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Chapter 24

Magnetism - II

Lecture 27

Quiz 4: Monday Nov. 1; Chaps. 22,23,24

• As with electric charges, like magnetic poles (N&N, S&S) repel and opposites (N&S) attract.

• As in electric polarization, both types of poles can attract unmagnetizediron, steel, nickel or cobalt objects by forming “induced magnets”

Magnetic Force between Poles

Induced Magnets

S N

S N

StrongMagnet

StrongMagnet

Unmagnetized iron can be induced to align by an external magnetic field, forming induced magnets.

NStrong magnetic poles attract unlike induced

magnet poles.

Permanent/Temporary Magnets

Difference between permanent magnet and temporary magnet• Permanent magnet

– alignment of domains remains once external magnetic field is removed

• Temporary magnet (induced magnet)– alignment of domains returns to random arrangement once

external magnetic field is removed

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Demo: Magnetizing IronMagnetic domains in iron nails are

induced to align by proximity of the strong magnet

Each nail becomes itself a magnet, which in turn magnetizes the nail below it, forming a chain.

When the strong magnet is removed, most of the domains un-align and nail lose most of their magnetization.

A weak and strong magnet repel each other. The greater repelling force is by the

A. stronger magnet.B. weaker magnet.C. Both the same.D. None of the above.

Magnetic PolesCHECK YOURSELF

Electric Currents and Magnetic Fields

Wire with loop

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ElectromagnetsElectric current in a coil of wire creates a

magnetic field similar to a bar magnet.

Current passing through loops of coiled wire

N

S

N

S

N

S

An electromagnet can be made stronger by

A. increasing the number of turns of wire.B. increasing the current in the coil.C. Both A and B.D. None of the above.

Electric Currents and Magnetic FieldsCHECK YOURSELF

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Practical Electromagnets

Electromagnet created by passing current through a coil of wire.

Electromagnet is stronger when an iron bar is inserted within the coil. Connect to

battery or power supply

WireCoil

N

S

Iron Bar

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Check YourselfWhen an object is charged with static electricity, is

that object also magnetized?

But isn’t iron magnetized when the electrons are aligned? If iron has electrons, then why isn’t it charged?

Magnetic Data StorageMagnetize small regions of iron N or S

• “Hard” disk drive:

• Magnetic ink on checks (& paper money)• Videotape, credit cards, ...

Earth’s Magnetic Field• Earth is itself a huge magnet.• The magnetic poles of Earth are

widely separated from the geographic poles.

• The magnetic field of Earth is not due to a giant magnet in its interior—it is due to electric currents.

• Most Earth scientists think that moving charges looping around within the molten part of Earth create the magnetic field.

• Earth’s magnetic field reverses direction: 20 reversals in last 5 million years.

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Earth’s Magnetic FieldThe configuration of the Earth's

magnetic field resembles a strong bar magnet located near the center of the Earth.

The magnetic South pole is near (but not right at) the geographic North pole (so North pole of compass attracted towards Northern direction).

There isn’t an giant underground magnetized chunk of iron; Earth’s interior is simply too hot.

N

S

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Origin of Earth’s Magnetic FieldEarth is an electromagnet with electric currents

deep below the surface. Moving charges, looping around within the molten part of the Earth, create the magnetic field.

These currents are possibly the result of thermal convection rising from the central core combined with the rotation of the Earth about its axis.

• Magnetic compass is a permanent magnet on a pivot

Magnetic Compass

Compass needle points toward “magnetic north pole” of Earth. Differs from direction of “true North.”

Magnetic Declination

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Magnetic Force on ChargesMoving electric charges can feel magnetic

force when traveling through magnetic fields.

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Television TubeElectron beams, deflected by magnetic

fields, are used to create TV images.

Electromagnets

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Magnetic Force on CurrentMoving charges in an electric current

experience a force when in a magnetic field.

LoudspeakersLoudspeaker cone oscillations created by variations in electrical current which cause an electromagnet to be pulled towards and away from a permanent magnet.

These oscillations cause cone of loudspeaker to vibrate with same frequency as oscillations in the electrical current.

Headphones work essentially the same way, they’re just smaller. Motors are similar,

Biomagnetism• Certain bacteria biologically produce single-domain grains

of magnetite (a compound equivalent to iron ore) that they string together to form internal compasses. – They then use these compasses to detect the dip of Earth’s

magnetic field. – Equipped with a sense of direction, the organisms are able to

locate food supplies.

• Pigeons have multiple domain magnetite magnets within their skulls that are connected with a large number of nerves to the pigeon brain. – Pigeons have a magnetic sense, and not only can they discern

longitudinal directions along Earth’s magnetic field, they can also detect latitude by the dip of Earth’s field.

– Other similar animals: Bees, butterflies, sea turtles and fish.

Magnetic Resonance Imaging• Non-invasive medical imaging method, like

ultrasound and X-ray.• Based on induced magnetization of proton

atomic magnets.

Abdomen Spine Heart / Coronary

MRI Systems

Large structure is electromagnet.

Static Magnetic Field

Longitudinal

Transverse

B0

z

x, y

Main magnetic field direction

Polarization of Proton Magnets• Proton atomic magnets are normally oriented

randomly.• In an applied magnetic field, the proton

magnets partly align with the applied field.• Alignment along polarizing field results in net

magnetization. No Applied Field Applied Field

B0

Key Points of Lecture 27Key Points of Lecture 27

Before Monday, read Hewitt Chap. 24 (second half).

Homework #18 due by 11:00 PM Friday Oct. 29.

Homework #19 due by 11:00 PM on Sunday Oct. 31.

• Magnetic force between atomic currents (poles)• Magnets: permanent, induced, electromagnets• Magnetic Data Storage• Magnetic Field of Earth; Magnetic Compass• Magnetic Force on Current in Wire• Biomagnetism & MRI