Proton moving in a magnetic field

[erodger]

Homework Statement

A proton (mass= 1.67×10-27 kg, charge= 1.6×10-19 C) traveling with speed 1×106 m/s enters a region of space containing a uniform magnetic field of 1.2 T.

The magnetic field is coming out of the screen.

What is the time t required for the proton to re-emerge into the field-free region?

Homework Equations

qvB = mv²/R

The Attempt at a Solution

I begin by solving for R using the above equation:

R= (mv)/(qB)

Since the proton moves in and then back out it completes a semi-circle. The distance traveled is then πR as it is half of the circumference of a circle.

The velocity does not change for the entire distance traveled so dividing distance by velocity should give me the time.

This comes out as incorrect though. I am unsure as to what I am doing wrong.

 

[SHISHKABOB]

in what direction is the magnetic field pointed relative to the initial velocity of the proton?

 

[erodger]

Oh sorry, I edited the post. It is coming out of the screen. So it is perpendicular to its motion which means it will create a net force on the proton and cause it to undergo the circular motion.

 

[SHISHKABOB]

ah, okay. Well, what did you get for your time?

 

[erodger]

Well the question involved some steps and one of the steps was to find the radius. It continued to tell me I am wrong. I decided to stop using that and work on it my own using my method and I managed to come up with the correct answer, which just involved me to not round to 2 sig figs but add on more decimal digits. My answer was 2.73*10^(-8). Before I was just putting in 2.7*10^(-8).

 

[erodger]

Thanks for trying to help.

 

[SHISHKABOB]

what did you get for your radius?

huh, actually that's the same time I got.

 

[erodger]

I got 8.698*10^(-3) for my radius. The computer kept telling me it was incorrect but I managed to get the right answer with this value so I do not know what is wrong...

 

[SHISHKABOB]

so the time you got was correct, but the radius is not? Maybe it wants radius in terms of some unit like cm?

 

[erodger]

No it was asking for metres which is what my answer is in.

 

[SHISHKABOB]

interesting... I don't know what to tell you :(

did you try the same thing with what you did with time? Try changing the number of decimal points, or how you round your numbers when you solve for it.Sometimes I have these same problems with online homework things. They usually accept answers within a certain range of the correct answer, but can be really stubborn sometimes :P