1. Magnetic Forces and Fields
Domain Theory

2. Lesson objectives You will be able to
Define a magnetic field and its direction.
Describe causes of magnetic fields.
Explain magnetic interacts.

3. Magnetism Basics
Lode stone (iron ore) when suspended on a string always aligned in a specific direction N - S
3 ferromagnetic substances: iron, nickel, and cobalt can be magnetized or attracted to a magnet

4. when a ferromagnetic substance is magnetized, it has a distinctive north and south pole
Polarity: having N and S poles

5. Law of Magnets 1. Like poles repel 2. Unlike poles attract
3. Neutral ferromagnetic materials are attracted by either pole.

6. Magnetic Field
A region of magnetic influence is called a magnetic field (it extends infinitely). The field can be represented by lines of magnetic force (flux).
The symbol for magnetic field is B.
A magnetic field is produced by a change in motion at the atomic level shown by electrons moving about a nucleus.

7. Domain Theory
Domain:
A region in a ferromagnetic material that behaves as a tiny magnet (caused by the spinning electrons). There are millions of such domains in the magnet.
Domain Theory:
The reason that an object is magnetic (has magnetic properties) is that all of its domains (regions of magnetic moments) are aligned in the same manner. If the domains are not aligned in the same direction, then the object doesn’t have magnetic properties.

8. Domain Theory e describes a magnetic field at the atomic level
2 kinds of electron motion about the nucleus
1. Electrons spin on their axes
2. Electrons revolve about the nucleus e

9. All atoms have this motion
not all materials are magnetic because magnetic field generated by 1 atom is cancelled out by the adjacent atom
When electron spins are such that the generated magnetic fields are in the same direction, a net magnetic field is produced
This occurs in ferromagnetic substances

10. In iron, atoms where the magnetic fields are aligned or in the same direction is called a domain
Carbon added to iron in the semi-molten state prevents domains from aligning in other directions - produces a permanent magnet

11. Bar Magnet and Field The magnetic field direction is described
as the direction that a free magnetic north
would move when placed near a fixed source magnet.

12. Interacting fields
B fields that interact have characteristic shapes.

13. Example 3
Indicate the direction of the magnetic field for each of the following. 1 N S 2 N S

14. Symbols
Coming out of the page. Think of an arrow or dart coming at you.
Going into the page. Think of an arrow or dart going away from you.

15. Moving Charges
Tried to prove that electricity and magnetism were not connected.
Discovered that they were when current carrying wire produced deflection in a compass

16. Magnetic Field of Current Carrying Wire
The shape of the magnetic field produced by the moving charges in a wire is circular. It grows weaker as one moves away from the wire.
What is the direction of the field around the current carrying wire?

18. First Left Hand Rule
For the direction of the B around a current carrying wire we let the thumb of the left hand indicate the direction of electron flow. The finger tips of the hand that circle the wire indicate the direction of the B field.

19. First Left Hand Rule Left hand is used for negative charges
Right for positive charges

20. First Left Hand Rule

21. Conventional Current
The flow of positive charge is referred to as conventional current. Some textbooks use this and talk in terms of right hand rules rather than left hand rules.