Electrostatics-determining velocities

[plutolover]

My physics teacher gave us a problem about a charged particle; it was a theoretical problem, so there weren't any figures. He said that the charge of the particle, the value of the magnetic field, and the mass was known. Then he said that we needed to find the velocity of the charged particle. I am not sure how to do this!


i used the formula: qvB=F and set that equal to ma=F. From this i got qvB=ma, however, I am not sure how to simplify the equation so that I am only solving for velocity in terms of q,B, and m! please help!

 

[rude man]

My physics teacher gave us a problem about a charged particle; it was a theoretical problem, so there weren't any figures. He said that the charge of the particle, the value of the magnetic field, and the mass was known. Then he said that we needed to find the velocity of the charged particle. I am not sure how to do this!


i used the formula: qvB=F and set that equal to ma=F. From this i got qvB=ma, however, I am not sure how to simplify the equation so that I am only solving for velocity in terms of q,B, and m! please help!

you need the correct formula for a!

 

[barryj]

I don't think this is a good problem. You cannot get the velocity from the charge, mass, and magnetic field. If thee is a velocity, the charge would go in a circle forever (I think)

 

[rude man]

I don't think this is a good problem. You cannot get the velocity from the charge, mass, and magnetic field. If thee is a velocity, the charge would go in a circle forever (I think)

You think right.

So why can't the charge go around in a circle forever with a certain constant velocity (actually speed)?

BTW the problem omitted that you also need the radius of the circle.

 

[plutolover]

you need the correct formula for a!

isn't a just delta v over time? then I would have time as an unknown variable. what do you mean by correct formula?

 

[rude man]

isn't a just delta v over time? then I would have time as an unknown variable. what do you mean by correct formula?

v is a vector. A vector can change its magnitude, its direction, or both, with time.

So the hint for you is that v changes its direction but not its magnitude v. If the particle moves in a circle with let's say constant speed v, what is |dv/dt|? Hint: think centrifugal acceleration.

Note that I use bold characters for vectors and regular for their magnitudes.

 

[haruspex]

Hint: think centrifugal acceleration.

Mmm.. centripetal?