Magnetism

What is magnetism?Magnetism is the properties and interactions of magnetsThe earliest magnets were found naturally in the mineral magnetite which is abundant the rock-type lodestone. These magnets were used by the ancient peoples as compasses to guide sailing vessels.Magnets produce magnetic forces and have magnetic field lines

Iron Filings lab

• Go to the various tables and make sketches of the images you see with iron filings and magnetic fields.

                                                        

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Magnets have two ends or poles, called north and south poles. At the poles of a magnet, the magnetic field lines are closer together. The more lines the greater the flux.

Unlike poles of magnets attract each other and like poles of magnets repel.

Magnetic Field Lines

• Are vector, have the symbol B, and the SI units are the Tesla (T). Gauss is another unit sometimes used.

• The arrows go from North to South

• They can up, down,left, right, into the page, or out of the page. Look at the examples drawn on the smart board…

The earth is like a giant magnet!The nickel iron core of the earth gives the earth a magnetic field much like a bar magnet.

How do compasses work?

Section Review p. 769

• Answer problem 1-3

What are magnetic domains?Magnetic substances like iron, cobalt, and nickel are composed of small areas where the groups of atoms are aligned like the poles of a magnet. These regions are called domains. All of the domains of a magnetic substance tend to align themselves in the same direction when placed in a magnetic field. These domains are typically composed of billions of atoms.

How do you make a magnet?

Magnetic Field of a current carrying wire

• A wire that has an electric current going through it will have a magnetic field around it. The direction of the current determines the direction of the field.

• The right hand rule will show you the field.

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Electricity and Magnetism – how are they related?

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

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Right hand rule in a current carrying wire

• See image on page 770

Magnetic Field of a current loop

• See images on page 771

• Solenoids – continuous loops of wire, they form strong magnets on the inside.

• How can you tell North / South?

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What is an electromagnet?When 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.

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Charged Particle in a Magnetic Field

• Stationary charges don’t interact with magnetic fields, but moving charges do.

• This force is maximum when the charges move perpendicular to the magnetic field.

Right Hand Rule

• B Magnetic Field 4 fingers• V velocity of moving charge thumb• F force on a positive charge palm of hand

A negative charge would have the opposite direction of force.

• Arrows into a page are drawn as X’s, arrows out of the page are drawn as points.

• Study Image on page 774• Answer questions on page 783 #35

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B, to the right

V, up the page

What would be the direction of force on an electron? A proton?

XForce is into the page, Magnetic field is down the page.

What is the direction of velocity for a positive charge?What is the direction of velocity for a negative charge?

Fmag= qvB

• Fmag = magnetic force on a charged particle (N)

Newton.

• Q= magnitude of charge (C) Coulomb

• V = velocity of charged particle(m/s)

• B= Magnetic Field (T) Tesla.

• Sample prob. P.774

• Practice probs 21A p.775 (1-3)

Magnetic Force on a current carrying wire

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• http://www.youtube.com/watch?v=tUCtCYty-ns

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http://www.youtube.com/watch?v=43AeuDvWc0k

2 parallel wires carrying current, they attract if the current is in the same direction

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2 current carrying wires repel if their currents run in opposite directions.

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Loudspeakers work by this idea

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Fmag = BIL

• Fmag = magnetic force (N)

• B = magnetic Field (T) Tesla

• I = current (A) Ampere

• L = length of conducting wire in the magnetic field.

• Sample prob. P. 778

• Prac. Probs 21B page 778 (1-3)

What is a galvanometer?A 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.

Galvanometers are used as gauges in cars and many other applications.

Galvanometer

• A simple instrument designed to detect electric current.

• When calibrated to measure current, it is an ammeter.

• When calibrated to measure voltage, it is a voltmeter.

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Electromagnetic Induction

• The production of an emf (electro-motive force – kinda like voltage) in a conducting circuit by a change in the strength, position, or orientation of an external magnetic field.

• Further explained on page 794

Faradays Law

• Hans Christian Oersted discovered magnetism and electricity are related.

• Magnetism is produced by the motion of electrons.

• Voltage is induced in a loop of wire if there is a change in the magnetic field in the loop.

• If a magnets in and out motion through a coil of wire is doubled, the voltage is doubled.

Faradays Law

• If the number of loop is doubled, the induced voltage is doubled.

• A rapidly changing magnetic field in any region of space induces a rapidly changing electric field. Which in turn induces a rapidly changing magnetic field.

• This generation and regeneration of electric and magnetic fields makes up electromagnetic waves.

Faradays Law

• The induced emf (electromotive force) in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit.

• It is the operating principle of transformers, inductors, many types of motors and generators.

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Lenz Law

• An induced current is always in such a direction as to oppose the motion or change causing it

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Induction

• Wires spinning in magnetic fields is what underlies all electric motors and electric generators.

• Most of the rest of chapter 22 deals with applications of this. Electric motors, electric generators, and transformers will be as far as we go down this road. Take AP Physics next year to get more.

What are electric motors?An electric motor is a device which changes electrical energy into mechanical energy.

Go to the next slide

How does an electric motor work?

Simple as that!!

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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:

YouTube animations

http://www.youtube.com/watch?NR=1&feature=endscreen&v=d_aTC0iKO68

http://www.youtube.com/watch?v=Xi7o8cMPI0E

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.

Mutual Inductance

• A measure of the ability of one circuit carrying a changing current to induce an emf in a nearby circuit.

• See page 814

• This only works in alternating current (ac)

• This explains ……..

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Transformers

• A device that changes one ac potential difference to a different ac potential difference. Only works for AC!

Power companies increase voltages for long distance transmission, then they must decrease voltages before going into your home.

We say the voltage has been stepped up or it has been stepped down.

Transformers

• A transformer can step up voltage but in no way can it step up energy and power.

• A transformer operates on ac because the magnetic field within the iron core must continually change.

Transformers

• They have 2 sides – primary and secondary

• Primary is the side closest (wired) to the generator

• Secondary is the side wired to the resistor or the consumer.

• A soft Iron core connects both sides.

Transformer Animation

http://www.youtube.com/watch?v=2-Ijjm7if5g&feature=related

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Transformer Equation

V2N1 = N2V1

V = voltage

N = number of turns or coils of wire

1 = Primary

2 = Secondary

Sample problem page 817

Practice 22D page 818 (1-3)

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This concludes our Magnetism topic