### Author Topic: Current Loop in magnetic field (How motor works!)  (Read 49042 times)

#### Fu-Kwun Hwang

• Hero Member
• Posts: 3062
##### Current Loop in magnetic field (How motor works!)
« on: February 01, 2009, 05:32:40 pm »
This simulation shows a current loop in uniform magnetic field.
The black arrows show the force due to magnetic field on the loop.
\$vec{F}= q vec{v}   imes vec{B}\$ or \$vec{F}= I int dvec{ell}   imes vec{B}\$
The magnetic field in in the +Z direction if Bz>0.
Then ,torque for the loop is calculated from definition \$vec{   au}=vec{r}   imesvec{F}\$

Because the current flow in the same counter-clockwise direction.
The force on the loop change sign during each helf cycle.
If we current reverse direction during each helf cycle, the following device can turn into a motor.
A voltage source is needed in the loop to drive the current.
-*-

P.S. The above simulation did not include the induced emf due to changing magnetic field in the loop.

You are welcomed to check out a very good animation DC motor operation principle

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#### Fu-Kwun Hwang

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• Posts: 3062
##### Re: Current Loop in magnetic loop (How motor works!)
« Reply #1 on: February 02, 2009, 12:16:43 am »
Registed user can get files related to this applet for offline access.
Problem viewing java?Add http://www.phy.ntnu.edu.tw/ to exception site list

Here is the real motor:

In order to make motor reach stable rotational motion. A friction force is introduced: F=-constant *b*omage;//where b is the width in y-direction
Even thought magnitude of the magnetic force is the same (black arrows),
the torque is a function of cos(w*t)

If the process is reversed: A loop rotating in a uniform magnetic field will become electric power generator.
Please check out A loop rotating in a magnetic field (How electric power generator works!)

Registed user can get files related to this applet for offline access.
Problem viewing java?Add http://www.phy.ntnu.edu.tw/ to exception site list

Full screen applet or Problem viewing java?Add http://www.phy.ntnu.edu.tw/ to exception site list
• 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.
• Upload worksheets as attached files to share with more users.
Let's work together. We can help more users understand physics conceptually and enjoy the fun of learning physics!

#### lookang

• Hero Member
• Posts: 1774
• http://weelookang.blogspot.com
##### Re: Current Loop in magnetic loop (How motor works!)
« Reply #2 on: February 02, 2009, 08:58:33 am »
This simulation shows a current loop in uniform magnetic field.The magnetic field in in the +Z direction if Bz>0.
1. Add vector field arrow(s) for vector Bz .

Because the current flow in the same counter-clockwise direction.

2. Add  arrow(s) line along the sides of the wires to aid visualization of I current and q current flow.
\$vec{F}= q vec{v}   imes vec{B}\$ or \$vec{F}= I int dvec{ell}   imes vec{B}\$

3. in the electron objects, can draw a negative sign -, to make it immediately association to electrons?

indeed  nice applets
can split posts into 2 separate applets for ease of discussion/collaboration etc.

thanks!

#### Fu-Kwun Hwang

• Hero Member
• Posts: 3062
##### Re: Current Loop in magnetic loop (How motor works!)
« Reply #3 on: February 02, 2009, 10:59:13 am »
I modified it according to your suggestion. However, I did not draw negative sign, because there are too many symbol shown already.
More checkboxs are added for user to decide which symbols s/he did not need! (velocity/field/force)

#### lookang

• Hero Member
• Posts: 1774
• http://weelookang.blogspot.com
##### Re: Current Loop in magnetic field (How motor works!)
« Reply #4 on: October 23, 2009, 01:36:18 pm »
i found these applets that can be referred to for improving the Ejs applet above.

http://www.magnet.fsu.edu/education/tutorials/java/dcmotor/index.html

http://www.walter-fendt.de/ph11e/electricmotor.htm

i try to remix from your applets, i will learn a lot from remixing.

#### lookang

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• Posts: 1774
• http://weelookang.blogspot.com
##### Re: Current Loop in magnetic field (How motor works!)
« Reply #5 on: October 29, 2009, 02:45:34 pm »
« Last Edit: October 29, 2009, 02:47:17 pm by lookang »

#### lookang

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• Posts: 1774
• http://weelookang.blogspot.com
##### Re: Current Loop in magnetic field (How motor works!)
« Reply #6 on: December 12, 2009, 01:26:45 am »
Hi prof hwang,[noeqn]
java.lang.ClassNotFoundException: users.ntnu.fkh.currentloopinbmotor_pkg.currentloopinbmotorApplet.class
at sun.plugin2.applet.Plugin2Manager.createApplet(Unknown Source)
at sun.plugin2.applet.Plugin2Manager\$AppletExecutionRunnable.run(Unknown Source)
Caused by: java.io.IOException: open HTTP connection failed:http://www.phy.ntnu.edu.tw/ntnujava/ejs4/users/ntnu/fkh/currentloopinbmotor_pkg/currentloopinbmotorApplet/class.class
at java.security.AccessController.doPrivileged(Native Method)
... 7 more
Exception: java.lang.ClassNotFoundException: users.ntnu.fkh.currentloopinbmotor_pkg.currentloopinbmotorApplet.class

#### Fu-Kwun Hwang

• Hero Member
• Posts: 3062
##### Re: Current Loop in magnetic field (How motor works!)
« Reply #7 on: December 12, 2009, 10:41:27 am »
Thank you! It is fixed now.

#### macfamous

• Newbie
• Posts: -5
##### Re: Current Loop in magnetic field (How motor works!)
« Reply #8 on: February 26, 2010, 02:11:08 am »
Sorry but I still get a little problem here .. may we have just a simple formula for this .. -*-

#### Fu-Kwun Hwang

• Hero Member
• Posts: 3062
##### Re: Current Loop in magnetic field (How motor works!)
« Reply #9 on: February 26, 2010, 12:03:49 pm »
The fomula for force and torque are provided in the previous posts.
May I know what kind of formula are you looking for?

#### lookang

• Hero Member
• Posts: 1774
• http://weelookang.blogspot.com
##### Re: Current Loop in magnetic loop (How motor works!)
« Reply #10 on: April 15, 2011, 11:32:32 am »
Here is the real motor:

In order to make motor reach stable rotational motion. A friction force is introduced: F=-constant *b*omage;//where b is the width in y-direction
Even thought magnitude of the magnetic force is the same (black arrows),
the torque is a function of cos(w*t)

<applet code="users.ntnu.fkh.currentloopinbmotor_pkg.currentloopinbmotorApplet.class" codebase="/ntnujava/ejs4/" archive="ejs_currentloopinbmotor.jar" name="currentloopinbmotor" id="currentloopinbmotor" width="626" height="628"></applet>

If the process is reversed: A loop rotating in a uniform magnetic field will become electric power generator.
Please check out A loop rotating in a magnetic field (How electric power generator works!)

Hi Prof!
i think i found a small bug that did not affect the simulation model because the inertia in your sim is always 1

the logic is
if
dcta/dt = omega

domega/dt = (getAlpha(cta) -bf*omega)/Inertia

because (getAlpha(cta) -bf*omega) = resultant torque

so magnetic torque = getAlpha(cta)
and magnetic force is F= N*I*L*B*sign2;

the a2 is actual the a2=N*I*L*B*b; (in my DC sim Ejs Open Source DC Motor Model by Fu-Kwun Hwang and lookang http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=1266) instead of a2=I*L*B*b/Inertia (in your current DC sim http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=912.msg3432#msg3432;
ignore my N because i have modified the sim into a multi turns DC motor.

is my interpretation of the physics DC model correct?
thanks!

In other words,
your model should be like this
dcta/dt = omega
domega/dt = (getAlpha(cta) -bf*omega)/Inertia
where a2=I*L*B*b; for your case, since your (or our) a2 is the torque*length b
need to modify code at 2 places.
« Last Edit: April 15, 2011, 11:40:06 am by lookang »