Magnetism, magnetic fields Notes

How Do Magnetic Fields Describe Regions Where Magnetic Forces Act?

1. Magnets can pull on some materials without touching them, so physicists describe the space around a magnet as a magnetic field.
2. A magnetic field tells us where magnetic effects can be felt, and (using a suitable detector) what direction the magnetic effect has at each point.

Magnetic Materials

1. Only certain materials show strong magnetic behavior.
2. The most common magnetic materials are iron, nickel, and cobalt.
3. Objects made from these materials can become magnetized.

Magnets and Magnetic Poles

1. Every magnet has two poles, called the north pole and the south pole.
2. Magnetic poles always exist as a pair.
3. It is not possible to have a magnet with only one pole.
4. When two magnets interact:
   1. Like poles repel (north–north or south–south)
   2. Opposite poles attract (north–south)

Magnetic Field Lines Show Direction And Relative Strength

1. A standard way to represent a magnetic field is with magnetic field lines.
2. These are not real "threads" in space.
3. They are a drawing tool that helps you visualize what the field is doing.
4. Magnetic field lines follow important rules:
   1. Outside a bar magnet, they go from the north pole to the south pole.
   2. They cannot cross (because the field at a point has only one direction).
   3. They do not start or stop in empty space (in this model, they begin and end at poles).
   4. Closer spacing means a stronger field.

Structure of a Bar Magnet

1. A bar magnet is a simple magnet with a north pole at one end and a south pole at the other.
2. The magnetic effect is not evenly spread along the magnet.
3. The poles are the most active regions.

Breaking a Magnet

1. When a bar magnet is cut into smaller pieces, each piece becomes a magnet.
2. Each new magnet still has both a north and a south pole.
3. Magnetic monopoles are not produced by cutting magnets.

Where Is The Field Strongest?

1. On a bar magnet diagram, the field lines are closest together near the poles, so the magnetic field is strongest near the poles.
2. This matches observations such as iron filings clustering most densely near the ends of the magnet.

Uniform and Non-Uniform Magnetic Fields

1. A magnetic field is described as uniform when its strength and direction are the same throughout a region.
2. In a uniform magnetic field, magnetic field lines are:
   1. Straight
   2. Parallel
   3. Evenly spaced
3. A non-uniform magnetic field has:
   1. Curved field lines
   2. Uneven spacing
   3. Changing strength with position

You Can Create A Uniform Magnetic Field In A Gap

1. Sometimes we want a region where the magnetic field is the same everywhere.
2. This is called a uniform magnetic field, and it is represented by straight, parallel, equally spaced field lines.
3. A near-uniform field can be created by bringing the north pole of one bar magnet close to the south pole of another.
4. In the gap, the field lines go straight across from N to S and are evenly spaced, showing constant direction and (approximately) constant strength.

Mapping Magnetic Fields With A Compass Is A Step-By-Step Process

1. Because a compass needle is a small magnet, it turns to align with the magnetic field at its location.
2. A plotting compass is a small compass used to trace field direction point-by-point.

Method: Tracing The Field Of A Bar Magnet

1. Place a bar magnet under a sheet of paper and draw its outline.
2. Put the plotting compass near one pole.
3. Mark a dot at the tip of the needle (showing the direction).
4. Move the compass so the tail of the needle sits on your dot.
5. Mark a new dot at the tip.
6. Repeat to create a chain of dots.
7. Join the dots smoothly to draw one field line.
8. Start in other places to build the complete field pattern.

Domain Theory Explains Why Some Materials Become Magnetic

1. Only some materials become strongly magnetic, especially iron, and also nickel and cobalt.
2. These materials can be understood using domain theory.
3. In an unmagnetized piece of iron, domains point in many different directions, so their magnetic effects largely cancel.
4. When magnetized, many domains become aligned, so their fields add together and produce a net magnetic field.

Applying Domain Theory To Common Situations

a) Cutting a bar magnet in half

1. Cutting a magnet does not create an isolated north pole or isolated south pole.
2. Each half still contains many aligned domains.
3. Each piece, therefore, becomes a smaller bar magnet with its own north and south poles (new poles appear at the cut ends).

b) Hitting a magnetized iron rod with a hammer

1. Hammering produces vibrations and mechanical shocks.
2. These disturb domain alignment, so domains become more randomly oriented.
3. With less alignment, the net field is weaker.

c) Heating a magnet sufficiently

1. Heating increases thermal motion inside the material.
2. If the temperature is high enough, thermal energy disrupts domain alignment, reducing (or destroying) the magnet's net magnetism.

The Earth Acts Like A Giant Magnet With A Tilted Field

1. Earth behaves like a giant bar magnet surrounded by a magnetic field.
2. Earth’s magnetic field is produced by the movement of molten iron inside its core.
3. The magnetic field extends far into space and surrounds the planet.

How a Compass Works

1. A compass needle is a small bar magnet.
2. The needle aligns itself with Earth’s magnetic field.
3. The north-seeking end of the needle points toward Earth’s magnetic South Pole.

Magnetic Poles Move And Sometimes Flip

1. Earth's magnetic poles are not fixed, they drift by about 50 km per year, and navigation by compass becomes very difficult near the poles.
2. Over geological time, Earth's magnetic field has also reversed (flipped direction) a few times in a million years, and the change can happen relatively quickly (possibly within about 100 years).

Earth's Magnetic Field Helps Protect The Atmosphere

1. The Sun emits many charged particles, streaming outward as the solar wind.
2. If these particles struck Earth's upper atmosphere directly, they could gradually strip it away.
3. Earth's magnetic field deflects many of these charged particles high above the atmosphere.
4. Some particles are guided along field lines toward the poles.
5. When they enter the atmosphere and collide with gas particles, they produce auroras, bright displays of light.