# Cartesian coordinates vectors

1. Feb 10, 2017

### The black vegetable

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

the answer given is the same but without the negative sign, I don't understand because the crossproduct of unit vectors
when using a Cartesian coordinates of the directions given by the right-hand rule? Is the positive z direction pointing out of the page if X and Y are as follows

apologies if this is in the wrong section, thanks for any help in advance

2. Feb 10, 2017

### BvU

Yes. $\hat {k} = \hat {\bf \imath } \times\hat {\bf \jmath}$ , so z points towards you, out of the screen.

Corkscrew rule I call it. Turn $\hat{\bf \imath }$ over the smallest angle towards $\hat {k}$. Corkscrew will go in the minus y direction : $$\hat \imath \times\hat k = -\hat \jmath$$

3. Feb 10, 2017

### TSny

The problem doesn't specify which of the two wires the charge -q is a distance b from. Does the answer change if you pick the other wire to be a distance b from -q?

4. Feb 10, 2017

### The black vegetable

Thank you both your answers, TSny I thought the same, closer to the top wire the combined magnetic field would be in the opposite direction . Maybe the question is just not very good