someone help me understand this..What is the direction of an electric field that will balance the weight of a 4.5 plastic sphere that has been charged to -6.2 ?

Question

someone help me understand this..What is the direction of an electric field that will balance the weight of a 4.5  plastic sphere that has been charged to -6.2 ?

anonymous · Answer

it's downward. why?

anonymous · Answer

The strength or magnitude of the field at a given point is defined as the force that would be exerted on a positive test charge of 1 coulomb placed at that point; the direction of the field is given by the direction of that force. [Wikipedia]
So, in case of a negative charge and if you want to compensate for the gravitation, the electric field has to point downward, i.e. the force on the _negative_ charge is upwards.

anonymous · Answer

force on the negative charge is upwards...why? can you explain with pic please?

anonymous · Answer

|dw:1328227506730:dw|

anonymous · Answer

Um, this comes from the definition of the electrical field. There's no real explanation why, it's just convention, how to define the direction of the electrical field.
But maybe you can think of a capacitor, where one side is charged negatively and the other side positive, then the direction of the field (in between the plates of the capacitor) would point from the positive charged plate to the negative. So a positive test charge in between would be accelerated in direction of the field, a negative test charge the opposite direction.

anonymous · Answer

o did you mean towards the neg charge when you said upwards? cuz i understand that. |dw:1328230811998:dw|