This magnet question confounds me

  • Thread starter lodovico
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Homework Statement


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



Homework Equations


Fc = mv^2/r
Fb= qvB


The Attempt at a Solution



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

[itex]\frac{1}{r}[/itex]=B
[itex]\frac{1}{B}[/itex]=r

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

i chose the (1/x) shaped graph
 

Answers and Replies

  • #2
berkeman
Mentor
60,578
10,893

Homework Statement


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



Homework Equations


Fc = mv^2/r
Fb= qvB


The Attempt at a Solution



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

[itex]\frac{1}{r}[/itex]=B
[itex]\frac{1}{B}[/itex]=r

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

i chose the (1/x) shaped graph

Yes, it is known as the Lamor Radius:

http://www-fusion.ciemat.es/fusionwiki/index.php/Larmor_radius

:smile:
 

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