Need some advice on this Electric Field question!

  • #1

Homework Statement


What are the magnitude and direction of the electric field that will balance the weight of an
a)electron b)proton?


Homework Equations


W=mg
E=F/q


The Attempt at a Solution


This is what I did:

W=mg=F
so E=mg/q

My only question is, since W is downward, when I do my calculations, do I have to make it:
W=-mg=F
or do I simply just put it as W=+mg

If yes, why do i put it as +, or why do i leave it as negative, this is what i'm a bit confused about.

Thanks in advance!
 

Answers and Replies

  • #2
tiny-tim
Science Advisor
Homework Helper
25,832
251
hi whatisphysics! :smile:

the question asks …
What are the magnitude and direction of the electric field that will balance the weight of …

… so you need to give the direction of E, ie "up" or "down" :wink:
 
  • #3
hi whatisphysics! :smile:

the question asks …


… so you need to give the direction of E, ie "up" or "down" :wink:

That's the thing, I don't know if when I'm calculating the force acted by the gravity, which is W=mg, whether it should be a Positive or a Negative. I know how to do the calculations, Im just a little bit confused about the signs for W=mg.

Thanks for replying so fast!
 
  • #4
tiny-tim
Science Advisor
Homework Helper
25,832
251
hmm … you're confusing me also! :redface:

if the up direction is positive, then W = -mg

if the down direction is positive, then W = +mg …

it's up (or down! :wink:) to you which direction you choose to be positive

alternatively, just say W + qE = 0, and then just by looking at the signs in that equation, you should be able to see whether E is parallel or opposite to gravity :smile:
 

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