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!