What happens to the negative charge

[negation]

A negative charge -q is placed at the origin.
Two positive charge of +4q are on both sides of the negative charge at distance d.
what happens?

 

[adjacent]

A negative charge -q is placed at the origin.
Two positive charge of +4q are on both sides of the negative charge at distance d.
what happens?

If they are placed simultaneously,then the negative charge will stay still,because the net force is 0 and the initial state of the negative charge was "rest".

 

[negation]

If they are placed simultaneously,then the negative charge will stay still,because the net force is 0 and the initial state of the negative charge was "rest".

Why is the net force = zero?

 

[Doc Al]

Why is the net force = zero?

What do you think the net force is?

 

[negation]

What do you think the net force is?

I have no idea. Can you shed some light?

 

[ZapperZ]

I have no idea. Can you shed some light?

Are you not able to calculate the force between two point charges using the straightforward Coulomb's law? Shouldn't this be something you already know BEFORE dealing with a problem such as "...radial electric field outside coaxial cable..."?

Zz.

 

[Doc Al]

I have no idea. Can you shed some light?

Use Coulomb's law to find the force that each positive charge exerts on the negative charge. But you should be able to use symmetry to figure out the net force on that center charge. No calculations needed.

Then you can consider the forces on the positive charges to see what happens to them.

 

[negation]

Use Coulomb's law to find the force that each positive charge exerts on the negative charge. But you should be able to use symmetry to figure out the net force on that center charge. No calculations needed.

Then you can consider the forces on the positive charges to see what happens to them.

Spot on. I'm looking for an interpretation without calculation.
I do know the calculation and the formula but in many cases, the question wants me to know what is happening without any empirical calculation.

 

[Doc Al]

Spot on. I'm looking for an interpretation without calculation.
I do know the calculation and the formula but in many cases, the question wants me to know what is happening without any empirical calculation.

Well, what's stopping you?

How do the forces exerted by each positive charge compare?

 

[negation]

Well, what's stopping you?

How do the forces exerted by each positive charge compare?

Both positive charge exerts an outward electric field in a radial direction. The electric field from the positive charge to the left of the origin points in the positive x-direction. The electric field from the positive charge to the right of the origin point to the negative x-direction.

Since the negative charge is at an equal distance from both positive charge, the electric force experienced by the negative charge from the left is +F and -F from the right.
F_net = -F+F=0?

 

[adjacent]

Both positive charge exerts an outward electric field in a radial direction. The electric field from the positive charge to the left of the origin points in the positive x-direction. The electric field from the positive charge to the right of the origin point to the negative x-direction.

Since the negative charge is at an equal distance from both positive charge, the electric force experienced by the negative charge from the left is +F and -F from the right.
F_net = -F+F=0?

Correct!

 

[Doc Al]

Both positive charge exerts an outward electric field in a radial direction. The electric field from the positive charge to the left of the origin points in the positive x-direction. The electric field from the positive charge to the right of the origin point to the negative x-direction.

Since the negative charge is at an equal distance from both positive charge, the electric force experienced by the negative charge from the left is +F and -F from the right.
F_net = -F+F=0?

Yes. Simple as that!

 

[negation]

Thanks guys!

 

[jbriggs444]

There is another argument that may be appealing:

The layout is symmetric about the plane midway between the positive charges. If you were able to predict motion in one direction then you would also be able to predict motion in the symmetric direction. The only case in which this does not result in a contradiction is when the two predicted motions are identical -- i.e. when there is no motion at all.