Download Goal: To understand what Electric Fields are

Goal: To understand Magnetism
Objectives:
1) To examine Magnetic fields and field lines
2) To understand Why magnetic fields are important
for the earth and how are they created on the
earth?
3) To understand Magnetic force
4) To understand Motions in a magnetic field
5) To learn about Magnetic domains
6) To learn How to BUILD YOUR OWN MAGNET!
Magnetic Fields
• Just like with electric fields and
gravitational fields, magnetic fields are an
indicator of the magnetic topography of the
region.
• One example to illustrate this is the
magnetic dipole or bar magnet.
Bar magnets
• All have 2 ends, North and South
• This is like + and – for charges.
• This creates a field where charges for from
the north pole to the south pole just like
they went from + charge (high potential) to
– (low potential)
Magnetic Field
Lines
• Much like we had electric field lines we
have magnetic field lines.
• These are the lines which charges would
follow (sort of).
• These are the lines that charges or
magnetic materials would follow
Earth’s magnetic field (NASA)
Magnetic Force
• The magnetic force is very complicated.
• With electric fields the force is towards
charges usually.
• However, with magnetic forces, the force
is PERPENDICULAR to the magnetic
field.
Magnetic Force Equation
Force = charge * velocity of charge X Magnetic field
• But V X B isn’t normal multiplication because V
and B are vectors, but for this course we will
ignore this, and assume everything is
perpendicular.
• So, the maximum force is q V B
• And the direction of the force is on the axis that V
and B DON’T use!
Motions in an electric field
• Now that we know what the force is, what
will the motion be like if the magnetic field
is constant?
• Hint: how does the force change with time
as the direction of the velocity of the
charge changes?
Motions in an electric field
• Now that we know what the force is, what will
the motion be like if the magnetic field is
constant?
• The charge will move in a circle!
• The force changes the direction of the velocity.
• However this changes the direction of the force.
• So, the charge will move in a circle
perpendicular to the magnetic field (around the
field lines).
• Any motion the particle has in the direction OF
the field will be constant (no force).
Cyclotrons
• A device which accelerates particles is
called a cyclotron.
• This is used mostly for protons (electrons
would just go in the opposite direction).
• This uses the fact that the radius of the
circle that a proton will move in just
depends on its velocity and the magnetic
field.
Magnetic Domains
• What causes one bar of iron to be a super
powered magnet, but another just to be a bar of
iron?
• Both have regions that are “magnetic”.
• However, for a magnet, a majority of these are
lined up in a specific direction of N vs S while for
a lump of iron is random.
• (Yes, this means that a bar magnet does not
have all the “North” magnetics on one side – and
if you cut it in half, you have 2 magnets!)
Build a Magnet!
•
•
•
•
Tired of loosing those screws?
Magnetize your screwdriver!
But how?
Well, just wrap a wire around the screwdriver,
then connect both ends of the wire to a power
supply (such as a battery, but not too powerful or
you might hurt yourself).
• Leave it on for some time, few minutes, and
when you turn it off you will have yourself a
magnetized screwdriver!
• No more dropping screws into your computer’s
power supply…
Conclusion
• We have learned about magnetic fields and
forces.
• We have learned how charges move in these
magnetic fields.
• We have discovered that magnetic fields, while
seemingly boring, are in fact VERY important to
life on the surface of the earth!
• Finally, we can now build our own magnets!