Direction of the net force on the charge

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I drew these pictures, so some letters came unclear, I will clarify them later in parentheses.
Chp9,46, the diagram represents two small charged spheres, A and B, 3 meters apart. Each sphere has a charge of +2.0*10^-6 coulomb. (The point in the middle is y)
If a positive charge is placed at point x, the direction of the net force on the charge will be
1) toward the left 2) toward the right.
I chose 2, because the two spheres are both positive, so when another positive charge came to point x, sphere A would react toward right because of the repelling; sphere B would point toward right as well, so Fnet will point toward right eventually. But answer was 1), How come it’s left?

2. Chp9,89 The diagram shows three small metal spheres with different charges. A has +10 coulombs, B has -10 coulombs, C has +5 coulombs.
Which vector best represents the net force on sphere B? The choices are in the digram. I chose 1 because the force exerted by A points to B, the force exerted by C points to B also, so consequently the net force should be 1), but answer was 4. If it were 4, then the force from A should have pointed to the left, and force from C should have pointed downward.

Thanks.
 

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  • #2
alphysicist
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Hi MIA6,

I think that you are just applying the directions to the wrong charges in these two problems.

I drew these pictures, so some letters came unclear, I will clarify them later in parentheses.
Chp9,46, the diagram represents two small charged spheres, A and B, 3 meters apart. Each sphere has a charge of +2.0*10^-6 coulomb. (The point in the middle is y)
If a positive charge is placed at point x, the direction of the net force on the charge will be
1) toward the left 2) toward the right.
I chose 2, because the two spheres are both positive, so when another positive charge came to point x, sphere A would react toward right because of the repelling;
It's true that sphere A would feel a force to the right. But the question is not asking about sphere A, it is asking about the charge at point X. Is the charge at X being pulled or pushed by sphere A? If it is being pushed, then the force it feels will be to the left.

sphere B would point toward right as well, so Fnet will point toward right eventually. But answer was 1), How come it’s left?

2. Chp9,89 The diagram shows three small metal spheres with different charges. A has +10 coulombs, B has -10 coulombs, C has +5 coulombs.
Which vector best represents the net force on sphere B? The choices are in the digram. I chose 1 because the force exerted by A points to B, the force exerted by C points to B also, so consequently the net force should be 1), but answer was 4. If it were 4, then the force from A should have pointed to the left, and force from C should have pointed downward.

Thanks.
Here they are asking for the force that B experiences. Is A pushing or pulling charge B? Since the force from A is attractive, the force that B experience is toward A, which is to the left. (And then you can think of the force from C the same way.)
 
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Hi,alphysicist, thanks for your explanations. Now I know what my mistake was.
 

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