Electric Force Question

[Dave12314]

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I came across this question:

What change is observed in the force exerted on charged particle P in a uniform electric field when it is moved closer to the plate on the left, so that the distance r separating them is reduced by half?

A. The force doubles, if the charge is negative.
B. The force doubles, if the charge is positive.
C. The force increases by a factor of 4, regardless of the type of charge.
D. The force does not change, regardless of the magnitude or sign of the charge.

The answer is D but I'm confused why it would not be A or B

I thought since E=V/d then if the distance is reduced by half then E would be increased by 2. Consequently, since F= Eq, I thought if E doubles than the force doubles.

Can someone explain why this would not be correct?

 

[Ssina]

so the trick is in the uniform, since E is uniform, it means it won't be stronger closer to left or right plate (i'm assuming there is a picture)... and since the F= q E... neither the q nor the E changed so the force doesn't change either.

 

[milski]

Uniform electric field means that the force exerted on the charged particle will be the same everywhere in that field.

E=V/d would imply a non-uniform field.

 

[SaintJude]

Yes, A uniform electric field is an electric field whose influence
over a charge is constant regardless of the point of the space taken
into consideration.

My teacher compared it to a "halo" of an angel. No matter where an angel is, it will have a halo that nothing can diminish. The analogy worked for me. So yes, F=qE. The electric field would change if the distance between the capacitor plates or the voltage applied to the plates changed. Otherwise, by definition, the electric field remains constant.

 

[Ssina]

on the following picture:
At P2: the positive charge is feeling a strong repelling force from the top positive plate (since it is closer so smaller r to this plate... due to F=kqq/r2) and a smaller attractive force from the opposite negative plate.

At P1: it is feeling a stronger attraction from the negative bottom plate and a smaller repelling force from the top positive (again from F=kqq/r2)

soo in effect we reduced one and increased the other force, but the Fnet is still the same!

 

[Ssina]

Yes, A uniform electric field is an electric field whose influence
over a charge is constant regardless of the point of the space taken
into consideration.

My teacher compared it to a "halo" of an angel. No matter where an angel is, it will have a halo that nothing can diminish. The analogy worked for me. So yes, F=qE. The electric field would change if the distance between the capacitor plates or the voltage applied to the plates changed. Otherwise, by definition, the electric field remains constant.

not sure if I follow that, nevertheless, any analogy with an angel in it... is cool 😀

 

[SaintJude]

Haha, yeah you're pictures are definitely much better than my analogies any day. And thanks for the Hb-dissociation curve picture too. Great explanation