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Thursday, March 7, 2019

A particle with mass and charge is moving from left to right in a uniform gravitational field and a uniform electric field.


Question 10
A particle with mass and charge is moving from left to right in a uniform gravitational field and a uniform electric field. The gravitational field is downwards. The gravitational force and the electric force on this particle act in opposite directions.

What could be the sign of the charge on the particle and the direction of the electric field?

sign of charge             direction of electric field
A         negative                                  down
B         negative                                  up
C         positive                                    left
D         positive                                    right





Reference: Past Exam Paper – June 2017 Paper 11 Q11





Solution:
Answer: A.


The gravitational field and force are downwards. Since we are told that the gravitational force and electric force are in opposite directions, the electric force must be upwards.

The electric field direction is from positive to negative. It represents the electric force acting on a positive test charge.


The direction of the electric field is the same as the direction of the force on a positive charge. So, the force on a negative charge would be opposite to the direction of the electric field.


If the electric field is downwards, it means that the force on a positive charge is also downwards. But this is in the same direction as the gravitational force. So, the charge needs to be negative when the electric field is downwards, so that the force on it is upwards. [A is correct]


If the electric field is up and the charge is negative, then the electric force on it is (opposite to the field) downwards. [B incorrect]

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