Electric Field of +4.0*10^-9 C in 1600 N/C Field

[Expresso]

A constant electric field exists in a region of Space. The field has a magnitude of 1600N/C and points due north. A point charge of +4.0 * 10^-9 C is then placed in this electric field. What is the magnitude and direction of the net electric field at a spot 13 cm due east of the charge?

http://img135.imageshack.us/img135/3221/46204214bm6.png

My question is, how do we know that the origin starts off positive? Thanks.

 

[berkeman]

A constant electric field exists in a region of Space. The field has a magnitude of 1600N/C and points due north. A point charge of +4.0 * 10^-9 C is then placed in this electric field. What is the magnitude and direction of the net electric field at a spot 13 cm due east of the charge?

http://img135.imageshack.us/img135/3221/46204214bm6.png

My question is, how do we know that the origin starts off positive? Thanks.

Not sure wht you mean by your question. All you have to do is form the vector sum of the constant background E-field, plus the radial E-field from the point charge, and sum it at the point shown.

 

[Expresso]

Do we assume the origin has a positive charge? Or is there something in the problem that tells us that? thanks.

 

[berkeman]

The "origin" can be anywhere. The background E-field is straight up (north), and they ask for the vector sum 13cm to the right of the charge (east). They are making the problem easy for you at first. Don't say "the origin has a positive charge", say "it makes the problem easiest if I choose the origin of my coordinate system right at the positive charge in this problem".