# Positron in a magnetic field help

JaeSun
some more basic homework help!!

can you guys point me in the right direction, where to start? (grrr, so freakin lost in this class)

11. A 22.5-eV positron (positively charged electron) is projected into a uniform magnetic field B = 455[mu]T with its velocity vector making an angle of 65.5 degrees with B (vector). Find (a) the period (b) the pitch p, and (c) the radius r of the helical path.

ill try and scan the diagram too

grr

thanks

im "learning" magnetic fields for this chapter.

here is the scan of the problem, and a figure to go with it:

http://storage.whiteazn.com/phy181_pr11.jpg [Broken]

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## Answers and Replies

JaeSun
was reading the chapter..and they had a section on circulating charges ... it had an equation relating qvB to centripetal acceleration of:

qvB=m(v^2)/r

problem is, in the question, to find r, we have to know v, which isnt given ?? and also, dont know the mass ?

grrrrr

any help!1??!

einai
Originally posted by JaeSun
was reading the chapter..and they had a section on circulating charges ... it had an equation relating qvB to centripetal acceleration of:

qvB=m(v^2)/r

problem is, in the question, to find r, we have to know v, which isnt given ?? and also, dont know the mass ?

grrrrr

any help!1??!

I think you can find v from the energy of the positron by using the kinetic energy formula, since E = 22.5 eV << the positron rest mass (0.5 MeV). The mass is equivalent to the electron mass.

JaeSun
ehh?

is the kinetic energy formula:

K=(1/2)mv^2 ?

and still lost on this part:

since E = 22.5 eV << the positron rest mass (0.5 MeV). The mass is equivalent to the electron mass.

einai
Originally posted by JaeSun
ehh?

is the kinetic energy formula:

K=(1/2)mv^2 ?

and still lost on this part:

since E = 22.5 eV << the positron rest mass (0.5 MeV). The mass is equivalent to the electron mass.

Yeah, that should be the right formula.

A positron basically is just an electron with a positive charge, so they have the same mass.

JaeSun
so whats the mass? 22.5 ??