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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