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Physics electric fields

A small rubber ball coated with a conducting paint carries a positive charge.
The ball is suspended in equilibrium from a vertical wall by an uncharged non-conducting thread of negligible mass. The wall is positively charged and produces a horizontal uniform electric field perpendicular to the wall along the whole of its length. The ball is on a string at a 30 degree angle to the wall and the string is then cut. Describe and explain the motion of the ball.

The mark-scheme says that the ball experiences a vertical and horizontal force and the motion of the ball is in a straight line with constant acceleration.
I am confused about this because if the ball is experiencing a vertical and horizontal force due to cutting the electric field lines then why doesn't the ball move in a parabolic shape like most particles in an electric field? Any help would be greatly appreciated because I think I am confused about when work is/isnt done in an electric field. :smile:
Original post by Kwj2021
A small rubber ball coated with a conducting paint carries a positive charge.
The ball is suspended in equilibrium from a vertical wall by an uncharged non-conducting thread of negligible mass. The wall is positively charged and produces a horizontal uniform electric field perpendicular to the wall along the whole of its length. The ball is on a string at a 30 degree angle to the wall and the string is then cut. Describe and explain the motion of the ball.

The mark-scheme says that the ball experiences a vertical and horizontal force and the motion of the ball is in a straight line with constant acceleration.
I am confused about this because if the ball is experiencing a vertical and horizontal force due to cutting the electric field lines then why doesn't the ball move in a parabolic shape like most particles in an electric field? Any help would be greatly appreciated because I think I am confused about when work is/isnt done in an electric field. :smile:


The ball has a resultant force on it, after the string is cut, equal to mg downwards, its weight, and Eq horizontally, the force due to the field E on its charge q.
If, as the question says, the electric field is uniform and horizontal, then the force Eq doesn't change in magnitude or direction as the ball moves.
Similarly, the force mg doesn't change.
So the ball moves with uniform acceleration acted upon by a constant force equal to the vector sum of mg and Eq.

The parabola would be relevant if the electric field was vertical and not horizontal as it states here.
Or if there was no electric field and the ball was given an initial horizontal velocity only.

Assuming you copied it correctly, the word 'constant' is missing from the mark scheme where it says 'the ball experiences a (constant) vertical and horizontal force'.
(edited 1 year ago)

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