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Aim: How can we explain the
3rd left hand rule of
magnetism?
Do Now:
How will the current flow?
To the left
rd
3
Left-Hand Rule
 Used any time you have a
charge moving through a
magnetic field.
 When current is placed
perpendicular to magnetic field,
a force will be produced on the
wire.
 Place thumb in direction of current, and
fingers pointing in direction of magnetic
field. Your palm will point in the direction
the force pushes.
Which direction is the force?
N
Current
S
Up
Describe the motion of the
moving charge
 To figure out the force on a positive
charge, use the right hand (or opposite
from negative charges)
 This is how Jay can smash particles
together
 http://www.youtube.com/watch?v=bEvLK
11jdJ8
 The resultant of the velocity and the force
can produce circular motion
Remember…
 Moving charges create magnetic fields.
 Your left-hand rules
 These concepts revolutionized the world
Force on a Current
Carrying Wire
F = IlxB
**The x is a cross product – this means the wire and field must
be perpendicular to use this formula
F = force (N)
I = electron flow (A)
l = length of wire (m)
B = Magnetic Field strength (Tesla = T
= N/Am)
Example Problem
The current flowing in a 2.0 m wire is 100
A. The wire is placed in a magnetic field
of 3.0 x 10-3 T such that it is oriented
perpendicular to the field. Calculate the
force experienced by the wire.
F = IlxB
F = 100 A(2.0 m)(3.0 x 10-3 T )
F = 0.6 N
Force on an Electron
An electron entering a magnetic field
experiences a force similar to that on a
wire (proton would go in the opposite
direction).
Since F = IlB and I = q/t
Then, F = qlB/t,
and l/t = v
Then the force on any charged particle is
F = qvB
Mass of an Electron
Since a particle is free to move, upon
entering a magnetic field, it will constantly
change directions in response to the force
so,x itx goes
x x x xinx a
x circle x x x x x x x x
x
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Mass of an Electron
Because the electron moves in a circular path the force can
also be considered a centripetal force, so
F = qvB = mv2/r
Therefore, q/m = v/Br
 1897: charge to mass ratio (q/m) was determined by J.J.
Thomson
 1909-1913: charge of an electron was determined by Millikin
with his oil drop experiment
 Mass of electron was then determined with q/m
What will happen?
S
x
x
x
x
x
N
S
N
Fingers – field
The wire will spin
Thumb – current
This is a motor
Palm – force
Axis of
rotation
Electric Motor
Converts electrical energy to
mechanical energy
Motor Demo
What will happen?
S
x
x
x
x
x
N
S
N
Palm – force
Current will be
generated
Thumb - current
This is a generator
Fingers – field
Axis of
rotation
Generator
Converts mechanical energy to
electrical energy
Generator Demo
AURORAS
This is what is
happening
 Known as the
northern lights
 Electrons from violent
storms on the Sun
enter the Earth’s
magnetic field
 The electrons collide
with gases in the
Earth’s atmosphere
 Color depends on the
This is what
type of atoms and
you see
molecules struck
Proton Aurora
Forms from
Reconnection
Event