The diagram shows an electric field pattern caused by two positive and two negative point charges of equal magnitude placed at the four corners of a square.

Question 9

The diagram shows an electric field pattern caused by two positive and two negative point charges of equal magnitude placed at the four corners of a square.

In which direction does the force act on an electron at point X?

Reference: Past Exam Paper – November 2016 Paper 11 & 13 Q31

Solution:

Answer: B.

The direction of the electric field is from positive to negative.

The electric field lines at any point show the direction of the force on a positive test charge. Since we are dealing with an electron (which is has a negative charge), the direction of the force on it would be opposite to the direction of the force on a positive charge (which is indicated by the direction of the field).

But when the field lines are curved, the direction of the force is along the tangent at that point. [A and C are incorrect] The tangent is parallel to the field lines.

D shows the direction of the force on a positive charge as this is in the same direction as the field at that point.

The direction of the force on an electron would be opposite, which is along B.