
lookang
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Posts: 1794 http://weelookang.blogspot.com  « Embed this message Reply #3 on: October 05, 2010, 10:38:01 am » posted from:SINGAPORE,SINGAPORE,SINGAPORE What is the trajectory in your mind if there is an electron inside a vacuum with magnetic field?
a beam of electrons in helix?

Assume the energy of the electron is 1keV: try to calculate it's velocity.
Assume the magnetic field is 1kG(Gauss)=0.1T(Tesla) and velocity is perpendicular to the magnetic field.
: try to calculate it radius and frequency.
0.5*m*v^2 = E
0.5*9.1*10-31*v^2 = 1*10^3*1.6*10^-19
v = 1.88*10^6 m/s

assume F = m*v^2/r is valid since perpendicular b and v
B*v*q = m*v^2/r
0.1*1.88*10^6*1.6*10^-19 = (9.1*10^-31*1.88*10^6)/r
r = 1.06x10^-4 m

assume circle path
2*pi*r / T = v
3.57x10^-10 s = T

With the above calculated data: try to think of the motion of the electron.
a beam of electrons in helix moving away from the circular axes?

If the velocity of the electron is not perpendicular: assume 1% of velocity is parallel to the magnetic field.
What do you think about the average motion of the electron will be look like?
look like a beam in the vz direction? a beam of electrons in helix motion moving away from the circular axes x and y?

Am i visualizing this.? http://www.thunderbolts.info/forum/phpBB3/viewtopic.php?p=27760&sid=9a745d78c23d14d82acc194a8389c7a2

not sure whether this is what you mean « Last Edit: October 05, 2010, 10:44:48 am by lookang » Logged
Fu-Kwun Hwang
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Posts: 3085   « Embed this message Reply #4 on: October 05, 2010, 10:17:50 pm » posted from:,,Taiwan Think about the magnitude of the physics quantity is very important when we study physics.

Did you notice that the radius is very small: $10^{-4}$ m
And the period of one resolution is very small,too. $T=3.57\times 10^{-10}$ s.
So the electron will make almost $3\times 10^9$ turns in one second.
There is no way you can observed such trajectory with your eye or ordinary device.
And the velocity is very large. $v=1.88\times 10^6$ m/s.
If only 1% of velocity component is along the field line, the electron will move along the field line with speed $u\approx 10^4$m/s $\approx 10$ km/s.

So on average, the electron will move along the field line.  That is also how electrons move between sourth pole and north pole-- following the field line of earth's magnetic field.

When the magnetic field is very strong, it will force electron moving back -- that need more physics analysis.
(or check out Helmholtz coil / particle trapped in magnetic mirror field) Logged
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