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# A2 fields- 2 quick questions watch

1. Hi,

There just a couple of fields questions that I'm not sure about.

1. Electric fields questions: I'm not sure why the acceleration of the electron decreases and the acceleration of the proton increases by looking at the diagram.

2. Gravitational field question. I'm not sure what equations to begin with/ combine to get the formulae.

Any help would be really appreciated.
Attached Images

2. 1) Do you know what the direction of the arrows means? remember that a proton and an electron have opposite charges, hence the forces acting on them due to the electric field will also act in opposite directions and since a resultant force will lead to an acceleration, the accelerations must also be in opposite directions.

2) Gravitational potential is the GPE per unit mass, so can be written as mgh/m = gh, where h is the change in distance. See if you can go from there

For a bit more on gravitational potential see https://isaacphysics.org/concepts/cp...tational_field
3. (Original post by solC)
1) Do you know what the direction of the arrows means? remember that a proton and an electron have opposite charges, hence the forces acting on them due to the electric field will also act in opposite directions and since a resultant force will lead to an acceleration, the accelerations must also be in opposite directions.

2) Gravitational potential is the GPE per unit mass, so can be written as mgh/m = gh, where h is the change in distance. See if you can go from there

For a bit more on gravitational potential see https://isaacphysics.org/concepts/cp...tational_field
Thanks so much. Is the electron decelerating because it is moving in the direction of the field but the force on it is in the opposite direction or is it decelerating because it is moving against the direction of the field?
4. (Original post by VioletPhillippo)
Thanks so much. Is the electron decelerating because it is moving in the direction of the field but the force on it is in the opposite direction or is it decelerating because it is moving against the direction of the field?

I'm guessing that both particles are moving to the left?
If so then yes, the force acting on the electron is in the opposite direction to its motion, so it will decelerate.

Both particles are moving in the direction of the field, but the direction of the field indicates the direction at which the electric force acts on a *positive* charge so for a negative charge, such as the electron, the force would be acting against it.
5. (Original post by solC)
I'm guessing that both particles are moving to the left?
If so then yes, the force acting on the electron is in the opposite direction to its motion, so it will decelerate.

Both particles are moving in the direction of the field, but the direction of the field indicates the direction at which the electric force acts on a *positive* charge so for a negative charge, such as the electron, the force would be acting against it.
The question says they're both starting from stationary - but notice the field lines get closer on the right of the diagram showing increasing field strength and weaker on the left side of the diagram.

The particle that goes left will accelerate at a reducing rate as it's displacement left becomes greater and it moves into an area of lower field strength. The particle that goes right will accelerate at an increasing rate as it's displacement right becomes greater.
6. (Original post by Joinedup)
The question says they're both starting from stationary - but notice the field lines get closer on the right of the diagram showing increasing field strength and weaker on the left side of the diagram.

The particle that goes left will accelerate at a reducing rate as it's displacement left becomes greater and it moves into an area of lower field strength. The particle that goes right will accelerate at an increasing rate as it's displacement right becomes greater.
Oh yes, apologies, I assumed that there was a previous part about them being set into motion in a particular direction however what you have said makes a lot more sense. Thanks for pointing that out

Yep, I (somehow) seem to have missed the 'non-uniform field' part...
7. (Original post by VioletPhillippo)
Thanks so much. Is the electron decelerating because it is moving in the direction of the field but the force on it is in the opposite direction or is it decelerating because it is moving against the direction of the field?
As others have said, that is sort of the case.

Remember that electric field lines represent the path that a positive (test) charge will follow if placed in the electric field. The proton has a positive charge and so it will move to the right - you can see that towards the right, the density of field lines increases. This means that the field strength is greater and so the Coulomb force on the proton will also be greater (recall ). And as we know , a greater force => greater acceleration.

A similar argument works for the electron. Hope that clarifies it fully for you

[Forgive my poor notation omitting vectors!]
8. (Original post by kingaaran)
As others have said, that is sort of the case.

Remember that electric field lines represent the path that a positive (test) charge will follow if placed in the electric field. The proton has a positive charge and so it will move to the right - you can see that towards the right, the density of field lines increases. This means that the field strength is greater and so the Coulomb force on the proton will also be greater (recall ). And as we know , a greater force => greater acceleration.

A similar argument works for the electron. Hope that clarifies it fully for you

[Forgive my poor notation omitting vectors!]
Thank you, I have a much better understanding now

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