BALDWIN 1

PHYSICS Mr. BALDWIN/Mr. JONESMAGNETISM April 4, 2013

AIM: What is magnetism?

DO NOW: Draw a bar magnet, labeling its poles and showing the magnetic field lines around it. Indicate the directions of the magnetic field lines

HOME WORK – Answer the handout

Magnets & Magnetic FieldsMagnets & Magnetic FieldsCertain types of mineral rock possessed unusual properties of attraction to the metal iron. One particular mineral, called lodestone, or magnetite

Magnets have two ends – poles – called north & south.

Like poles repel; unlike poles attract.

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Lodestone (also called Magnetite) is a naturally-occurring "permanent" magnet mineral. By "permanent," it is meant that the material maintains a magnetic field with no external help.

The characteristic of any magnetic material to do so is called retentivity.

Ferromagnetic materials are easily magnetized.

Paramagnetic materials are magnetized with more difficulty.

Diamagnetic materials actually tend to repel external magnetic fields by magnetizing in the opposite direction.

Magnets & Magnetic FieldsMagnets & Magnetic Fields

However, if you cut a magnet in half, you don’t get a north pole and a south pole – you get two smaller magnets.

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Magnetic FieldsMagnetic fields can be visualized using magnetic field lines, which are always closed loops.http://phet.colorado.edu/en/simulation/magnets-and-electromagnets

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Earth’s Magnetic FieldsEarth’s Magnetic Fields

The Earth’s magnetic field is similar to that of a bar magnet.

Note that the Earth’s “North Pole” is really a south magnetic pole, as the north ends of magnets are attracted to it.

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BALDWIN 8

PHYSICS Mr. BALDWIN/Mr. JONESMAGNETISM April 5, 2013

AIM: What is magnetism? (Cont’d)

DO NOW: Draw the magnetic field lines around current carrying wire (conductor) that carries a current in the downward direction.

HOME WORK –

Magnets and Magnetic FieldsMagnets and Magnetic Fields

A uniform magnetic field is constant in magnitude and direction.

The field between these two wide poles is nearly uniform.

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Electric Currents Produce Magnetic Fields

Experiment shows that an electric current produces a magnetic field.

RHR 1: Right Hand “Curl” Rule

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Electric Currents Produce Magnetic Fields

The direction of the field is given by a right-hand rule.

http://phet.colorado.edu/en/simulation/faraday

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BALDWIN 12

PHYSICS Mr. BALDWIN/Mr. JONESMAGNETISM 04/19/23

AIM: How does an electric field affect moving charges?

DO NOW: Draw the magnetic field around a wire that is carrying a current that is flowing out the board.

http://phet.colorado.edu/en/simulation/faraday

HOME WORK – Read Electromagnetic Induction Pg. 135

Force on an Electric Current in a Magnetic Field; Definition of B

A magnet exerts a force on a current-carrying wire. The direction of the force is given by a right-hand rule.

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Force on an Electric Current in a Magnetic Field; Definition of B

The force on the wire depends on the current, the length of the wire, the magnetic field, and its orientation.

This equation defines the magnetic field B.

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DO NOW:

If a wire, carrying a current flowing out the board, is placed in a uniform magnetic field pointing from south to north, in which direction would it be deflected?Draw a diagram illustrating your findings.

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Force on an Electric Current in a Magnetic Field

Unit of Magnetic Filed: the Tesla, T.

1 T = 1 N/A·m

Another unit sometimes used: the gauss (G).

1 G = 10-4 T.

1T= 10,000 G

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Force on Electric Charge Moving in a Magnetic Field

The force on a moving charge is related to the force on a current:

Once again, the direction is given by a right-hand rule.

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F qvB

BALDWIN 18

PHYSICS Mr. BALDWINMAGNETISM April 19, 2023AIM: What is electromagnetic induction?

(How does a transformer work?)

DO NOW: What do you think is the purpose of an electrical transformer (substation)?

HOME WORK – Revise your magnetism notes for TEST!!!

Solenoids and Electromagnets

A solenoid is a long coil of wire. If it is tightly wrapped, the magnetic field in its interior is almost uniform:

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Solenoids and Electromagnets

If a piece of iron is inserted in the solenoid, the magnetic field greatly increases. Such electromagnets have many practical applications.

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Applications: Galvanometers, Motors, Loudspeakers

A galvanometer takes advantage of the torque on a current loop to measure current.

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Applications: Galvanometers, Motors, Loudspeakers

An electric motor also takes advantage of the torque on a current loop, to change electrical energy to mechanical energy.

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Applications: Galvanometers, Motors, Loudspeakers

Loudspeakers use the principle that a magnet exerts a force on a current-carrying wire to convert electrical signals into mechanical vibrations, producing sound.

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Ferromagnetism: Domains

Ferromagnetic materials are those that can become strongly magnetized, such as iron and nickel.

These materials are made up of tiny regions called domains; the magnetic field in each domain is in a single direction.

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Ferromagnetism: Domains

When the material is unmagnetized, the domains are randomly oriented.

A magnet, if undisturbed, will tend to retain its magnetism. It can be demagnetized by shock or heat.

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PHYSICS Mr. BALDWINMAGNETISM April 19, 2023AIM: What is the purpose of a transformer?

DO NOW: Which one is a step-up transformer and why? Which one is the step-down transformer and why?

HOME WORK – Revise your magnetism notes for TEST!!!

Electromagnetic Induction & Faraday’s Law

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Induced EMF

Almost 200 years ago, Faraday looked for evidence that a magnetic field would induce an electric current with this apparatus:

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Induced EMFHe found no evidence when the current was steady, but did see a current induced when the switch was turned on or off.

Therefore, a changing magnetic field induces an emf.

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Transformers & Transmission of Power

A transformer consists of two coils, either interwoven or linked by an iron core. A changing emf in one induces an emf in the other.

The ratio of the emfs is equal to the ratio of the number of turns in each coil:

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Step-Up Transformers

This is a step-upstep-up transformer – the emf in the secondary coil is larger than the emf in the primary:

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Step-Down Transformers

This is a step-step-down down transformer – the emf in the secondary coil is smaller than the emf in the primary:

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PHYSICS Mr. BALDWINMAGNETISM April 19, 2023AIM: How do you calculate the voltage and

current output of a transformer?

DO NOW: (QUIZ)

• The primary coil of a transformer has 2.0 x 102 turns. It is connected to a 1.0 x 102 V source. Calculate the number of turns on the secondary coil needed to supply a voltage of 6.0 x 102 V.

HOME WORK – Revise your magnetism notes for TEST!!!

Transformers & Transmission of Power

Energy must be conserved; therefore, the power transmitted across a transformer is conserved, thus the ratio of the currents must be the inverse of the ratio of turns:

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Transformers & Transmission of Power

Note: Transformers work only if the current is changing; this is one reason why electricity is transmitted as an alternating current (ac).

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TEST YOURSELFTEST YOURSELF

1. The voltage across a transformer primary coil that has 50 turns is 25 V.

a. What type of transformer is it?

b. What is the output voltage across the secondary coil, which has 20 turns?

2. The voltage across the input terminals of a transformer is 140 V. The primary has 20 loops and the secondary has 10 loops.

a. What type of transformer is it?

b. The voltage the transformer puts out is…

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• A 180.0 W transformer has an input voltage of 12.0 V and an output current of 4.0 A. What is the ratio of Voutput to Vinput?

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BALDWIN 38

PHYSICS Mr. BALDWINMAGNETISM April 19, 2023

AIM: How are charges affected by magnetic fields?

DO NOW: If a wire, carrying a current flowing out the board, is placed in a uniform magnetic field pointing from south to north, in which direction would it be deflected?Draw a diagram illustrating your findings.

HOME WORK: prepare for TEST!!!!!!!!!!

Force on Electric Charge Moving in a Magnetic Field

If a charged particle is moving perpendicular to a uniform magnetic field, its path will be a circle.

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Force between Two Parallel Wires

The magnetic field produced at the position of wire 2 due to the current in wire 1 is:

The force this field exerts on a length l2 of wire 2 is:

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Force between Two Parallel Current Carrying Wires

Parallel currents attract; antiparallel currents repel.

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