Electric Fields Help
Why is the force -EQ considered when the real force is acting downward on the charge? Is the negative sign due to this or due to the displacement being in the direction opposite to that of the force?
So is the force actually acting in the other direction or is the negative sign just showing that negative work is done? Also, since the electric field being the negative of the potential gradient is always true, how to account for the case of a negative charge, when the work done would be positive due to an upward force in this setup?
Original post by Sricheta
So is the force actually acting in the other direction or is the negative sign just showing that negative work is done? Also, since the electric field being the negative of the potential gradient is always true, how to account for the case of a negative charge, when the work done would be positive due to an upward force in this setup?
Due to Newton's third law, the field does exert a positive force downwards on the charge; while the charge exerts an equal and opposite force on the field which is upwards. since the charge is exerting a negative force upwards it must also be doing negative work in the upwards direction.
Original post by Kingemperor07
Due to Newton's third law, the field does exert a positive force downwards on the charge; while the charge exerts an equal and opposite force on the field which is upwards. since the charge is exerting a negative force upwards it must also be doing negative work in the upwards direction.
I am actually finding it difficult to get my head round this 😅. But what about the work done on the charge, which is taking it to a higher potential?
Original post by Sricheta
I am actually finding it difficult to get my head round this 😅. But what about the work done on the charge, which is taking it to a higher potential?
change in potential energy (electrical, gravitational, elastic ...) when moving an object from A to B is either
- *negative* of the work done by conservative force associated with the field OR
- work done by external force needed to move object without acceleration. To move object without acceleration external force must be equal and opposite to the force exerted on the object by the field.
W = (field) (displacement) cos phi
phi is angle between field and displacement.
change in PE = -W
phi = 180, B is at a higher potential than A, work done by the field is negative but change PE is positive.
phi = 0, B is at a lower potential than A, work done by the field is positive but change PE is negative.
W = (external force) (displacement) cos phi
phi is angle between external force and displacement.
change in PE = W
phi = 0, B is at a higher potential than A, work done by the external force and hence change in PE are both positive.
phi = 180, B is at a lower potential than A, work done by the external force and change in PE are both negative.
(edited 11 months ago)
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