Negative charge held stationary between two plates.

[Clinicall]

I took a written exam, and I'll try to post the exact question up if it's too confusing, but I'll explain it as best I can here.

There is a Capacitor (who's plates are parallel to the ground) holding a negative charge stationary between the two plates, and the question asks what is the direction of the Electric Force.

Now, I know that the charge must have a force acting on it downwards due to gravity (mg), and that there must be a force acting upwards to counteract the force of gravity on a charge to make it stationary. My question is how do I know in which direction the Electric force is going? The correct answer was downwards, (and from what I think I remember, if that charge had been positive, the Force and the EF would be in the same direction, so it would have been directed upwards.)

Can someone perhaps explain this to me better? Is there something similar to the right hand rule, or will it basically be in the same direction if the charge is positive and opposite if its negative?

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[milski]

You have to make the distinction here between a field and a force. A force is what really acts on the object/particle, a field is a description of what that force would be for a given particle in a given place.

Let's start with the force first. The particle is stationary. That means that the net force on it is zero. Since the gravity is pointed downwards (obviously), it follows that the force from the electric field will be pointed up. By definition F=qE or in other words, the electrical field is in the direction of the force which would act on a positive charge.

Since you have a negative charge, the force on it will be in the opposite direction of the field and since the force is up, the field will have to point down.

 

[Clinicall]

I suppose I'm understanding it from the equation point of view, but if you have a point charge, I guess I'm just imagining an electric field, and that it is pointing in all directions.

I'm suppose I'm just struggling to envision the electric field and corresponding forces. I have a good grasp on magnetism and the right hand rule though.

Can I just assume that if I have a point charge, the EF will always be in the same direction as the force, and that an opposite charge will have an EF opposite to the force?

 

[yoyohomieg5432]

I suppose I'm understanding it from the equation point of view, but if you have a point charge, I guess I'm just imagining an electric field, and that it is pointing in all directions.

I'm suppose I'm just struggling to envision the electric field and corresponding forces. I have a good grasp on magnetism and the right hand rule though.

Can I just assume that if I have a point charge, the EF will always be in the same direction as the force, and that an opposite charge will have an EF opposite to the force?

the electric field in a capacitor( the definition of 2 charge separated plates is essentially a capacitor) is not pointing in all directions. it is pointing from the positively charged plate to the negatively charged plate.

you know an electron in an electric field will want to move towards the positive charged plate (this is against the direction of the electric field).

so, imagine the top plate is + charge and the - plate is down charge. gravity is pulling the charge down. now, since the top plate is positive charged, the force due to the electric field will move the charge up towards the top plate. however, in this case since the particle is stationary the force acting up on the particle equals the force of gravity acting downward.

 

[milski]

I suppose I'm understanding it from the equation point of view, but if you have a point charge, I guess I'm just imagining an electric field, and that it is pointing in all directions.

I'm suppose I'm just struggling to envision the electric field and corresponding forces. I have a good grasp on magnetism and the right hand rule though.

Can I just assume that if I have a point charge, the EF will always be in the same direction as the force, and that an opposite charge will have an EF opposite to the force?

I see what's confusing you. Let's take a step back a start with the basics. The field is created by a charge (or a set of charges). Just the field on its own is not associated with any forces. It's only when you introduce another charge in that field, that a force is created on the newly introduced charge. Charges are not affected by the field they create themselves - that would be the equivalent of raising yourself in the air by pulling your hair up.

You are correct that a point charge will create a field which is straight lines in all directions away from the charge (for q>0) or into the charge (for q<0). But that's not the interesting field in the problem. To exert a force on the charge the field needs to be created by another (set) of charges. In this case, it is created by the charged plates. Charged plates are a typical case of a uniform electric field - it is straight lines, perpendicular to the plates, going from one plate to the other.

 

[gettheleadout]

I see what's confusing you. Let's take a step back a start with the basics. The field is created by a charge (or a set of charges). Just the field on its own is not associated with any forces. It's only when you introduce another charge in that field, that a force is created on the newly introduced charge. Charges are not affected by the field they create themselves - that would be the equivalent of raising yourself in the air by pulling your hair up.

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