The Student Room Group

Changes in electrostatic potential of a moving object...

"An object is moving in a straight line from a positively charged area (A) to a negatively charged area (B). Explain how the electrical force and electrostatic potential energy experienced by the object will vary when it is neutral (has no charge)."

My answer:
Neutral objects are attracted by both positive and negative charges. This means that the object's electrostatic potential energy will increase as it moves away from point A, until it is the same distance from A as it is from B. It will then decrease as it moves closer to B.

So basically.. work is done for it to move away from A? The positively charged point A exerts attractive forces on the object etc.? Or am I completely barking up the wrong tree...? :colondollar:
Original post by Aivicore
"An object is moving in a straight line from a positively charged area (A) to a negatively charged area (B). Explain how the electrical force and electrostatic potential energy experienced by the object will vary when it is neutral (has no charge)."

My answer:
Neutral objects are attracted by both positive and negative charges. This means that the object's electrostatic potential energy will increase as it moves away from point A, until it is the same distance from A as it is from B. It will then decrease as it moves closer to B.

So basically.. work is done for it to move away from A? The positively charged point A exerts attractive forces on the object etc.? Or am I completely barking up the wrong tree...? :colondollar:


I would more-or-less agree with your answer (though the question is a bit vague, I think) except for one thing: you haven't stated why a neutral object is attracted to +ve and -ve charge.

And in reality the details of the potential change will depend on how much +ve charge is in A, and how much -ve charge in B, and their distance apart.
Reply 2
A neutral object won't feel any electrostatic force, nor have any electric potential energy, so I wouldn't expect any variation as it moves.
Original post by mik1a
A neutral object won't feel any electrostatic force, nor have any electric potential energy, so I wouldn't expect any variation as it moves.


At first sight, this would appear to be the case, but in fact, a neutral object contains charges, which are displaced from their equilbrium positions in the presence of an external E field. This causes one part of the object to become more +ve than another part, and the +ve and -ve regions then feel a force in the E field.

This mechanism is called "electrostatic induction": http://en.wikipedia.org/wiki/Electrostatic_induction
Reply 4
Original post by atsruser
At first sight, this would appear to be the case, but in fact, a neutral object contains charges, which are displaced from their equilbrium positions in the presence of an external E field. This causes one part of the object to become more +ve than another part, and the +ve and -ve regions then feel a force in the E field.

This mechanism is called "electrostatic induction": http://en.wikipedia.org/wiki/Electrostatic_induction


Is it not always true that neutral objects contain charges; what if it is a neutron?

Maybe it is me being picky, but the question explicitly says 'has no charge' which is a stronger constraint than what is strictly necessary for a neutral object ('has no net charge':wink:. I think it is really ambiguous. An object with no charge cannot be polarised.
Original post by mik1a
Is it not always true that neutral objects contain charges; what if it is a neutron?

Maybe it is me being picky, but the question explicitly says 'has no charge' which is a stronger constraint than what is strictly necessary for a neutral object ('has no net charge':wink:. I think it is really ambiguous. An object with no charge cannot be polarised.


I think that you're being very picky, since it's far more likely (IMHO) that the question is referring to a macroscopic object rather than a neutron (or an object constructed solely from neutrons :-)

I don't like the question though. I'm not sure if it was written by someone who didn't know about polarisation, or if it's aimed at finding out if the reader knows about polarisation themselves, or maybe if it was written by someone who did know about polarisation, but is happy with some simple-minded but incorrect answer ("neutral objects feel no electric force blah blah ..").
Reply 6
I can't think of any situation where positive and negative charges can possible stay separated like the question asks unless they are themselves bound in some crystalline structure or a doped semiconductor, in which case the only kind of neutral object that would feasibly be able to pass through them would be microscopic (alpha, neutron, photon, neutrino...)
Original post by mik1a
I can't think of any situation where positive and negative charges can possible stay separated like the question asks unless they are themselves bound in some crystalline structure or a doped semiconductor, in which case the only kind of neutral object that would feasibly be able to pass through them would be microscopic (alpha, neutron, photon, neutrino...)


I can't see what you mean - why can't positive and negative charges stay separated? The obvious layout would be a neutral object moved about between the well-separated plates of a charged capacitor.
Reply 8
The question specifies the object is moving from a positively charged area to a negativly charged area. In the case of a capacitor, the only areas containing charges are on the surface of the plates, so no object could move through this.

Anyway, the electric field in a standard capacitor is uniform so the polsrisation of an object wouldn't vary as it moved from one side to the other.

Basically I think the question is very poorly written.
Original post by mik1a
The question specifies the object is moving from a positively charged area to a negativly charged area. In the case of a capacitor, the only areas containing charges are on the surface of the plates, so no object could move through this.

You seem to be overthinking this - the question is clearly only interested in the effect of the electric field on an object - it's asking about the potential - it's not concerned with how the charges that produce the field get there. We're not worried about moving a physical object through solid plates. The question is merely supposing a region of positive and negative charge separated by some distance.


Anyway, the electric field in a standard capacitor is uniform so the polsrisation of an object wouldn't vary as it moved from one side to the other.


Well, by "standard capacitor" I guess you mean a parallel plate capacitor. Your statement is true as long as the plates are close together, and as long as we don't go too near the edge of the plates. If we have a parallel plate capacitor (which is only a couple of charged separated metal plates, after all) we can ensure that our field is non-uniform by a) separating them a long way or b) rotating one plate relative to the other. So it's trivial to make a non-uniform field out of a pair of metal plates.

Also, the E fields in a spherical or cylindrical capacitor are already non-uniform - they go as 1/r^2 or 1/r respectively - so your statement doesn't really make sense unless we only consider a very narrow range of physical arrangements to be a capacitor.

Basically I think the question is very poorly written.

I agree with this. It's a stupid question.

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