Chapter 14

Magnetism

14.1 Properties of Magnets14.2 Electricity and

Magnetism14.3 Producing Electric

Current

MagnetismKey Question:

How do magnets and compasses work?

14.1 Properties of MagnetsIf a material is magnetic, it has the ability

to exert forces on magnets or other magnetic materials nearby.

A permanent magnet is a material that keeps its magnetic properties.

14.1 Properties of MagnetsAll magnets have

two opposite magnetic poles, called the north pole and south pole.

If a magnet is cut in half, each half will have its own north and south poles.

Whether the two magnets attract or repel depends on which poles face each other.

14.1 Properties of Magnets

Magnetic forces can pass through many materials with no apparent decrease in strength.

14.1 Properties of Magnets

14.1 Properties of Magnets

Magnetic forces are used in many applications because they are relatively easy to create and can be very strong.

Large magnets create forces strong enough to lift a car or a moving train.

14.1 Magnetic FieldsThe force from a

magnet gets weaker as it gets farther away.

Separating a pair of magnets by twice the distance reduces the force by 8 times or more.

14.1 Magnetic Fields

A special kind of diagram is used to map the magnetic field.

The force points away from the north pole and towards the south pole.

14.1 Magnetic FieldsYou can actually

see the pattern of the magnetic field lines by sprinkling magnetic iron filings on cardboard with a magnet underneath.

14.1 Magnetic Field LinesA compass needle

is a magnet that is free to spin.

Because the needle aligns with the local magnetic field, a compass is a great way to “see” magnetic field lines.

14.1 Geographic and magnetic poles

The planet Earth has a magnetic field that comes from the core of the planet itself.

The names of Earth’s poles were decided long before people understood how a compass needle worked.

14.1 Geographic and magnetic poles

The compass needle’s “north” end is actually attracted to Earth’s “south” magnetic pole!

14.1 Declination and “true north”

Because Earth’s geographic north pole (true north) and magnetic south pole are not located at the exact same place, a compass will not point directly to the geographic north pole.

The difference between the direction a compass points and the direction of true north is called magnetic declination.

Magnetic declination is measured in degrees and is indicated on topographical maps.

14.1 Declination and “true north”

Magnetic declination is measured in degrees and is indicated on topographical maps.

Most good compasses contain an adjustable ring with a degree scale used compensate for declination.

14.1 Declination and “true north”

14.1 Earth’s magnetismStudies of

earthquake waves reveal that the Earth’s core is made of hot, dense molten metals.

Huge electric currents flowing in the molten iron produce the Earth’s magnetic field.

The gauss is a unit used to measure the strength of a magnetic field.

The magnetic field of Earth (.5 G) is weak compared to the field near the ceramic magnets you have in your classroom. (300- 1,000 G).

For this reason you cannot trust a compass to point north if any other magnets are close by.

14.1 Earth’s magnetism

Today, Earth’s magnetic field is losing approximately 7 percent of its strength every 100 years.

If this trend continues, the magnetic poles will reverse sometime in the next 2,000 years.

14.1 Earth’s magnetism

14.1 Magnetic MaterialOnly some metals are attracted by Magnets,

or can be made into a permanent Magnet. Ea: Iron, cobalt, and nickel.

Electrons have magnetic properties. In most material these properties cancel each other out.

In Iron, cobalt, and nickel, large groups of electrons align their magnetic poles, and create groups with most of their poles aligned. We call these groups Magnetic Domains.

These domains act like small magnets, with a north and south pole.

An iron nail contains an enormous numbers of these magnetic domains, but they are all aligned randomly.

In presence of an external field, these domains may align with the field.

14.1 Magnetic Material

14.2 Electricity and Magnetism Electromagnets

Key Question:How are electricity and magnetism

related?

14.2 ElectromagnetsElectromagnets

are magnets that are created when there is electric current flowing in a wire.

The simplest electromagnet uses a coil of wire wrapped around some iron.

14.2 Right hand ruleTo find the north

pole of an electromagnet, use the right hand rule.

When the fingers of your right hand curl in the direction of the wire, your thumb points toward the magnet’s north pole.

14.2 Moving Charge and Magnetic Field

Electricity and Magnetism – how are they related?

When an electric current passes through a wire a magnetic field is formed.

14.2 ElectromagnetsWhen an electric current is passed

through a coil of wire wrapped around a metal core, a very strong magnetic field is produced. This is called an electromagnet.

Solenoid

14.2 Doorbells

A doorbell contains an electromagnet.

When the button of the bell is pushed, it sends current through the electromagnet.

14.2 Using Electromagnets to Make Sound

• The electromagnet

changes electrical energy

to mechanical energy that vibrates the

speaker cone to produce sound.

Fleming’s Left Hand Rule

14.2 Making an Electromagnet Rotate

14.2 Making an Electromagnet RotateThe forces exerted on

an electromagnet by another magnet can be used to make the electromagnet rotate.

A current carrying wire

creates a magnetic field. When in a magnetic field, it will feel a force due to the forces between magnetic poles.

If you change the orientation of the electromagnet, it will continue to rotate.

14.2 Making an Electromagnet Rotate

14.2 GalvanometerA galvanometer is an electromagnet that

interacts with a permanent magnet. The stronger the electric current passing through the electromagnet, the more is interacts with the permanent magnet.

The greater the current passing through the wires, the stronger the galvanometer interacts with the permanent magnet.

A Simple Electric Motor

A simple electric motor also includes components called brushes and a commutator.

The brushes are conducting pads connected to the battery.

The brushes make contact with the commutator, which is a conducting metal ring that is split.

The brushes and the commutator form a closed electric circuit between the battery and the coil.

14.2 Electric Motors

Making the Motor Spin

Step 1. When a current flows in the coil, the magnetic forces between the permanent magnet and the coil cause the coil to rotate.

14.2 Electric Motors

Making the Motor Spin

Step 2. In this position, the brushes are not in contact with the commutator and no current flows in the coil.

The inertia of the coil keeps it rotating.

14.2 Electric Motors

Making the Motor Spin

Step 3. The commutator reverses the direction of the current in the coil.

This flips the north and south

poles of the

magnetic field

around the coil.

14.2 Electric Motors

Making the Motor Spin

Step 4. The coil rotates until its poles are opposite the poles of the permanent magnet.

The

commutator

reverses the

current, and

the coil keeps

rotating.

14.2 Electric Motors

From Mechanical to Electrical Energy

We have seen how electricity can produce a magnetic field, but a magnetic field can also produce electricity! How?

What is electromagnetic induction?

Moving a loop of wire through a magnetic field produces an electric current. This is electromagnetic induction.

A generator is used to convert mechanical energy into electrical energy by electromagnetic induction.

Carefully study the next diagrams:

14.3 Producing Current

14.3 Producing Current

14.3 Producing Current

14.3 Direct current versus alternating current AC vs DC : What’s the difference?

Direct current is electrical current which comes from a battery which supplies a constant flow of electricity in one direction.

Alternating current is electrical current which comes from a generator. As the electromagnet is rotated in the permanent magnet the direction of the current alternates once for every revolution.

Go to this website and click the button for DC then for AC to visually see the difference between the two.

You can see that the DC source is a battery – current flows in one direction. The AC source is the generator and the current alternates once for each revolution.

14.3 TransformersA transformer is composed of two different coils

of wire around opposite sides of an iron core. As you know, passing an alternating current through a coil of wire that surrounds a metal core induces a varying magnetic field in the core. This field will cause a responding current flow in a secondary coil wound around the opposite side.

What makes transformers so useful is that if you change the number of turns from one side to the other, you change the voltage in the wire on the right! Transformers can change a high voltage to a lower one, or a low voltage to a higher one.

First let's look at a transformer that converts a low voltage to a high one. This is called a step-up transformer.

The right side has four times more turns.

the voltage on the right has increased four times (from 100 V to 400 V).

The voltage has been stepped up by a factor of four.

14.3 Transformers

If you increase the number of turns on the right, the voltage coming off the transformer will increase in proportion.

Because current is inversely proportional to voltage, you can see that stepping up the voltage pays a price ... the current on the right is only a quarter of what it was on the left. Step-up transformers increase the voltage, but decrease the current. In our example above, the current went from 10 A to 2.5 A, a reduction of by a factor of four.

Interestingly, you can also see that the power supplied to the primary coil (1000 W) is equal to the power delivered to the secondary coil. This assumes there is no loss of power due to heat (which of course is the ideal case ... generally not true in real life).

Now let's look at a transformer which reduces the voltage. This is called a step-down transformer.

14.3 Transformers

Using the numbers in the example above, you can see that the right side has one fifth the number of turns. As a result, the voltage on the right is only one-fifth as large. The voltage has been stepped down by a factor of five (1000 V down to 200 V).

14.3 Transformers

If you decrease the number of turns on the right, the voltage coming off the transformer will decrease in proportion.

There are other factors which can affect how a transformer operates, such as the number of turns used altogether, the size and type of wire, and the type of metal used for the core. This allows for transformers which can work with very tiny or very large voltages.

14.3 Transformers