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Help in mathematical physics

  1. #1
    Nashita's Avatar Full Member
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    Help in mathematical physics

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    Assalamualaikum

    I am a first year student, my course of study is physics honours.

    In Laplace's eqn in spherical polar coordinates with azimuthal symmetry , while solving it using the method of separation of variables why do we equate the radial part and the theta part with a constant ?

    P.S. I looked for it in Google but no where it's written why they equate it with a constant!
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    Re: Help in mathematical physics

    wa'alaikum as'salaam,

    have you tried youtube? It's a good place to go for these questions.

    you can even ask questions under the video of interest.

    https://www.youtube.com/results?sear...sperical+polar

    laplace equation sperical polar - YouTube
    Enjoy the videos and music you love, upload original content, and share it all with friends, family, and the world on YouTube....
    | Likes Nashita, Alamgir liked this post
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    Nashita's Avatar Full Member
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    Re: Help in mathematical physics

    format_quote Originally Posted by Zzz_ View Post
    wa'alaikum as'salaam,

    have you tried youtube? It's a good place to go for these questions.

    you can even ask questions under the video of interest.

    https://www.youtube.com/results?sear...sperical+polar

    laplace equation sperical polar - YouTube
    Enjoy the videos and music you love, upload original content, and share it all with friends, family, and the world on YouTube....
    Yes I have tried YouTube, I think I need to see some more videos on this topic.

    Thank you so much!
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    Re: Help in mathematical physics

    I'm not entirely sure of what you're talking about, but if it is what it sounds like to me, the reason is the following:

    By doing it in those coordinates, you can rearrange the equation so that all the stuff with the variable r is on one side of the equals sign, and all the stuff with the theta is on the other side. Let's call the stuff on both sides as follows (i.e. left-hand side is a function of r only, a right-hand side a function of theta only):

    LHS(r) = RHS(theta)

    Thing is, the equation is true for any values of r and theta, you can literally plug in any two and it's still true. If you, then, keep r constant and change theta, then LHS(r) remains so too, and because of it being equal to it, RHS(theta) will be constant too, even as you change theta. From that, it follows that RHS(theta) must be equal to a constant and not change value depending on theta. The same argument in reverse for r and LHS(r). We can call this constant C.

    Thus, we get LHS(r) = RHS(theta) = C.

    Which we can split up into two single-variable differential equations that we can solve separately:
    LHS(r) = C
    RHS(theta) = C


    I hope that makes sense.
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    Re: Help in mathematical physics

    format_quote Originally Posted by Futuwwa View Post
    I'm not entirely sure of what you're talking about, but if it is what it sounds like to me, the reason is the following:

    By doing it in those coordinates, you can rearrange the equation so that all the stuff with the variable r is on one side of the equals sign, and all the stuff with the theta is on the other side. Let's call the stuff on both sides as follows (i.e. left-hand side is a function of r only, a right-hand side a function of theta only):

    LHS(r) = RHS(theta)

    Thing is, the equation is true for any values of r and theta, you can literally plug in any two and it's still true. If you, then, keep r constant and change theta, then LHS(r) remains so too, and because of it being equal to it, RHS(theta) will be constant too, even as you change theta. From that, it follows that RHS(theta) must be equal to a constant and not change value depending on theta. The same argument in reverse for r and LHS(r). We can call this constant C.

    Thus, we get LHS(r) = RHS(theta) = C.

    Which we can split up into two single-variable differential equations that we can solve separately:
    LHS(r) = C
    RHS(theta) = C


    I hope that makes sense.
    Absolutely correct! Thank you so much .
    | Likes Grandad liked this post
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    Re: Help in mathematical physics

    format_quote Originally Posted by Nashita View Post
    Yes I have tried YouTube, I think I need to see some more videos on this topic.

    Thank you so much!
    Can I uncross my eyes now? :
    Help in mathematical physics

    'Sometimes, silence is the best answer for a fool.' (Alī ibn Abī Tālib‎)
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    Re: Help in mathematical physics

    format_quote Originally Posted by Grandad View Post
    Can I uncross my eyes now? :
    -__________-


    What do you mean??
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    Re: Help in mathematical physics

    format_quote Originally Posted by Grandad View Post
    Can I uncross my eyes now? :
    Sorry. My post was meant for Brother Futuwwa's post. In the UK, whenever someone comes up with a complex statement that the reader is not smart enough to understand he (or she) will say: 'I've gone cross-eyed over this'.....meaning, argghhh what on earth was that??? It's a light-hearted joke against oneself (and a compliment to the other).

    I hope this help.
    Help in mathematical physics

    'Sometimes, silence is the best answer for a fool.' (Alī ibn Abī Tālib‎)
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    Re: Help in mathematical physics

    format_quote Originally Posted by Grandad View Post
    Sorry. My post was meant for Brother Futuwwa's post. In the UK, whenever someone comes up with a complex statement that the reader is not smart enough to understand he (or she) will say: 'I've gone cross-eyed over this'.....meaning, argghhh what on earth was that??? It's a light-hearted joke against oneself (and a compliment to the other).

    I hope this help.
    LOL Help in mathematical physicsHelp in mathematical physicsHelp in mathematical physicsHelp in mathematical physicsHelp in mathematical physicsHelp in mathematical physics

    This forum thread is for those who understand math and physics, if you aren't into math or physics it's obvious that you won't be able to understand anything clearly!
    And this forum thread is on Laplace's equation and solving it by spherical polar coordinates. If you want to know more about Laplace's equation then Google it!
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    Re: Help in mathematical physics

    format_quote Originally Posted by Grandad View Post
    Sorry. My post was meant for Brother Futuwwa's post. In the UK, whenever someone comes up with a complex statement that the reader is not smart enough to understand he (or she) will say: 'I've gone cross-eyed over this'.....meaning, argghhh what on earth was that??? It's a light-hearted joke against oneself (and a compliment to the other).

    I hope this help.
    You could so easily confuse a stupid (old) person!
    Help in mathematical physics

    'Sometimes, silence is the best answer for a fool.' (Alī ibn Abī Tālib‎)
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    Re: Help in mathematical physics

    I studied engineering but physics is still my passion. It is totally a different story..
    Help in mathematical physics

    “Either seem as you are or be as you seem” Rumi
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