# This magnet question confounds me

1. Mar 19, 2013

### lodovico

1. The problem statement, all variables and given/known data
Today, my teacher gave us some practice AP multiple choice and my friend and I were debating between answer choices H and J (changed from A and B to remove any confusion) the question as i remember it:
blah blah... B-field into the page, constant velocity v, radius R and charge q. which graph shows the relationship between B-field and radius.. something with radius getting smaller as B-field gets larger

Choice H: this is a straight line coming from +y crossing the +x axis (basically a straight negative slope)

Choice J: the (1/x) graph with x>0

2. Relevant equations
Fc = mv^2/r
Fb= qvB

3. The attempt at a solution

Fc=Fb
$\frac{mv^2}{r}$=qvB
$\frac{mv}{r}$=qB
since mv and q are constants, is it ok to treat them as 1? that is what i did so...

$\frac{1}{r}$=B
$\frac{1}{B}$=r

since on the graphs r was the y-axis and B was the x-axis

i chose the (1/x) shaped graph

2. Mar 19, 2013

### Staff: Mentor

Yes, it is known as the Lamor Radius: