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BioPhysicist
BioPhysicist
01:00
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I had an idea on how to solve this, but I can't come up with any of the answer choices. How should one approach this?
 
1 hour later…
Wolgwang
Wolgwang
02:20
@BioPhysicist By calculating torque about A after the string is cut and...
$\tau=I\alpha$ where $I=\dfrac{ml^2}{3}$
$\tau=(1.6)(g)(0.32)=(1.6)\frac{(0.64)^2}{3}\alpha$
[ : • COM ]
$\alpha=23.57$
$a_B=\alpha r=(23.57)(0.64)=15.08≈(A)14.7$
 
3 hours later…
BioPhysicist
BioPhysicist
05:11
@Wolgwang What if you picked a different point to calculate the torques/angular acceleration about?
Wolgwang
Wolgwang
@BioPhysicist Rod will be revolving about A...
So we can't take any other point (?)
S.M.T
S.M.T
06:14
@JohnRennie Hello sir
John Rennie
John Rennie
@S.M.T Hi :-) I busy dealing with a couple of questions at the moment, but I will be around later.
S.M.T
S.M.T
06:37
@JohnRennie K. I understand sir. I hope you’re doing okay. The new variant omicron of covid is bad. Take care sir.
S.M.T
S.M.T
07:06
@JohnRennie Sir , Room for S.M.T & John rennie sir. Do join this room.
I wanted to send my invite here. By mistake , I sent you wrong to biosphere
BannedUser
BannedUser
07:49
are there any good physics book where it tells about derivation of every law theories equation it mentions ?
Books for undergraduate students
I want to teach myself physics
Prateek Mourya
Prateek Mourya
08:28
Hello @JohnRennie sir
John Rennie
John Rennie
@PrateekMourya Hi :-)
Prateek Mourya
Prateek Mourya
Sir can you discuss the problem of field lines of three charges placed at vertices of equilateral triangle
To generalize to drwa field lines ror any system
John Rennie
John Rennie
You can do this mathematically, but it's quite a hard calculation. Or we can use an approximation to get an idea of how the field lines will look.
Prateek Mourya
Prateek Mourya
Approximation
John Rennie
John Rennie
For an approximation imagine replacing the electric field by water flowing out of positive charges and into negative charges.
Give me a moment and I'll draw a diagram ...
user image
Near to the charges the water will flow out equally in all directions. OK so far?
Prateek Mourya
Prateek Mourya
08:37
Yes
John Rennie
John Rennie
But water is incompressible, so in between the charges the water flows will crash into each other and deflect each other. So you'll get a flow looking something like this (it will rtake a moment to draw the diagram):
Prateek Mourya
Prateek Mourya
Can you drwa only main diagrams
John Rennie
John Rennie
user image
Prateek Mourya
Prateek Mourya
Ok
John Rennie
John Rennie
Another way of looking at this is that the field lines show the direction of the force on a charge.
Right in the middle the net force on a charge would be zero because the forces from the three charges would sum to zero and cancel.
Prateek Mourya
Prateek Mourya
08:44
Yes
John Rennie
John Rennie
So there cannot be any field line passing through the central point.
You can think of the field lines repelling each other and curving to avoid each other.
Does this help make it clearer?
Prateek Mourya
Prateek Mourya
Yes....
Does this comes with practice
Of using analogy of water out of tap
John Rennie
John Rennie
It is something you get used to as you see examples.
Prateek Mourya
Prateek Mourya
Ok
John Rennie
John Rennie
Mathematically you have to vector sum the fields from the three charges, then at any point the field line shows the direction of the summed vector field.
Prateek Mourya
Prateek Mourya
08:48
Hmmm
John Rennie
John Rennie
But that calculation is typically hard.
Well not "hard" because you are only summing three vectors, but it gets messy.
Prateek Mourya
Prateek Mourya
But where at which main points
We must be interested
John Rennie
John Rennie
You'd have to compute the sum everywhere to get an idea of how the field lines look.
Prateek Mourya
Prateek Mourya
Well that needs a computer
John Rennie
John Rennie
Yes.
It isn't a calculation you'd do by hand, and you wouldn't be asked to do it in the JEE.
You just need to have an idea of how the field lines would look, and for that using the analogy of flowing water is good.
Prateek Mourya
Prateek Mourya
08:51
But here this three charge sys
John Rennie
John Rennie
Yes ... ?
Prateek Mourya
Prateek Mourya
Where we should calculate the vecyor sum
John Rennie
John Rennie
Are you asking me how to do this mathematically? Or how to draw the approximate diagram?
Prateek Mourya
Prateek Mourya
How did you know that field lines would look like this
John Rennie
John Rennie
It's kind if obvious.
The field lines start by radiating out symmetrically from the charges.
And they cannot cross each other, so they have to curve to avoid each other.
Prateek Mourya
Prateek Mourya
08:55
If they were from opposite charfes
John Rennie
John Rennie
If you look at my diagram then there isn't any other way the field lines could be to obey those restrictions.
Prateek Mourya
Prateek Mourya
What would they do to avoid each other
The two points are key idea?
John Rennie
John Rennie
If you have opposite charges then field lines flow out of positive charges and into negative charges.
Prateek Mourya
Prateek Mourya
@JohnRennie if they were from opposite charges
John Rennie
John Rennie
@PrateekMourya I'm not sure what you mean ..
Do you mean take our triangle and make two of the charges positive and one negative?
Prateek Mourya
Prateek Mourya
08:58
Sir i have to submit my phone now
Thanks sir
John Rennie
John Rennie
OK :-)
Ashish Ahuja
Ashish Ahuja
09:12
@caramalizedTomato pathfinder..

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