The magnetic force on a charged particle
1) depends on the sign of the charge on
the particle.
2) depends on the velocity of the particle.
3) depends on the magnetic field at the
particle's instantaneous position.
4) is at right angles to both the velocity
and the direction of the magnetic field.
5) is described by all of these.
Electric Force:


FE  qE
Magnetic Force:

 
FB  qv  B
Figure 27.17
Cyclotron motion
• Radius of orbit
mv
R
qB
• Period – time for one complete orbit.
2 R
v
T
Velocity Selector
Figure 27.21a
Mass Spectrometer
mv
r
qB
Figure 27.33
A positively charged particle moves with speed v in
the positive x direction. A uniform magnetic field of
magnitude B exists in the negative z direction. You
want to balance the magnetic force with an electric
field so that the particle will continue along a
straight line. The electric field should be in the
1) positive x direction.
2) positive z direction.
3) negative y direction.
4) negative x direction.
5) negative z direction.
Currents in Magnetic Fields
• Current:
I  qnvd A

 
FB  qv  B
– Each charge q feels a magnetic force:
– Force on a wire segment of length dl is the force
on all of the charges in that segment:
dQ  qn(Vol )  qn( Adl )



dF  dQv d  B


 qn( Adl ) v d  B
 
 Idl  B
A current I flows in a wire that is oriented
as shown. Which of the vectors represent
the magnetic field that results in a
maximum force on the wire?