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Distance at which electric field causes measurable change

  1. Aug 24, 2014 #1

    I was wondering if there is a way in which it would be possible to calculate the distance at which an electric field would need to be to polarize a neutral object or mass m, to a point where the object being like a rod, aligns with the field. I guess this is dependent on the mass, however i am not sure how to calculate the minimum distance at which a change would be able to be seen and measured experimentally. Any help would be appreciated.

    Thank you
  2. jcsd
  3. Aug 24, 2014 #2


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    and it would depend on the field strength :smile:
  4. Aug 25, 2014 #3
    Hello daveen,

    Thank you for the response. How do i determine at what point the field strength is not strong enough to cause any change due to polarization on the object which can be seen with the naked eye? because by equations, the electric field expands to infinity, and only then it is possible to say that it goes to 0, but before that, any distant neutral object should be polarized. am i correct? does it make sense what i am trying to ask?
  5. Aug 25, 2014 #4


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    Electromagnetic forces vary as [itex] \frac{1}{r^2} [/itex], so they have infinite range. Therefore regardless of the distance of the rod from the field source, there will be a polarization and so a change of orientation. So you only can choose an order of magnitude and ask in what distance, the polarization and the change of orientation is of this order of magnitude.
  6. Aug 25, 2014 #5
    I understand the range is infinite, but for example if i try and put a neutral object away from a charge creating an electric field of small magnitude, even though the effect of the field should have infinite range, practically, it would not be possible to actually see a measurable effect on the neutral object being so far away.

    So at what distance would an electric field stop having a measurable effect on an object during a lab experiment? obviously this depends on the electric field strength and the size of the object and the charge and other factors, but is there a way to make this into an equation or something similar?
  7. Aug 25, 2014 #6


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    Your answer is hidden in my post under the disguise of choosing an order of magnitude for the effects. In fact it depends on you that what is that distance. Imagine you were so smart and rich that for any order of magnitude, you could build equipment to measure the effects to that order of magnitude, then that distance would be infinite. But because that's impossible, you should consider your equipment to see to what precision you can measure the effects. Then the order of magnitude is chosen and you can calculate to what distance you can measure the effects by equipments with such precision.
  8. Aug 25, 2014 #7
    Is there a way to calculate the magnitude to which a piece of equipment is able to measure?

    If not, would it be wrong to assume a really small value of change given by the equipment compared to something and use that as a starting level?

    what i mean is, for example, if i have a neutral object on a scale with a charged object far from it, and the scale only measures a force smaller or equal to 1% of the neutral object's weight, would i be wrong in making that a starting point for the rest of the experiment, where i test other shorter distances and the relative forces, to find a relationship?
  9. Aug 25, 2014 #8


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    That's not something you want to calculate. Either the manufacturer gives it to you or you simply realize it by looking at the LCD screen, or the pointer moving on a calibrated sheet or anything. Also the one who is reading the number given by the equipment may affect the precision of the measurement. Environmental effects like temperature, light, vibrations of the equipment etc. may affect it too.
    Use a parameter. You don't need to assign it a value. Just give it a name like p!!!
  10. Aug 25, 2014 #9
    The scale is can measure to 0.001g, ( not sure about the actual accuracy) and the neutral spheres that i am using in the experiment which will be polarized are between 1g and 5g. Other factors such as humidity, light and vibration are being minimized or kept constant.
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