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Today’s agenda:
Review and some interesting consequences of
F=qvxB.
You must understand the similarities and differences between electric forces and magnetic
forces on charged particles.
Magnetic forces on currents and current-carrying wires.
You must be able to calculate the magnetic force on currents.
Magnetic forces and torques on current loops.
You must be able to calculate the torque and magnetic moment for a current-carrying wire
in a uniform magnetic field.
Applications: galvanometers, electric motors, rail guns.
You must be able to use your understanding of magnetic forces and magnetic fields to
describe how electromagnetic devices operate.
Reminder: signs
F = qv  B
Include the sign on q, properly account for the directions of any
two of the vectors, and the direction of the third vector is
calculated “automatically.”
F = q vB sinθ
If you determine the direction “by hand,” use the magnitude of
the charge.
Everything in this equation is a magnitude.
The sign of r had better be +!
mv
r=
qB
Reminder: left- and right-hand axes
This is a right-handed
coordinate system:
y
z
This is not:
z
x
y
For the magnetism part of physics 2135, you MUST use righthand axes.
And you’d better use your
right hand when applying the
right-hand rule!
x
Handy way to “see” if you have drawn right-hand axes:
y
Z
?
y
x
z
x
z
?
y
x
I personally find the three-fingered axis system to often (but not
always) be the most useful way to apply the right-hand rule.


“In F = IL  B and F = qv  B does it matter
which finger I use for what?”

You’ll learn about F = IL x B
later in today’s lecture.
F = IL  B
F = qv  B
 
 


No, as long as
you keep the
right order. All
three of these
will work:
 












This works:


This doesn’t:

Switching only two is wrong!

“The right-hand rule is unfair! Physics is discriminating against
left-handers!”
No, you can get the same results with left-hand axes and lefthand rules. See this web page.
But Physics 24 does discriminate against left-handers!
This is Captain Jack Crossproduct.
He visits our classes occasionally
(see the physics on the blackboard
behind him).
You don’t want to see what he
does with his scimitar when he
sees a left hand used for the right
hand rule!
The right hand rule is just a way of
determining vector directions in a cross
product without having to do math.
Magnetic and Electric Forces
The electric force acts in the direction of the electric field.
FE = qE
The electric force is nonzero even if v=0.
The magnetic force acts perpendicular to the magnetic field.
FB = qv  B
The magnetic force is zero if v=0.

FB  v = 0  =  q 0  B = 0
Magnetic and Electric Forces
The electric force does work in displacing a charged particle.
FE = qE
E
+
F
D
WF =F  D =FD = qED
The magnetic force does no work in displacing a charged
particle!
B v
FB = qv  B
WF =F  ds =0
+ds
Amazing!
F
not really