Need some advice on this Electric Field question

[whatisphysics]

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!

 

[tiny-tim]

hi whatisphysics!

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"

 

[whatisphysics]

hi whatisphysics!

the question asks …


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

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, I am just a little bit confused about the signs for W=mg.

Thanks for replying so fast!

 

[tiny-tim]

hmm … you're confusing me also!

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

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

it's up (or down! ) 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

 

[rwisz]

I would advise the student to keep in mind that the weight of an object is always directed towards the center of the Earth, which is considered to be the negative direction. Therefore, when calculating the electric field that balances the weight of an electron or proton, it is important to take into account the direction of the weight and assign it a negative sign. This is because the electric field must be in the opposite direction of the weight in order to balance it. So in this case, the correct equation would be E = -mg/q.

Additionally, it is important to remember that the electric field is a vector quantity, meaning it has both magnitude and direction. The direction of the electric field for an electron will be opposite to that of a proton, since they have opposite charges. Therefore, for an electron, the electric field will be directed upwards, while for a proton it will be directed downwards.

I would also suggest reviewing the concept of electric fields and weight, as well as practicing more examples to gain a better understanding of how to approach these types of questions. It is always helpful to draw diagrams and visualize the situation to better understand the direction and magnitude of the electric field.