# Help with unit vector for a magnetic field

• guyvsdcsniper
In summary, the conversation discusses the evaluation of the cross product of the velocity of a charge and the unit vectors associated with a point. The vector r represents the position of the point relative to the proton, not the position of the proton itself. The question is why there should be a magnetic field and what information is missing from the problem statement and the provided picture. It is mentioned that the charge of the proton is not given. However, the problem can still be solved with the available information.

#### guyvsdcsniper

Homework Statement
What is the magnetic field at the position of the dot in the figure?
Relevant Equations
Cross Product
So when evaluation the cross product of the velocity of the charge and the unit vectors associated with the point I am getting

v x r = j x [ i + j].
Well j x j is 0.
j x i = -k, but yet the answer is positive. Why is this?

The vector r is the position of the point relative to the proton, not the position of the proton.

Why should there be a magnetic field? What is given in this problem that you did not include in the Homework Statement and is not shown in the picture you posted? We are not even told the sign of the charge.

kuruman said:
Why should there be a magnetic field? What is given in this problem that you did not include in the Homework Statement and is not shown in the picture you posted? We are not even told the sign of the charge.
The picture shows a proton at (1,1) with a given velocity and asks for the magnetic field at (-1,-1). It seems pretty clear to me.

SammyS
Orodruin said:
The picture shows a proton at (1,1) with a given velocity and asks for the magnetic field at (-1,-1). It seems pretty clear to me.
Yes, I see that now that I switched browsers.

kuruman said:
Yes, I see that now that I switched browsers.
Yes, I agree that we can figure out the problem which likely is to be solved.

However, I agree with your previous post in that a clear problem statement would be a great help. Especially one written as text and readable with any browser.