Magnetism

Magnets and Magnetic Fields• Naturally occurring• Attract or repel each

other• Attract certain metals

(iron, cobalt, nickel)• Point in a given direction

relative to the Earth when allowed to hang freely

• North and South poles- there are NO monopole magnets!

• Opposite poles attract

Just as an electric charge is surrounded by an electric field, a moving electric charge is also surrounded by a

MAGNETIC FIELD.Similar to how a planet orbits the

sun and rotates on its own axis, an electron orbits the nucleus and spins on its own axis.

The MAGNETIC FIELD is due to both the orbital motion as the electron orbits the nucleus and the spin of each electron around its own axis

For elements like iron, cobalt, and nickel, all the electrons in those atoms spin in the same direction, instead of opposite directions, which would cancel the effect. Therefore, these atoms are naturally “magnetic”.

If the magnetic fields of large clusters of those atoms, called “domains” are aligned, the whole material is magnetized.

Not every piece of iron is a magnet. Only those pieces whose domains are all aligned!

B

Magnetic fields are created by

Magnetic materials ( those spinning electrons!)

A magnetic field, B, is represented by magnetic field lines that point

OUT of the north poleand

INTO the south poleN

S

Magnetic fields are also created by

Current-carrying wires (a lot of moving charges!)

The direction of the magnetic field, B, is determined using a “right-hand rule”. The magnetic field encircles the wire.

Magnetic fields encircle

Current-carrying wires

Some people are afraid to live near electric lines for fear of the magnetic fields around the wires. There is no definitive evidence of danger due to living near these magnetic fields.

• The Earth behaves like a giant bar magnet. This is due to the movement of Earth’s molten interior (lots of moving charges).

• The magnetic poles do NOT line up with the geographic poles.

• Under the North geographic pole lies a South magnetic pole!

• The direction of Earth’s magnetic field is not rigidly fixed and moves around from year to year.

• The poles have completely reversed numerous times (about every 100,000 years) according to geological evidence.

Magnets and Current-Carrying Wire

• A wire with current running through it produces a magnetic field around the wire.

• Electromagnets (or solenoids) are magnetic only when current flows.

A MOTOR is a machine that converts electricity into mechanical motion. Most motors use electromagnets to produce rotation. A motor has a rotor spinning with coils of current carrying wires passing magnets.

Michael Faraday is credited with building the first electric motor

A generator does the reverse of a motor: it converts mechanical motion into electricity!

Examples: a wind or water turbine, steam engine

Who is credited with making the first generator?

Yes, Michael Faraday!

B

Magnetic fields are created by

Magnetic materials ( those spinning electrons!)

A magnetic field, B, is represented by magnetic field lines that point

OUT of the north poleand

INTO the south poleN

S

Magnetic fields are also created by

Current-carrying wires (a lot of moving charges!)

The direction of the magnetic field, B, is determined using a “right-hand rule”. The magnetic field encircles the wire.

The “Flip-Side”

Currents in wires

produce

magnetic fields.

AND

Changing magnetic fields

produce

currents in wires….

Michael Faraday

Michael Faraday studied both electricity and its relationship to magnetism.

We remember the “Faraday cage” that protects its contents from static charges. The unit for capacitance is the Farad.

He also made the first electric motor and generator and transformer!

Faraday’s Law states that a changing magnetic field will “induce” a current in a wire.

Changing magnetic fields induce a current in wires: The metal detector

One application of Faraday’s law is the security metal detector at the airport. As a person passes through the detector, which is a large electromagnet, any metal on their person will change the magnetic field in the detector, inducing a current that signals the presence of metal!

Changing magnetic fields induce a

current in wires - Another application of

Faraday’s Law:The Electric Guitar

The “pickup” in an electric guitar contains a coil of wire wrapped around a magnet.

The vibration of the guitar string causes the magnet to vibrate.

The changing magnetic field induces a current in the coil of wire.

The current is sent to the amplifier to produce the sound.

Changing magnetic fields induce a current in wires: Electric Generators (and alternators)

All generators use rotation of an apparatus with wires and magnets. The rotation causes the magnetic fields to be constantly changing. Changing magnetic field induce current in the wires = electricity!

Magnetic Forces

* Current-carrying wires act like magnets.* Magnets exert forces on other magnets.Therefore…• Magnets exert forces on current-carrying wires.The magnitude of the magnetic force is given by

F = iL x BWhere B is the magnetic field, measured in Tesla, Ti is the current, measured in ampsL is the length of the wire, measured in meters.The current flowing through the wire and the magnetic field, B,

must be perpendicular to each other. Only PERPENDICULAR components contribute to the magnetic force! Yes, we’re back to vector components!

The Force on a Current-Carrying Wire in a Magnetic Field

The Direction of the Magnetic Force, F = iL x B

Force is a vector and always has a direction associated with it. The direction of the magnetic force is determined by a “right-hand rule”

1. Point your thumb in the direction the current is flowing.

2. Point your fingers in the direction of the magnetic field.

3. Your palm will face the direction of the magnetic force on the wire.

Speaker and Magnets

1. There is a magnet in the back of most speakers.

2. The wires going into the back of a speaker carry current from the amplifier.

3. The coil of wire experiences a magnetic force which pushes it back a forth against the paper cone.

4. The cone vibrates producing sound.

Magnetic Fields created by current carrying wires

If two parallel wires have currents traveling in opposite directions, the magnetic fields generated by those currents cause these wires to repel each other.

However, if two parallel wires have currents traveling in the same direction, the magnetic fields generated by those currents between the wires results in the wires attracting each other.

i, current

i, current

F, force

F, force

i, current

i, current

F, force

F, force

James Clerk Maxwell

Maxwell showed that electricity and magnetism were all part of the same phenomenon- the electromagnetic force! He wrote 4 very famous equations showing that relationship and describing electromagnetic waves

Maxwell’s four equations:

Don’t these look like fun?

The Force on a Moving Charge in a Magnetic Field

Just as current-carrying wires (lots of moving charges) experience a force in a magnetic field,

a single electrical charge also experiences a force if it is moving in a magnetic field.

e-

Magnetic force

The Force on a Moving Charge in a Magnetic Field

The magnitude of the force is given by

F = qv x B

Where B is the magnetic field in Tesla,

q is the electric charge in Coulombs

v is the velocity in meters per second

Only perpendicular components of v and B contribute to the magnetic force.

CRT’s and Magnets(TV and computer monitors- NOT flat

screen)1. Electrons are produced

at the cathode.

2. They move through an electromagnet.

3. The electromagnet pushes them in different direction.

4. The hit the phosphorescent screen, making it glow.

Particle AcceleratorsThis magnetic force is used to accelerate

particles in laboratories like CERN, in Switzerland, to study fundamental particles and their interactions in its Large Hadron Collider.

The magnetic force on a moving charge will be a centripetal force, causing the charge to move in a circular pathway.

Magnetic Force = centripetal Force

qv x B = mv2 / rThe direction of the Force is given by the

right-hand rule:

Thumb = v (velocity)

Fingers = B (magnetic field)

Palm = Force on a POSITIVE charge

Mass SpectrometerA mass spectrometer is a machine that can

determine the mass of a subatomic charged particle by measuring the radius of the circle it moves in while under the influence of a magnetic field.

qv x B = mv2 /r

How the Earth’s Magnetic Field protects us

There are storms on the surface of the sun that are known as solar flares or sun spots.

These storms eject electrons, ions, and atoms out through the corona toward space. This is called the “solar wind” or “cosmic radiation”.

Earth’s Magnetic Field

The Magnetosphere acts like a shield around the Earth to protect us from the solar wind.

http://player.discoveryeducation.com/index.cfm?guidAssetId=F27D04F3-021B-461E-8633-6C539412324C&blnFromSearch=1&productcode=US

When the solar wind is particularly intense, some of these charged particles penetrate the Earth’s Magnetosphere.

These charged particles gets trapped in Earth’s magnetic field lines, orbiting around and around the field lines and traveling from pole to pole. They can cause disruption of signal from satellites and can even cause the destruction of sensitive equipment inside satellites.

But… it also can cause the atmospheric molecules near the field lines to glow.

This causes the auroras:

Aurora Borealis, the Northern lights

Aurora Australis, the Southern lights.