Magnetic Forces and Fields Domain Theory

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

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

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

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

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.

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.

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

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

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

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.

Interacting fields B fields that interact have characteristic shapes.

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

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.

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

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?

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.

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

First Left Hand Rule

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.