Magnetism Section 1

Preview

Section 1 Magnets and Magnetic Fields

Section 2 Magnetism from Electricity

Section 3 Magnetic Force

Magnetism Section 1

What do you think?

• An iron nail is attracted to an iron magnet but not to another nail. Two magnets can attract each other.• Is either end of the nail attracted to either end of the

magnet? • Is either end of one magnet attracted to either end of

the other magnet? Explain.• Both are made of iron, but the magnet behaves

differently. Why?• How does the nail change when near the magnet so

that it is attracted?

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Magnetism Section 1

Properties of Magnets• Magnets attract metals classified as

ferromagnetic.– Iron, nickel, cobalt

• Magnets have two poles, north and south.– Like poles repel each other.– Opposite poles attract each other.

• When free to rotate, the north pole points toward the north.

Magnetism Section 1

Click below to watch the Visual Concept.

Visual Concept

Magnetic Poles

Magnetism Section 1

Magnetic Domains• In ferromagnetic materials,

groups of atoms form magnetic domains within the material.

• In a paper clip or nail, the domains are randomly arranged.

• In a magnet, the domains are more aligned.

Magnetism Section 1

Magnetic Domains• What would happen to the

domains?– They would better align.

• How would the paper clip be different afterward?– It would behave as a magnet.

• Would it remain magnetized?– The domains would gradually

become more randomly oriented.

• Suppose you rubbed a paper clip repeatedly in one direction with the north pole of a magnet.

Magnetism Section 1

Magnetic Fields• What object is used to detect a gravitational field?

– Any mass - when released it falls in the direction of the field

• What object was used to detect an electric field?– A positively charged test particle - when released it

moves in the direction of the field• What object would be used to detect a magnetic

field?– A compass - the north pole points in the direction of the

magnetic field

Magnetism Section 1

Magnetic Fields• Compass needles show

the direction of the field.– Out of the north and into

the south• The distance between

field lines indicates the strength of the field.– Stronger near the poles

• The field exists within the magnet as well.

Magnetism Section 1

Magnetic Flux• Flux measures the number of

field lines passing perpendicularly through a fixed area.– More flux near the poles

Magnetism Section 1

Click below to watch the Visual Concept.

Visual Concept

Representing the Direction of a Magnetic Field

Magnetism Section 1

Earth’s Magnetic Field• The north pole of a magnet

points toward the geographic north pole or Earth’s south magnetic pole.– Opposites attract

• The magnetic poles move around.

• The magnetic and geographic poles are about 1500 km apart.

Magnetism Section 1

Earth’s Magnetic Field• Which way would a compass

needle point in the U.S.?– Toward the north and slightly

downward into Earth– Field lines go into Earth as

seen in the diagram; they are not parallel to the surface.

• Earth’s poles have reversed many times in the past, as evidenced by core samples showing differing magnetic field directions.

Magnetism Section 1

Now what do you think?

• An iron nail is attracted to an iron magnet but not to another nail. Two magnets can attract each other.• Is either end of the nail attracted to either end of the

magnet? • Is either end of one magnet attracted to either end of

the other magnet? Explain.• Both are made of iron but the magnet behaves

differently. Why?• How does the nail change when near the magnet so

that it is attracted?

Magnetism Section 2

What do you think?

• Electromagnets are used every day to operate doorbells and to lift heavy objects in scrap yards. • Why is the prefix electro- used to describe these

magnets?• Is electricity involved in their operation or do they create

electricity?• Would such a magnet require the use of direct current

or alternating current?• Why?

Magnetism Section 2

Magnetism from Electricity• A compass needle held near a

current carrying wire will be deflected.– Electric current must produce a

magnetic field.– Discovered by Hans Christian

Oersted• Many compasses placed

around a vertical current carrying wire align in a circle around the wire.

Magnetism Section 2

Right-Hand Rule• To find the direction of the

magnetic field (B) produced by a current (I):– Point your right thumb in the

direction of the current– Curl your fingers and they will show

the direction of the circular field around the wire.

Magnetism Section 2

Click below to watch the Visual Concept.

Magnetic Field of a Current-Carrying Wire

http://www.youtube.com/watch?v=bSge-qDcS4Y&fe

ature=endscreen&NR=1

Magnetism Section 2

Magnetic Fields• Use the right hand rule to

decide what direction the magnetic field would be at points A, B, and C.

• Since magnetic fields are vectors, how would the net field appear in the center of the loop?

C B A

Magnetism Section 2

Click below to watch the Visual Concept.

Visual Concept

Magnetic Field of a Current Loop

Magnetism Section 2

Magnetic Field Around a Current Loop• Magnets and loops of

wire have magnetic fields that are similar.

• Solenoids are coils of wire similar to the single loop.– More loops strengthens the

field– Placing an iron rod in the

center strengthens the field as well

• Called an electromagnet

Magnetism Section 2

Now what do you think?

• Electromagnets are used every day to operate doorbells and to lift heavy objects in scrap yards. – Why is the prefix electro- used to describe these

magnets?• Is electricity involved in their operation or do they create

electricity?– Would such a magnet require the use of direct current

or alternating current?• Why?

Magnetism Section 2

Homework• p.682/1-4 and p.686/1-3

Magnetism Section 3

What do you think?

• When watching a television with a CRT, an image is created on the screen by beams of electrons striking red, green, and blue phosphors on the screen. • How are these beams aimed at the right phosphors?• Why does holding a magnet near the screen alter the

image and sometimes permanently damage the screen?

• How often does the TV produce a new still image for you to see?• How do these still images create movement?

Magnetism Section 3

Charged Particles in a Magnetic Field• Magnetic fields exert a magnetic force on moving

charged particles.– Force is greatest when the movement is perpendicular to the

magnetic field– Force is zero when the particle moves along the field lines– Force is in between these values for other directions

• When the movement is perpendicular, the magnetic force is:

Fmagnetic = qvB– where q is the charge, v is the velocity, and B is the magnetic

field strength.

Magnetism Section 3

Charged Particles in a Magnetic Field• So, the magnetic field (B) can be determined from the

force on moving charged particles as follows:

• SI unit: Tesla (T)– where T = N/(C•(m/s)) = N/(A•m) = (V•s)/m2

Magnetism Section 3

Charged Particles in a Magnetic Field• The right-hand rule for the force

on a moving charged particle– Thumb in the direction a positive

particle is moving– Fingers in the direction of the

magnetic field– The force will be in the direction of

your palm• For negative particles, the force

is out the back of your hand.

Magnetism Section 3

Click below to watch the Visual Concept.

Visual Concept

Force on a Charge Moving in a Magnetic Field

Magnetism Section 3

Classroom Practice Problems• An electron moving north at 4.5 104 m/s enters

a 1.0 mT magnetic field pointed upward.– What is the magnitude and direction of the force on

the electron?– What would the force be if the particle was a proton?– What would the force be if the particle was a neutron?

• Answers: – 7.2 10-18 N west– 7.2 10-18 N east– 0.0 N

Magnetism Section 3

Magnetic Force as Centripetal Force• Use the right-hand rule to

determine the direction of the force.

• Which direction would the force be when the charge is at the top? the left side? the bottom?– Always directed toward the

center– Because of this magnetic

force, the charge moves in a circle.

– The force is centripetal.

Magnetism Section 3

Current-Carrying Wires• Magnetic forces also exist on the

moving charges in current-carrying wires.– The right-hand rule to is used to

determine the direction, as shown in the diagram.

– The magnitude of the force is as follows:

Magnetism Section 3

Parallel Current-Carrying Wires• Current carrying wires create a magnetic

field which interacts with the moving electrons in the nearby wire.– Currents in the same direction produce

attraction.– Currents in opposite directions cause the wires

to repel.• Use the-right hand rule to verify the

direction of the force for each of the four wires shown.

Magnetism Section 3

Classroom Practice Problem• A 4.5 m wire carries a current of 12.5 A from

north to south. If the magnetic force on the wire due to a uniform magnetic field is 1.1 103 N downward, what is the magnitude and direction of the magnetic field?

• Answer: 2.0 101 T to the west

Magnetism Section 3

Applications - Cathode Ray Tube• Televisions and computer

monitors use CRTs.• A magnetic field deflects

a beam of electrons back and forth across the screen to create an image.

Magnetism Section 3

Applications - Speakers• The forces on electrons

as they move back and forth in the coil of wire cause the coil to vibrate.

• The coil is attached to the paper cone, so sound waves are produced by the vibration.

Magnetism Section 3

Click below to watch the Visual Concept.

Visual Concept

Galvanometer

Magnetism Section 3

Now what do you think?

• When watching a television with a CRT, an image is created on the screen by beams of electrons striking red, green, and blue phosphors on the screen. – How are these beams aimed at the right phosphors?– Why does holding a magnet near the screen alter the

image and sometimes permanently damage the screen?

– How often does the TV produce a new still image for you to see?• How do these still images create movement?