NoteARific: Centripetal
When a moving charge enters a magnetic field, its path will begin to curve since there is a
force acting on it.
•
Assuming the field is uniform, and big enough, the force acting on it.
..
F
m
= B q v
will be constant and acting at right angles to the particles velocity.
•
Since it acts at right angles to the initial velocity of the particle, that component
of the velocity won’t change.
o
Think of projectile motion examples where the horizontal component
never changed even though the vertical did.
•
The particle will follow a path that (if it continued) would be circular.
•
The force acting on the charge (F
m
) must therefore be the same as the centripetal
force (F
c
)
F
m
= F
c
B q v = mv
2
/r
Bq
mv
r
=
•
Which allows you to calculate the radius of the circular path the particle would
follow.
•
On the other hand, if you know the radius of the path and a few other values, you
could calculate the particle’s mass (or charge).
•
You don’t have this formula written this way on your data sheet, so get really
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 Fall '08
 RABE
 Physics, Electron, Charge, Force, Electric charge

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