Magnetism

Magnetism

Natural Attraction without pheromones

History of Magnets More than 2000 years ago, rocks called

lodestones were found in the region of Magnesia in Greece.

In the 12th century, the Chinese used them for navigating ships.

What are magnets?

Most materials Have paired up electrons moving in opposite

directions. The field created by one moving charge is

canceled by the other. No magnetic field is created.

What are magnets? Any charges in motion

produce a magnetic field.

Some materials like Iron, Nickel, or Cobalt Have a single electron or paired electrons

spinning in the same directions. The magnetic field created by one electron is not

canceled by the other. An atomic sized magnet is created.

Why is Fe magnetic and Al not? What makes a good magnet?

Every spinning electron is a tiny magnet. A pair of electrons spinning in the same direction is a

stronger magnet. A pair of electrons spinning in opposite directions work

against one another; the magnetic fields cancel. Fe has 4 unpaired electrons spinning in the same direction.

Cobalt has 3. Nickel has 2. Aluminum has one unpaired electron.

Temporary Magnets What happens when you place a magnet

next to a nail? This is because the magnet causes the nail

to become polarized; the nail becomes a magnet.

This is temporary; if you pull the magnet away, the nail loses its magnetism.

Permanent Magnets Permanent magnets are produced in the same manner as

the nail; however, due to the microscopic structure of the material, the magnetism becomes more permanent.

Most permanent magnets are made of ALNICO, an iron alloy containing 8% Aluminum, 14% Nickel, and 3% Cobalt.

Some rare earth elements, such as neodymium and gadolinium, produce strong permanent magnets.

Magnetic Domains In magnetic materials, neighboring atoms pair up

to form large groups of atoms whose net spins are aligned.

These groups are called domains. When a piece of iron is not a magnet, the

domains point in random directions.

Magnetic Domains If the non-magnetized iron is placed in a strong

magnetic field, the domains will line up in the direction of the field.

In temporary magnets, the domains will return to their random orientation after the field is removed.

In permanent magnets, the domains will remain aligned.

1 domain = 1 quadrillion (1015) atoms

Magnetic Poles

All magnets have two regions “poles” that produce magnetic forces.

They are named like this because if you take a magnet and suspend it from the middle (so that it can swing freely), it will rotate until the north pole of the magnet points north and the south pole points south.

Like poles repel. Opposite poles attract.

No less than two. North and South

cannot be separated. If a magnet is broken,

poles aren’t separated; two smaller magnets are formed.

Magnetic Fields

You have probably noticed that forces between the magnets (both attraction and repulsion), are felt not only when the magnets are touching each other, but also when they are held apart.

In the same way that gravity can be described by a gravitational field, magnetic forces can be described by the magnetic fields around magnets.

Magnetic Fields Force lines around a magnet Always flow from N to S Circular in path

Magnetic Field Demo What kinds of magnetic fields are produced

by pairs of bar magnets?

MagneticField Lines

The shape of the magnetic field is

revealed by magnetic field

lines.

Magnetic Field Lines Magnetic field lines are the same as

electric field lines in that both are stronger when lines are drawn closer together. So the magnetic field is stronger at the poles

The magnetic field lines have arrows going from north to south.

Magnetic field lines do not cross because the magnetic field cannot go in two directions at once.

Common Uses of Magnets Magnetic recording media: VHS tapes, audio cassettes, floppy disks, hard disks. Credit, debit, and ATM cards Common television and computer monitors Speakers and Microphones Electric motors and generators Compasses Magnets can pick up magnetic items (iron nails, staples, tacks, paper clips) that are

either too small, too hard to reach, or too thin for fingers to hold. Some screwdrivers are magnetized for this purpose.

Magnets can be used in scrap and salvage operations to separate magnetic metals (iron, steel, and nickel) from non-magnetic metals (aluminum, non-ferrous alloys, etc.).

Magnetic levitation transport, or maglev, is a form of transportation that suspends, guides and propels vehicles (especially trains). The maximum recorded speed of a maglev train is 361 mph.

How to demagnetize a magnet Heating a magnet past its Curie temperature -

the molecular motion destroys the alignment of the magnetic domains. 768°C for Iron

Hammering or jarring – the mechanical disturbance tends to randomize the magnetic domains.

Placing the magnet in an alternating magnetic field.

Ferrofluid

a mixture of tiny iron particles covered with a liquid coating that are then added to water or oil.

Used in car suspensions, cancer detection, loud speakers

video

Earth’s Magnetic Field Earth is a huge

magnet. This is possibly due

to the molten Iron core.

The magnetic field around Earth is called the Magnetosphere

Earth’s Magnetic Field Magnetic north pole is different than geographical north pole. There is about a 25 ̊difference from geographic north pole to

magnetic north pole, this is called magnetic declination In addition, the north pole

of a magnet is attracted to earth’s north pole because that is the magnetic south pole.

The south pole of a magnet is attracted to the earth’s south pole because that is the magnetic north pole.

Magnetosphere

Extends several tens of thousands of km into space.

Protects Earth from solar winds.

Dynamo Theory

The dynamo theory proposes a mechanism by which a celestial body such as the Earth generates a magnetic field.

In the case of the Earth, the magnetic field is induced and constantly maintained by the convection of liquid iron in the outer core.

Magnetic field of Earth is not stable The magnetic poles of Earth wander up to 15 km

every year. Based upon the study of lava flows throughout the

world, Earth's magnetic field reverses at intervals, ranging from tens of thousands to many millions of years, with an average interval of approximately 250,000 years.

The last reversal is theorized to have occurred 780,000 years ago.

After a being put into a strong magnetic field a temporary magnet has the following configuration. If the field if turned off, which pole is the north pole of the magnet?

A. Left poleB. Right poleC. middleD. No poles

Auroras

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

This occurs near the magnetic poles. These charged particles collide with electrons of the atoms in

our atmosphere and transfer their energy. The colors of the lights are determined by the type of gases in

the atmosphere. O2 releases green light; N2 releases red light

aurora borealis (northern lights); aurora australis (southern lights)

Animal Migration

Some animal species do have the ability to detect the magnetic field, & they use it to make their migrations.

Bats and sea turtles use magnetic information to find their way.

We're not 100 percent sure how animals detect the magnetic field, but small particles of magnetite have been found in the brains of some species. Those particles may be reacting to the magnetic field and activating nerves in such a way as to send orientation information to the animal's brain.

Bacteria & Magnets Some bacteria have a chain of magnetite as part of their

internal structure They use this magnetite to find their way in swamps Bacteria in the northern hemisphere have magnetite that

are opposite in polarity than the bacteria with magnetite in the southern hemisphere.

Animal Migration Video

Below show the domains of a magnet, which pole is the north pole of the magnet?

A. Left poleB. Right poleC. middleD. No poles

Magnetic field Units

We can determine the magnetic field by measuring the force on a moving charge:

The SI unit of magnetic field is the Tesla (T).Dimensional analysis: 1 T = 1 N·s / (C·m) = 1 V ·s / m2

Sometimes we use a unit called a Gauss (G):1 T = 104 G

The earth’s magnetic field is about 0.5 G

BF

qv

sin v

B

Understanding the magnetic force requires usto work in THREE dimensions.

So let’s use a new notation to depictthe forces in the TWO dimensional world.

x x x x xx x x x xx x x x xx x x x xB into the page

. . . . .

. . . . .

. . . . .

. . . . .B out of the page

What should you use-- dots or x’s?

The tail ofan arrow.

x

The x’s

The tip ofan arrow.

.The dots

Magnetic Forces1. Forces on moving charges

2. Forces on currents in wires or fluids

• A charged particle in a static (not changing with time) magnetic field will experience a magnetic force only if the particle is moving.

• q is charge,v is velocity, B is the magnetic field B, angle , F is the magnitude of the force on the charge is:

|F| = | q v B sin | = q vBOr

B = F magnetic qv

F B v B= =q qv sin

This force acts in the direction perpendicularto the plane defined by the vectors v and Bas indicated by the right-hand rule!

WARNING: cross-product(NOT simple multiplication!)

DON’T FORGET:Forceshave

directions!

A particle with a charge of 8 C is moving at 2,000 m/s when it enters a magnetic field perpendicular to its direction of motion that has a magnitude of 4 Tesla. What is the magnitude of the force exerted

on the particle?

A. 64,000 NB. 1000 NC. 62.5 ND. 4000 N

Right Hand Rule• Draw vectors v and B

with their tails at the location of the charge q.

• Point fingers of right hand along velocity vector v.

• Curl fingers towards Magnetic field vector B.

• Thumb points in direction of magnetic force F on q, perpendicular to both v and B.

BvqF

=

A current-carrying wire is placed in a magnetic field, as shown below. Indicate the direction of the force

exerted on the wire.

A. upB. downC. rightD. left

Force on parallel wires

• Each of two parallel wires with current I, experiences an attractive magnetic force that diminishes as one over the distance separating the wires: F I1 I2 L / b.– L = length – We use this proportionality to define the unit of current

• The force on wire 2 is equal to current in wire 2 times magnetic field from wire 1 times length of wire 2.– Magnetic field generated by a current diminishes as one

over distance from wire (1/d)

bI1

I2

Magnetic Field from a Wire

• The magnetic field lines from a current form circles around a straight wire with the direction given by another “right hand rule” (thumb in direction of current, finger curl around current indicating direction of magnetic field).

1Icurrent fromr distance a field-B

1

=

=Il

FB magneticI1

Br

I2

The magnetic force of a straight 1.5 m segment of wire carrying a current of 9 A is 3.0 N. What is the magnitude of the component of the magnetic field

that is perpendicular to the wire?

A. 40.5 TB. 2 TC. .22 TD. 4.5 T

Example A loudspeaker is an electromechanical device

which converts an electrical signal into sound.

Loudspeakers are used in numerous applications

from hearing aids to air raid sirens Loudspeakers are the most variable elements in

any audio system, and may be responsible for marked audible differences between otherwise identical systems.

Microphone Audio Circuit Speaker

The Main Components

Magnetic Forces due to Electric Current• Current is charges in motion• Causes force on magnet• Example: Compass near wire with current

wire

current

Side View Top View

B Fields of Current Distributions

By winding wires in various geometries, we can produce different magnetic fields.

For example, a current loop ( to plane, radius r, current emerging from plane at top of loop):

I

Magnetic field at center of loopB = m0 I / (2r)

Magnetic field far from loop:B I·(Area of loop) / r3

Solenoids

If we stack several current loops together we end up with a solenoid:

In the limit of a very long solenoid, the magnetic field inside is very uniform:

B=m0 n In = number of windings per unit length,I = current in windings

B 0 outside windings

Galvanometer

• Current in coil is finite, due to non-zero resistance of coil

• Magnetic field produces torque on current in coil.

• Needle swings until magnetic torque is balanced by torsion of spring

A current-carrying wire is placed in a magnetic field, as in the diagram below. Find the

direction of the Force applied to the wire.

A. upB. downC. rightD. Out of the page

A current-carrying wire is placed in a magnetic field, as in the diagram below. Find the

direction of the Force applied to the wire.

A. upB. downC. rightD. Out of the page

As current goes through a wire below, it produces the magnetic field shown below.

The red arrows show the direction a compass point around the wire. Which

direction is the current flowing?A. UpB. DownC. No currentD. Not enough info

wire

Side View Top View

Electromagnets Use current to “make” a

magnet Usually add a piece of iron to

increase the field Higher the current and higher

the number of loops increases the strength of the magnet

Electric motor A magnet and an electromagnet are set up

to rotate around an axle. Poles attract – electricity is converted into

mechanical energy

Induction Just as a moving charge can create a

magnetic field, a moving B can create a moving charge – current

Induced EMF and Current The changing magnetic field causes an

induced current. Since a source of emf (voltage) is needed to produce a current (remember Ohm’s Law), the moving magnet acts like a source of emf. So we would say the moving magnet induces an emf in the coil, producing an induced current.

Magnetic Flux Magnetic Flux deals with the magnetic

field and the surface through which it passes. It is a measure of the quantity of magnetism that goes through a surface.

Magnetic Flux The flux only depends on the portion of the

magnetic field that is perpendicular to the area in which is passes through. So when the magnetic field direction is not perpendicular to the area the equation for flux becomes

Φ = B A cos θThe SI unit of magnetic flux is the Weber (Wb)

What is the magnetic flux of a 0.230 T uniform magnetic field through a rectangular area of 0.5x0.2 meters that is at a 45 degree angel?

A. .023 WbB. .016 WbC. .099 WbD. .039 Wb

EMF and FluxSo, an EMF [ε] is created due to the rate change of flux [Φ]. How can the flux change? The coil can spin (Δθ) The coil can change area (ΔA) The coil can move out of the field The magnetic field can change strength (ΔB)Since flux depends on B, A and θ, that means that any of them can be changing to produced an induced emf ε.

Faraday’s Law of Induction

t = B

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

The SI unit for EMF is Volts (V)EMF, potential difference and voltage are the same things. They all produce current in a closed circuit.

A single loop of wire with an area of 0.09 m2 enters a magnetic field of 0.186 T in a time of 0.09 seconds perpendicularly. What emf is induced in the loop?

A. .01674 VB. .00150 VC. .186 VD. Huh?

Electric Generator Opposite of electric

motor Move wire through a B

field and generate electricity

Converts mechanical energy into electric energy

Transformer Allows us to increase current or voltage at

the expense of the other. When we need high voltage we step it up. When we need high current we step it

down. Using current from B field

Transformer Formula

2

1

2

1

VV

NN

=

N1 = Number of turns in the primary coil

N2 = Number of turns in the secondary coil

V1 = Voltage in the primary coil

V2 = Voltage in the secondary coil

A step-up transformer has a primary voltage of 12-Volts with 2 loops around the primary terminal. If the secondary terminal has 500 loops, what voltage is produced by the transformer?

A. .048 VB. 3000 VC. 83 VD. 21 V

A loop of wire that has a cross-sectional area of 0.75m2 is brought completely into a magnetic field (B=1.1 T) in a time of 0.1 seconds. What is the induced emf?

A. .825 VB. .0825 VC. 8.25 VD. .147 V