magnetism mark lesmeister dawson high school physics © 2013 mark lesmeister/pearland isd

Magnetism

Mark Lesmeister

Dawson High School Physics© 2013 Mark Lesmeister/Pearland ISD

Acknowledgements

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MAGNETS AND MAGNETIC FIELDS

Section 1

Mini-experiment 1

• Examine the bar magnet. Determine whether it will attract aluminum foil, paper clips, or both.

• Pick up a paper clip with the bar magnet. Use the other end of the paper clip to pick up another paper clip. While holding the first paper clip, gently slide the magnet away.

• Determine what effect each end of the bar magnet has on the compass. Note: The compass is also a magnet; the red end of the compass is the north end, because it points north.

Magnets

• All magnets have two poles.– Unlike poles attract.

– Like poles repel.

– No magnetic monopole, that is a North without a South or a South without a North, has ever been found.

N S N S

N S S N

S N N S

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Magnets

• Only certain materials exhibit magnetic properties.

• For example, magnets always attract ferrous materials.

• Some materials can be made into permanent magnets.

• Soft magnets, like iron, are easy to magnetize, but lose their magnetism easily.

• Hard magnets, like nickel and cobalt, are hard to magnetize, but retain their magnetism a long time.

The Magnetic Field

• The magnetic force is a field force.

• The force can be represented by field lines.– Lines show the

direction the north pole of the magnet seeks.

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The Magnetic Field

• Symbol for field strength is B.

• The unit for magnetic field is the tesla (T).

• 1 T = N/(C m/s) = N (A m)

Mini-experiment 2

• Using the compass, map out the approximate magnetic field surrounding the bar magnet.

• Using the magnetic field sensor, determine what happens to the strength of the magnetic field as you get farther away from the magnet.

Magnetic Field of a Bar Magnet

N S

© 2013 Mark Lesmeister/Pearland ISD

Magnetic Field Inside a Bar Magnet

N S


Drawing Magnetic Fields in Three Dimensions.

• Magnetic fields often must be represented in 3 dimensions.

• represents a field in the plane of the page.

• represents a field going into the page.

• represents a field going out of the page.

Geomagnetism

• The Earth has a magnetic field.– Since north poles of

magnets are attracted to south poles, the Earth’s south magnetic pole is in the northern hemisphere, and vice versa.

Geographic AxisMagnetic Axis

SN

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Geomagnetism

• The magnetic poles are not perfectly aligned with the geographic poles.– Declination is the

difference between the direction to the true pole and magnetic pole.

– The declination changes, so maps must be updated regularly.

Geographic AxisMagnetic Axis

ELECTROMAGNETISM AND MAGNETIC DOMAINS

Section 2

Representations of the Magnetic Field

• In Equations:– The symbol

for magnetic field strength is B.

– The unit for magnetic field strength is the Tesla (T).

• In Symbols:• represents a

field going into the page.

• represents a field going out of the page.

• Field LinesN

S

Mini-experiment 3

– Wrap the wire around the four posts of the tangent galvanometer as shown in class.

– Connect one clip of the wire to the positive terminal of the power supply.

– Place the compass on the platform, and orient the device so the compass needle is aligned with the plane of the coil of wire.

– Connect the other clip to a 10 Ohm resistor, and connect the other side of the resistor to the negative terminal of the power supply.

– Gradually raise the current in the circuit, and observe the compass needle. Do not exceed 5 Volts.

Magnetic Field of a Long Wire

• A wire carrying a current produces a magnetic field.

• The magnitude of the field of a long wire is given by

Source: Stannered, from Wikipedia Commons, http://en.wikipedia.org/wiki/File:Electromagnetism.svg

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Right-hand Rule

• The direction of the field is given by the right hand rule:– Grasp the wire with

the right hand so that the thumb points in the direction of the current.

– The fingers will then curl in the direction of the magnetic field.

Source: Wikipedia Commons, http://en.wikipedia.org/wiki/File:Manoderecha.svg

Magnetic Field of a Current Loop

• A loop of current produces a magnetic field similar to a bar magnet.

Mini-Experiment 4

• Connect the solenoid (long, tightly wound coil), the 10 Ohm resistor and the power supply together in series.

• Insert the iron core into the solenoid.• Observe the magnetic field created by the

solenoid using the compass.• Observe what happens to the field as the

current is increased.• Do not exceed 5 V unless instructed to do

so.

Magnetic Field of a Solenoid

• A solenoid produces a strong magnetic field by combining several loops.

• The strength of the magnetic field depends on the amount of current flowing through the loop.


Magnetic Field in the Center of a Solenoid

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What makes a magnet?(The classical explanation.)

• In the atom, moving charges produce magnetic fields.– Usually, these fields cancel each

other out.– In some substances, these fields

add up, giving the atom a net magnetic field.

• In some substances, large numbers of these fields can become aligned, forming domains.

electron

neutron

proton

What makes a magnet?(The classical explanation.)

• In an unmagnetized substance, the domains are arranged randomly.

• When the domains are exposed to a magnetic field, they can align with the field, producing a permanent magnet.

Source: Ra’ike, Wikipedia Commons, http://en.wikipedia.org/wiki/File:Weiss-Bezirke1.png

MAGNETIC FORCE ON A MOVING CHARGE

Section 3

The Cross Product

The magnitude of the cross product C=A X B is C = AB sin θ.

The direction of a cross product is given by the right hand rule.The fingers of the right

hand point in the direction of the first vector, and curl in the direction of the second vector.

The thumb indicates the direction of the cross-product.

A

B

C

CBA


Charged particles in a magnetic field

• A charged particle with velocity v moving through a magnetic field B experiences a force Fmagnetic = qv X B

• Fmagnetic = q v B sin θ

• The unit for magnetic field is the tesla (T).• 1 T = N/(C m/s) = N/ (A m)

vq

B

Fmagnetic

Example Problem

• A proton moving north at 4.5 x 104 m/s enters a 1.0 mT magnetic field pointed upward.– What is the magnitude and direction of the force

exerted on the proton?• 7.2 x 10-18 N East

– What would the force be if the particle were an electron?• 7.2 x 10-18 N west

– What would the force be if the particle were a neutron? • 0 N

Path of a Charged Particle in a Mag. Field

• A charged particle moving perpendicular to a uniform magnetic field experiences a force perpendicular to the velocity.

• This will produce circular motion.

mq

Protection from the Solar Wind

• The Sun gives off charged particles which interact with the magnetic field of the Earth.

• The particles spiral around the magnetic field lines of the Earth.


Auroras

• Charged particles from the sun get trapped in the Earth’s magnetic field.

• When those particles are decelerated by the magnetic force, light energy is emitted.

Source: http://upload.wikimedia.org/wikipedia/commons/a/aa/Polarlicht_2.jpg

Velocity Selector

• A particle of charge q and velocity v passes through crossed electric and magnetic fields.

• Which way should the electric field point so the electric force is opposite the magnetic force?


Velocity Selector

• What is the motion if the magnetic force is bigger than the electric force?


Velocity Selector

• What is the motion if the magnetic force is bigger than the electric force?

• What is the motion if the electric force is bigger than the magnetic force?


Velocity Selector

• A particle of charge q passes undeflected through crossed electric and magnetic fields.

• If the magnetic field strength is B and the electric field strength is E, what is the velocity of the particle?


MAGNETIC FORCE ON A CURRENT:ELECTRIC MOTORS

Section 4

Magnetic force on a current carrying conductor

conductor theoflength L

conductor in thecurrent I

strength field magneticB

where

BILFmagnetic

• A current carrying conductor experiences a force in a magnetic field.

• The direction of the force is given by the right hand rule.

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IFMAGNETIC

B (into page)


Electric Motors

• A current carrying conductor experiences a force in a magnetic field.

• This is the principle behind an electric motor.

Source: http://en.wikipedia.org/wiki/Image:Electric_motor_cycle_2.pngLicensed under GNU Free Document License

L

IFMAG

B

http://en.wikipedia.org/wiki/Image:Electric_motor_cycle_2.png

Practice 2

• 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 x 103 N downward, what is the magnitude and direction of the magnetic field?

• Answer: 2.0 x 101 T, to the west.

Magnetic Force Between Long Parallel Conductors

• A long current carrying conductor produces a magnetic field.

• Another long parallel current placed nearby will experience a force caused by its current and the magnetic field produced by the other wire.

• Two parallel conducting wires exert a force on each other.

F1-2


Magnetic Force Between Long Parallel Conductors

• The currents will attract each other if they are in the same direction.

• They will repel each other if they are in opposite directions.

F1-2

F1-2

ELECTROMAGNETIC INDUCTION: GENERATORS AND TRANSFORMERS

Section 5

Mini-experiment 5

• Connect the posts of the larger coil to the terminals of the meter with the provided wires. The meter is a sensitive current meter.

• Insert the bar magnet into the middle of the coil. Observe what happens to the meter when you let the magnet rest in the coil. Slowly move the magnet in and out of the coil, and observe the meter. Move the magnet in and out of the coil more rapidly, and observe the meter.

Magnetic Fields and Currents

• Just as currents produce magnetic fields, moving magnets can produce currents.

• Any change in the magnetic field passing through a loop can induce a voltage in the loop known as an emf.

• This voltage can cause a current to flow in the loop.

• The current is called an induced current.

Inducing a Current

• There are several ways to change the magnetic field passing through a loop.– Move the loop in or out of the magnetic field,

by moving either the loop or the field.


Inducing a Current

• There are several ways to change the magnetic field passing through a loop.– Change the size or orientation of the loop.

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Inducing a Current

• There are several ways to change the magnetic field passing through a loop.– Change the strength of the field (perhaps by

changing the current that is producing it).

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Faraday’s Law

• The emf produced by a change in the magnetic field going through a loop of area A is given by

A

B

θ

t

AB

)cos(

Lenz’s Law

• Lenz’s Law gives the direction of the induced current:– The magnetic field of

the induced current will be opposite the change in the applied magnetic field. A

B

θ

Generators

• A generator converts some other form of energy, usually kinetic energy, into electric energy.

• The KE can be produced many ways.

• The emf produced is continuously changing (AC).

N S N S

Turn here.

The magnetic field through this loop changes.

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Types of Current

• Direct current is usually produced from batteries.

• AC current is produced in a generator. 0 0.01 0.02 0.03 0.04 0.05 0.06

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Transformers

• A current flowing through the primary coil produces a magnetic field.

• If the current is AC, the magnetic field will alternate as well.

• The changing magnetic field produces an emf in the secondary coil.

Source: BillC on en.wikipedia (http://en.wikipedia.org/wiki/File:Transformer3d_col3.svg)

Transformers

• A transformer is used to change the voltage of AC current.

• The ratio of the number of turns in each coil equals the ratio of the voltages.

•

Source: BillC on en.wikipedia (http://en.wikipedia.org/wiki/File:Transformer3d_col3.svg)

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magnetism mark lesmeister dawson high school physics © 2013 mark lesmeister/pearland isd

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