# Velocity of proton following a helical path? need a bit of help

1. Jul 22, 2011

### yjk91

velocity of proton following a helical path?? need a bit of help

1. The problem statement, all variables and given/known data
A proton with an initial velocity given by (4.0 + 3.0 + 3.0)(105 m/s) enters a magnetic field given by (0.62 T). Describe the motion of the proton.

The proton will follow a helical path with a velocity of 3e5 m/s along the Z -axis, with the circular motion in the XY -plane having a speed of ?? and radius of ??

3. The attempt at a solution

so i got the velocity of the proton along Z axis but i'm not sure how to get the XY axis

my idea was that i could just square x and y velocity then rad it but it didn't work

and would i be able to get the Radius by using this formula?

2. Jul 23, 2011

### ehild

Re: velocity of proton following a helical path?? need a bit of help

What is the direction of the magnetic field?
Are you sure you copied the velocity along z correctly? It can be 305 m/s, but 3e5 is impossible if it means 300000 m/s.

Do you know the formula for the Lorentz force, the magnetic field exerts on a moving charged particle?

ehild

3. Jul 23, 2011

### yjk91

Re: velocity of proton following a helical path?? need a bit of help

yeah it's to the z

no we have not learned the Lorentz force yet
can't you find the velocity the ciruclar motion by doing x^2 + y^2 and rad it
but it's not the answer

4. Jul 23, 2011

### yjk91

Re: velocity of proton following a helical path?? need a bit of help

oh nvm it is haha

5. Jul 23, 2011

### yjk91

Re: velocity of proton following a helical path?? need a bit of help

qvB = mv^2/R or R = mv/qB.

i'm using this to find the R

so 1.6*10^-27 * 5 * 10^5 / (1.6*10^-19 * 0.62 * .62T)

and i got 0.008065 m and it's not the answer

5 * 10^5 is the velocity on the xand y axis
i tried using the z axis but that's not even close

help?

6. Jul 23, 2011

### ehild

Re: velocity of proton following a helical path?? need a bit of help

What is what? does 105 mean 105 m/s? And 3e5 means 3˙105 m/s?
So 5˙105 m/s is the speed of the proton in the x,y plane.
The mass of proton is

$m_{p}\ =\ 1.673\ \times\ 10^{-27}\ kg$

Using 1.6 instead of 1.67 is bad.

The charge is
$e\ =\ 1.602\ \times\ 10^{-19}\ C$

It does not hurt using more then necessary significant digits during calculations.
Do the calculation again, with the more accurate proton mass. It should be all right.

ehild

7. Jul 23, 2011

### yjk91

Re: velocity of proton following a helical path?? need a bit of help

oh thanks
i'll remember

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