What is the magnitude and direction of the electric field?

[johnnyies]

Homework Statement

A small object of mass 3.80 g and charge -18uC (-18 x 10^-6 or microcoulombs) "floats" in a uniform electric field. What is the magnitude and direction of the electric field?

Homework Equations

F = ma
F = Eq

The Attempt at a Solution

I already have the magnitude down. It's 2070 N/C

Why does the electric field point down though? I don't fully understand the direction of electric fields with positive or negative charges in them. can someone explain this to me?

 

[kuruman]

Positive charges move with the electric field lines and negative charges move against the electric filed lines. So if the electric field is down, a negative charge will experience an upward force against gravity and therefore float.

 

[johnnyies]

so if the electric field is down, that means it's of a negative charge?

 

[kuruman]

Say at a point P in space you are told that the electric field is straight down and that it is produced by a single charge. This can mean one of two things:

(a) There is a positive charge directly above point A or
(b) there is a negative charge directly below point A.

Now if I place a negative charge at point A it will experience a force up regardless of whether (a) or (b) is the case.

 

[johnnyies]

it wasn't very clearly explained in my book. Much thanks!