Magnetism and ElectricityPhysics 6th Six Weeks

Hans Oersted

Michael Faraday

James Clerk Maxwell

Intro to Magnetism

• Magnets are materials that attract items made from iron, cobalt, and nickel

• Materials that are attracted to a magnet are called “ferrous materials” and those that are not are called “nonferrous materials”.

• Early humans discovered that there were naturally occurring magnetic ores (rich in iron) which the Ancient Greeks referred to as “lodestones”.

• The word “magnetism” comes from the fact that a large number of lodestones came from the Ancient Greek region of “Magnesia”.

A lodestone & iron nails

Magnetic Poles

• All magnets, no matter what shape they have, have two poles – commonly referred to as a North Pole and a South Pole.

• Even if a magnet is broken, the remaining pieces will have two poles.• Poles always exist in pairs – and are referred to as a magnetic dipole• You can never have a North without a South – there are no magnetic monopoles

• Opposite poles attract and like poles repel one another

Magnetic Fields

• The space around a magnet through which the invisible magnetic force is exerted is known as the Magnetic Field

• Magnetic fields are the result of moving electric charge.

• In all materials each spinning electron acts as a tiny magnet, yet in most materials the spin of the electrons of the atoms that make them up cancel each other out, making them largely not magnetic.

• In the atoms that make up iron, cobalt, and nickel however, several electrons happen to spin in the same direction and thus their magnetic fields build upon one another.

• Overall, the magnetic fields of these atoms build together, allowing these materials to be attracted or repulsed by magnetic fields.

The Earth’s Magnetic Field & Solar Weather

• The Sun is constantly emitting a stream of highly energetic particles (the “Solar Wind”) as well as highly energized, high frequency EM waves (“Cosmic Rays”).

• The Earth’s magnetic field shunts the Solar Wind and Cosmic Rays away from the surface.

• The strongest EM waves that make it through are UV A and UV B, whereas UV C, X-Rays, & Gamma Rays that are part of the Cosmic Rays bounce off into space.

• Planets like Mars that lack an active magnetic field are constantly bombarded with the dangerous aspects of the Sun’s output, and it is highly unlikely life could endure on their surfaces as a result.

• Furthermore, without a magnetic field, the atmosphere of the Earth would be stripped away. There is evidence that Mars had a much larger atmosphere, and recent data from probes has detected a comet like “tail” extending from Venus as its atmosphere (much thicker than our own) is being stripped away due to its extremely weak magnetic field.

Cosmic Wind and Auroras

Animation of the “Aurora Australis” as taken by the

NASA IMAGE satellite in 2005

• Solar Wind reacting with the Earth’s Magnetic Field (our “Magnetosphere”) produces aurora “storms” around the poles where the magnetic field is the strongest.

• Just like electric charged excites molecules in tube of Neon causing a Neon light to glow, the excited solar wind particles excite particles in the upper atmosphere causing them to glow. Oxygen and Nitrogen in our atmosphere act as the “Neon” in a “Neon light” and glow when energized electrons hit them – and pass back out again – weaker than before.

• In times of particularly intense solar wind activity, auroras may spring up with greater intensity.

Magnetism: Electron Spin

• Electrons are surrounded by an electric field• As electric charges move they produce a magnetic field• Electrons are little packets of negative charge (they are electric charges)• The movement of electrons in a material causes magnetism.• Electrons, like planets do around the Sun, both rotate or spin, and revolve around

the nucleus.• An experiment conducted by Albert Einstein and Dutch physicist Wander de Haas in

1915 led to the discovery that the spin of electrons was the most direct cause of magnetic charge. This leads each electron to act as a mini-magnet.

• In the majority of elements, the electrons spin in such a way that they cancel each out and that is why something like carbon (the “lead” in your pencil) isn’t magnetic.

• However, in 3 elements (which happen to be next to each other on the Periodic Table as Atomic Numbers 26 – 28 respectively) the spins are aligned so that the magnetic field of each of a few of their electrons add together. These are the only naturally magnetic elements Iron, Cobalt, and Nickel

Magnetic Domains• Clusters of magnetically aligned atoms (many

billions) form in magnetic substances. These clusters are called Magnetic Domains.

• When a magnetic material is in the presence of a nearby magnetic field, the domains line up and it becomes a Temporary Magnet. Another word for temporary magnetism is paramagnetism

• Very strong magnetic fields cause the domains in magnetic materials to line up permanently becoming a Permanent Magnet. Another word for permanent magnetism is ferromagnetism

• Permanent Magnets can lose their magnetic behavior if they are heated or receive a significant impact force (such as from a hammer or explosion)

• Temporary magnets usually stop behaving like a magnet when they are removed from the magnetic field

Magnetic Domains, continued

• The temperature at which a magnetic material loses its magnetism (that is when it is heated to the point that the domains jumble up again) is known as the Curie Temperature.

• The temperature is named for the French physicist Pierre Curie who discovered it.

• At the Curie Temperature, the movement of particles competes with the any alignment of magnetic charges to the point where alignment is lost.

• Each magnetic material has a different Curie Temperature

Magnetic domains below the Curie Temperature

Magnetic domains above the Curie Temperature

Magnetic Domains used to store information on a Hard

Drive

Recording of single magnetizations of bitson a 200 MB HDD-platter

Diagram labeling the major components of a computer HDD

Close up of HDD Head on platter(note mirror image on platter surface)

How are Magnetic Fields Produced?: Magnetism, Electric Charge, and EM Waves

• Therefore, as electric current flows through a wire, it creates a magnetic field around it

• Recall that a vibrating electric charge (an electron) makes electromagnetic waves.

• All forms of light are electromagnetic waves

• As an electric field moves, it creates a magnetic field and that magnetic field produces an electric field and so on and so on in a rippling braid.

Magnetism Basics

Hans Oersted

• The Danish scientist Hans Oersted discovered in the early 1820’s that magnetism and electricity were related in a now famous experiment.

• In the experiment, Oersted found that when a compass was placed near an electrified wire the needle was deflected by the electric field in the nearby wire.

• When the current was reversed, the needle was deflected in the opposite direction. When the electric flow was stopped the needle returned to normal.

• Oersted knew that electricity must produce magnetism, but he was unsure as to how exactly that occurred at that time.

• Oersted was the first to find experimental evidence of the link between magnetism and electricity

• Before Oersted, electricity and magnetism were considered to be

unrelated.

Hans Oersted…continued

• After Oersted’s experiments, it was evident that electric current could produce magnetism, many than wondered if the opposite were true, if magnetism could create electricity.

• The magnetic field around a current-carrying wire creates a circular pattern around the wire and the direction depends upon the flow of the current. The strength of the magnetic field depends upon the strength of the electric current.

Dr. Hewitt on Hans Oersted’s Experiment

Michael Faraday

• Born in 1791 to a poor blacksmith’s family & never attended much in the way of formal schooling past a rudimentary education.

• Apprenticed with a book binder.

• Was given tickets to see Sir Humphry Davy give a lecture by a customer of the book binder. Faraday later bound his notes and gave them to Davy as a gift.

• Was taken on as a helper of scientist, Sir Humphry Davy (who discovered Calcium, Chlorine, Sodium, & Potassium and proved diamond was made of carbon)

• Faraday became head of the “Royal Institute” the prestigious scientific academy in Great Britain following Sir Humphry Davy’s death in 1829

• Faraday is famous for establishing a link between electricity & chemical reactions, for inventing both the electric motor & electric generator, for discovering the link between electricity and light, and being the first to make an electric current from a magnetic field.

Sir Humphry DavyMichael Faraday

Michael Faraday: Part 1 (From “Einstein’s Big Idea”)

Michael Faraday: Part 2 (from “Einstein’s Big Idea”)

Faraday’s Principle of Electromagnetic Induction

• In the early 1831 Michael Faraday found that by moving a loop of conductive wire through a magnetic field, an electric current was caused to flow through the wire.

• Also he discovered that by moving a magnet through a coil of conductive wire, electric current was produced.

• Faraday was the first to produce an electric current from a magnetic field

• Principle of Electromagnetic Induction – Electric current may be produced by moving a loop of conductive wire through a magnetic field, or by moving a magnet through a conductive wire loop.

• Faraday’s Law – The induced voltage in a coil is proportional to the number of loops and the rate at which the magnetic field changes within those loops.

Using Electromagnetic Induction to Generate Electricity

• Generator – device that produces electricity by rotating a coil of wire in a magnetic field.

• The coil is rotated by an outside source of mechanical energy.

• Different methods are employed to create the mechanical energy such as falling water, wind, steam turbines, etc.

• In the large generators used in most power plants, an enormous magnet rotates around a coil of wire.

• Before the invention of the generator, the only way to produce electric current was by dissolving expensive metals in acid in voltaic cells (batteries)

Producing AC Current

• As the wire coil rotates through the magnetic field of the permanent magnet in the generator, current flows through the coil.

• As the coil rotates the current switches direction of flow (thus AC current is produced).

• The current periodically changes directions as the electromagnetic coil continuously changes turns.

The Principle behind the first electric generator (from “Einstein’s Big Idea”)

Generator vs. Electric Motor• Electric Motor – device that changes

electrical energy into mechanical energy. • Electric motors are essentially generators in

reverse. Invented by Michael Faraday

Michael Faraday’s 1st Electric Motor

• On 4th September 1821, Michael Faraday discovered that a vertically mounted wire carrying an electric current would rotate continuously round a magnet sticking out of a bowl of mercury. He named this phenomenon electro-magnetic rotations.

• Faraday was investigating electro-magnetism, which describes the relationship between electricity and magnetism. Hans Christian Oersted had shown that a magnetic compass needle could be caused to move by a wire carrying an electric current. Faraday showed that it was possible to produce continuous motion from the interaction of electricity and magnetism.

Making “Faraday’s Motor”

Electromagnets

• Definition – a temporary magnet made by placing a piece of magnetic material inside of a current-carrying coil of wire.

• The more loops, the more magnetic force

• The larger the core the more force

• More current = more force

• An electromagnet has a north and south pole like any other magnet

• The domains in the magnetic material within the electromagnet only behave like a magnet when the electric current is flowing.

Faraday and Maxwell: Electricity and the Speed of Light

James Clerk Maxwell • Faraday had already championed the idea that electricity & magnetism are connected – however he lacked the math to back it up at the time.

• In 1846, Faraday had shown experimentally, that an electrified wire could bend a light wave.

• Scottish scientist, James Clerk Maxwell, through experimentation came up with the theory – inspired by Faraday – that electromagnetic energy (electromagnetism) spreads out as waves, at the speed of light.

• The idea was further supported by “Maxwell’s Equations” which modelled how magnetism and electricity become one another at the speed of light

• By the way…in 1861, Maxwell also produced the first color photograph

“We have strong reason to conclude that light itself—including radiant heat and other radiation, if any—is an electromagnetic disturbance in the form of waves propagated through the electromagnetic field according to electromagnetic laws.”—James C. Maxwell

Control Light with Magnets! The Faraday Effect

Maxwell’s Equations…don’t worry it is far too early in your physics career to try and

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