Stuck on a few magnetic field questions

[NutriGrainKiller]

Problem 1:

A particle with mass 1.81*10^-3 kg and a charge of 1.22*10^-8 C has, at a given instant, a velocity v = 3.00*10^4 m/s jhat.

What is the magnitude of the particle's acceleration produced by a uniform magnetic field B=1.63T ihat + 0.980T jhat?

here is what i did:

knew to find the acceleration I needed to use a=QE/M, but since I didn't have E I had to find it. Used F(mag field)=Q*V*B*Sin(theta). I found theta from drawing the ijz axis and using trig, I got 59 degrees.

After I solved this I plugged it into F=QE to get E. Then plugged that into a=QE/M, and got an incorrect answer..not sure where I went wrong.


Problem 2:

A particle with charge 6.40 *10^-19 C travels in a circular orbit with radius 4.68 mm due to the force exerted on it by a magnetic field with magnitude 1.65 T and perpendicular to the orbit.

a) What is the magnitude of the linear momentum P of the particle?

b) What is the magnitude of the angular momentum L of the particle?

for a:
equations I used:

P=MV
R=MV/QB -> V= RQB/M

plugged 2nd into 1st, M cancelled, answer wasn't correct. eh?


for b:

not sure i know the equation for this problem...same as in classical physics?

thanks guys!

 

[topsquark]

Problem 1:

here is what i did:

knew to find the acceleration I needed to use a=QE/M, but since I didn't have E I had to find it. Used F(mag field)=Q*V*B*Sin(theta). I found theta from drawing the ijz axis and using trig, I got 59 degrees.

After I solved this I plugged it into F=QE to get E. Then plugged that into a=QE/M, and got an incorrect answer..not sure where I went wrong. Problem 2: for a:
equations I used:

P=MV
R=MV/QB -> V= RQB/M

plugged 2nd into 1st, M cancelled, answer wasn't correct. eh?for b:

not sure i know the equation for this problem...same as in classical physics?

thanks guys!

1) There's nothing wrong with your first problem except that you seem to be insisting there's an electric field. There isn't. The Lorentz force says:
\mathbf{F}=q\mathbf{E}+q\mathbf{v}X\mathbf{B}. Since there is no E, the first term drops out. Then you calculated F, as you did. Now simply apply F=ma.

2) Part a) looks right, I'm not sure why things aren't working. What answer did you get and what does your book say? For part b) I would assume that you aren't being relativistic or quantum mechanical, so yes use the Classical formula.

-Dan

 

[NutriGrainKiller]

1) There's nothing wrong with your first problem except that you seem to be insisting there's an electric field. There isn't. The Lorentz force says:
\mathbf{F}=q\mathbf{E}+q\mathbf{v}X\mathbf{B}. Since there is no E, the first term drops out. Then you calculated F, as you did. Now simply apply F=ma.

2) Part a) looks right, I'm not sure why things aren't working. What answer did you get and what does your book say? For part b) I would assume that you aren't being relativistic or quantum mechanical, so yes use the Classical formula.

-Dan

thanks, figured the first one out!

for 2a i got 4.94×10−18