physics magnetism

Question

An electron travels at a constant speed of 3.40 × 106 m/s towards the left of this page. It then enters a uniform magnetic field and experiences a maximum force of 4.65 × 10-8 N that points towards the top of this page.

a) What is the magnitude of the magnetic field?

b) What is the direction of the magnetic field?

 

I'm having trouble and would appreciate an explanation and answer. I am getting an answer of -85477.9 for the answer for a, but im not sure. and i don't know how to determine the direction. Please correct me.

Answer 1

(a)

 

Magnetic force:

 

F = -ev×B

 

The force is maximum when v is perpendicular to B.  Then its magnitude is

 

F = evB ⇒

 

B = F/(ev) = (4.65 x 10^-8) / [(1.6 x 10^-19)(3.40 x 10⁶)] T = 85477.9 T

 

You have the correct magnitude in teslas.

 

(b)

 

The force points towards the top of the page (given).  v points to the left, but -ev points to the right.  Then by the right hand rule, B must point perpendicular to the page and into the page.  That way, -ev×B lies in the plane of the page and points up the page.

 

Does this help?