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B
I
F
FB = ILB
Magnetic induction
Magnetic flux density
Magnetic field (strength)
[B] = [F/IL]
= Ns/Cm = Tesla
*1 Tesla = 104 gauss
Length of the section in Bfield
B

I
FB = ILB┴
FB = IL(Bsin)
(1)
(2)
(3)
(4)
A net force on the loop
A net torque on the loop
A net force and torque
nothing, zip
X
S
N
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/mothow.html
DC Electrical Motor
X
X
Example
The earth’s magnetic field in Colorado
is 0.59 G. It is directed at an angle
of 67° below horizontal. Find the
force on 5.0 m of an east-west wire
carrying a westward current of 20 A.
North-South wire?
In Colorado,
B = 0.59 G = 5.9 x 10-5 T
X
F
S
67 degree
N
Force on a 5 m E-W wire carrying a 20 A west-bound current?
S
F = ILB
= (20 A)(5 m)(5.9x10-5 T)
= 5.9 x 10-3 N
X
N
I
F
FB = ILBsin
on a wire
FB = qvBsin
F
on a charge
dF
dL
F
F = I L B┴
dF = I (dL) B
= (dQ/dt) dL B
= dQ (dL/dt) B
= dQ v B
F = Q v B
force on a moving charge
Lorentz force
Finding the Direction of
Magnetic Force




The direction of the
magnetic force is
always perpendicular
to both v and B
F = qvB sin
Fmax occurs when v is
perpendicular to B
F = 0 when v is
parallel to B
Right Hand Rule




Hold your right hand
open
Place your fingers in
the direction of B
Place your thumb in
the direction of v
The direction of the
force on a positive
charge is directed out
of your palm
• If the charge is
negative, the force is
opposite that
determined by the right
hand rule
My Old TV
Nam June Paik
http://www.paikstudios.com/
X
x
X
q
x
x
x
x
x
x
Xq
x
x
F
x
X
x
x
x
X
x
x
qx
x
x
x
x
q
x
x
v
x
q
X
x
F = q v B┴
Force by a magnetic field on a moving charge is always
Perpendicular to the direction of motion.
NO WORK DONE BY THE FIELD!!!
X
x
X
x
X
x
r
x
x
x
x
x
x
x
x
x
x
x
v
X
x
x
q
x m
X
x
x
x
B
FB = q v B
Force by B-field
This force causes a circular motion.
Centrepetal force = FB
Fc = mv2/r = q v B = FB
r = mv/qB
Since there is no work done by the field, in vacuum,
The charge will make a circular motion forever!!!
Bubble Chamber
B = 0.36 T
r = 0.4 m
Mass m?
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A univalent ion with mass m
Came into the field region with
Speed v = 6 x 105 m/s.
Mass spectrometer
An object with +e charge
And mass m kg