Very fast right hand rule question

[michaelw]

The electrons in the beam of a television tube have a kinetic energy of 2.40×10–15 J. Initially, the electrons move horizontally from west to east. The vertical component of the earth’s magnetic field points down, toward the surface of the earth, and has a magnitude of 2.00×10–5 T.

Just when i thought i had this right hand rule thing down, i come across this
heres what i did
used left hand, fingers down, thumb pointing right, force = out of page
yet the answer is the force is directed south??

what did i do wrong?

 

[siddharth]

The charge of an electron is negative. :)
so f=qv*b. when q is negative, force is in the opposite direction.

If you ask me, instead of thinking in terms of this rule, try think in terms of vectors and cross products

 

[michaelw]

i have no clue how to do cross products :/ and i used left hand since its an electron :)

 

[Doc Al]

heres what i did
used left hand, fingers down, thumb pointing right, force = out of page
yet the answer is the force is directed south??

what did i do wrong?

You are just interpreting the answer incorrectly. I assume your picture (mental or otherwise) shows East as going to the right of the page, and down as going down the page? The answer, given that, is out of the page--but out of the page = South! (Always translate the answer back into the language of the question.)

Since you seem to be having quite a time with the right hand rule, check out the links I provide here: https://www.physicsforums.com/showpost.php?p=389039&postcount=8