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# really basic question about coulomb's law

Posted on 2010-08-15
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Hi

I think i know the answer to this already but I'm just asking as a means to consolidate it really.

Formula = q1.q2.k /d^2

You are often asked to use this formula to find, for example, the force of q1 on q2. This can make you think that the force of q2 on q1 is somehow different because the wording in the question doesnt suggest they are one and the same thing.

But the fact of the matter is that when you have 2 point electric charges the force of q1 on q2 is the same magnitude (opposite) direction as the force of q2 on q1 even though say q1 could have a large value for charge and q2 doesn't

I hope thats right :)

Edit: i was also thinking the same thing about electric fields. When books describe an electric field they talk about the electric force on the test charge from the source charge and it 'sounds' as though the force is one directional. But again i hope i'm right in presuming that whatever total force the source charge exerts on the test charge (not the force per coulomb but the total force on all of the test charge) then the source charge feels that back?
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Question by:andieje
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d-glitch earned 2000 total points
ID: 33440392
>> when you have 2 point electric charges the force of q1 on q2 is the same
magnitude (opposite) direction as the force of q2 on q1

That is correct, forces are always equal and opposite.

The difference in magnitude of the charges is not relevant.  But a difference in the masses might be.  If one of the charges is very heavy, you might be able to treat it
as stationary.

This is also true in the case of electric fields.  Older televisions and oscilloscopes used
electric deflection plates to bend a beam of electrons.  The force of the electrons on
the plates is negligible.
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Author Closing Comment

ID: 33440509
thanks
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