can someone help me with this physics problem? (magnetic field)

[skim milk]

Question:
In the figure below, a magnetic field of 0.8 Tesla points into the page. A particle of charge -33 nC and mass 0.7 ug is shot into the field as shown in the figure with a velocity of 0.6 km/s. What is the radius of the orbit (in meters)? (The radius is a positive quantity.) Be careful with the powers of 10. Hint: k=kilo, m=milli, u=micro, n=nano and the SI units for B-field, charge, mass, length and time are: tesla (T), coulomb (C), kilogram (kg), meter (m) and second (s), respectively.

Figure:
The magnetic field is going into the page and the velocity (negative) is pointed up



I know we use the equation r=mv/qB

I converted them to the right SI units and plugged them into the equation and I keep getting the wrong answer.
What am I missing/doing wrong?

 

[sciencewhiz]

well, what are the values you converted to, what is your answer, and what is the correct answer?

 

[skim milk]

Originally posted by: sciencewhiz
well, what are the values you converted to, what is your answer, and what is the correct answer?

i don't know the correct answer

for the charge i used -33*10^-9
for mass, 0.7*10^-6
for velocity, 600 m/s
for B, 0.8 tesla


all into the equation r=mv/qB

I get a very large negative number.

 

[Chronoshock]

Right hand rule? (Didn't read the post)

 

[skim milk]

Originally posted by: Chronoshock
Right hand rule? (Didn't read the post)

yea i know, but i need to find the radius

 

[Goosemaster]

Originally posted by: Chronoshock
Right hand rule? (Didn't read the post)

lol....(inside thing ignore this)

 

[fawhfe]

First off, you should note that the equation you have is the result of the ciruclar motion eq:

ar=v^2
Replace a by |F|/m and |F| with |q(v X B)| and you get
|q(v X B)|*r/m=v^2

Most importantly, with the circular motion equation, what matters is the MAGNITUDE of the force (directed towards the center). In fact, the sign will only tell you whether the particle goes around in a circle to the right or left of its initial course in this case. So, in this case, just disregard the negative sign.

Other than that, you forgot that the mass is in micro-GRAMS, not micro-kilograms. So its actually 10^-9, not -6.