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Thread: What factors could change electrostatic force?

  1. #1 What factors could change electrostatic force? 
    ems
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    I have measured the force between a charged sphere and an earthed plate, and changed the separation.
    The graph of force vs. separation I have obtained has a steeper gradient than it should (haven't been able to attach image of the graph). Can anyone think of any reasons why this might happen?
    Any help would be greatly appreciated


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    Brassica oleracea Strange's Avatar
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    Without more detail of how you are doing this, it is hard to say. My first guess would be measurement error. How are you measuring the force? Also, are you sure the charge is constant and not leaking away during the experiment, for example. Are you making multiple measurements (both while increasing the distance and decreasing it)?

    Also, the inverse square law only applies to point sources. With a flat plate and a large sphere, you would need a more complex model to calculate the force.


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    KJW
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    If the sphere and plate are conductive, then redistribution of charge over the surfaces can lead to a modified force law in comparison to that of a charge distribution within non-conductive materials.
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    There are no paradoxes in relativity, just people's misunderstandings of it.
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    ems
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    Quote Originally Posted by Strange View Post
    Without more detail of how you are doing this, it is hard to say. My first guess would be measurement error. How are you measuring the force? Also, are you sure the charge is constant and not leaking away during the experiment, for example. Are you making multiple measurements (both while increasing the distance and decreasing it)?

    Also, the inverse square law only applies to point sources. With a flat plate and a large sphere, you would need a more complex model to calculate the force.
    Thank you, I am using a more complex model to calculate force - but wrote this here for simplicity. Multiple measurements were taken, but the forces were quite small and therefore the forcemeter used fluctuated - maybe it's due to this? But then I'd expect my results to fluctuate randomly about the actual line, rather than linearly at a steeper gradient.

    Thanks for your help
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    ems
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    Quote Originally Posted by KJW View Post
    If the sphere and plate are conductive, then redistribution of charge over the surfaces can lead to a modified force law in comparison to that of a charge distribution within non-conductive materials.
    Yes, I didn't mention this in my first post, but I did use a modified force law in my calculations I think the difference in my results may be either to do with the conditions of the environment or the apparatus used, but I'm not entirely sure
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    Quote Originally Posted by ems View Post
    I have measured the force between a charged sphere and an earthed plate, and changed the separation.
    The graph of force vs. separation I have obtained has a steeper gradient than it should (haven't been able to attach image of the graph). Can anyone think of any reasons why this might happen?
    Any help would be greatly appreciated
    What was the (your) expected relationship? Were you looking for an inverse square relationship?
    Charges are going to move on the sphere too so the attraction could rise greater than expected as the charges will not be covering the sphere evenly.
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    ems
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    Quote Originally Posted by Robittybob1 View Post
    What was the (your) expected relationship? Were you looking for an inverse square relationship?
    Charges are going to move on the sphere too so the attraction could rise greater than expected as the charges will not be covering the sphere evenly.
    Yes, an inverse square relationship was expected. Please could you explain to me in a little more detail why the charges would move on the sphere, and how this would cause the attraction to rise greater than expected? Thank you!
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    Quote Originally Posted by ems View Post
    Quote Originally Posted by Robittybob1 View Post
    What was the (your) expected relationship? Were you looking for an inverse square relationship?
    Charges are going to move on the sphere too so the attraction could rise greater than expected as the charges will not be covering the sphere evenly.
    Yes, an inverse square relationship was expected. Please could you explain to me in a little more detail why the charges would move on the sphere, and how this would cause the attraction to rise greater than expected? Thank you!
    Was that what you were getting?
    They move to where they have the greatest attraction so the net attraction is greater than if they hadn't moved.
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    Quote Originally Posted by ems View Post
    Quote Originally Posted by KJW View Post
    If the sphere and plate are conductive, then redistribution of charge over the surfaces can lead to a modified force law in comparison to that of a charge distribution within non-conductive materials.
    Yes, I didn't mention this in my first post, but I did use a modified force law in my calculations I think the difference in my results may be either to do with the conditions of the environment or the apparatus used, but I'm not entirely sure
    Your response doesn't appear to pay enough attention to what KJW has said (he's right, as always). Charges are mobile and, in equilibrium, will be in whatever configuration minimizes total energy.

    The dimensions of the plate, relative to the spacing between plate and sphere, will also make a difference. Since you have given us no details at all, it's hard to say whether insufficient plate size is also contributing to the discrepancy between expectation and observation.
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