Homework Help: Physics

Posted by Sandra on Sunday, March 14, 2010 at 11:53am.

An electron and a proton execute uniform circular motion in the presence of a uniform magnetic field.

If the radius of the circle is the same for both particles, which of the following statements is true?

A)Both particles move in the same direction; the electron moves faster than the proton.
B)Both particles move in the same direction; the proton moves faster than the electron.
C)The electron and the proton move in opposite directions but they move with the same speed.
D)The electron and the proton move in opposite directions; the electron moves faster than the proton.
E)The electron and the proton move in opposite directions; the proton moves faster than the electron.

Physics - drwls, Sunday, March 14, 2010 at 12:20pm
The electron and proton have opposite charges and very unequal masses. The opposite charges meas that the proton bends to the right while moving through a B field that is pointed up, while an electron bends the opposite way. Thus the move in circles in opposite directions.

That leaves you with choices C, D and E.

The relationshp between charge, velcoity and mass is:

m V^2/R = q V B
R = m V/(q B)
Since the R's and B are the same, as well as q (except for sign), V is inversely proportional to m. The electrons travel 1836 times faster.
Physics - bobpursley, Sunday, March 14, 2010 at 12:20pm
BqV=mv^2/r

Bqr/v = m

so if m is greater, v must be smaller. Obviously, they are in opposite directions.

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