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Electromagnetism

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by

Elizabeth Moreno

on 27 February 2013

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Transcript of Electromagnetism

How are electricity and magnetism related? Electromagnetism An electric current produces a magnetic field...
this relationship is called electromagnetism. How can we observe electromagnetism? When ever there is electricity, there is magnetism. Electromagnets are magnets that are created when there is electric current flowing in a wire. Electric current and magnetism Right hand rule To find the north pole of an electromagnet, use the right hand rule.

When the fingers of your right hand curl in the direction of the wire, your thumb points toward the magnet’s north pole. Solenoids A solenoid is a coil of wire with a current.

How can you change the strength?

By looping, or winding, the wire around the magnet!

The strength will increase as the number of loops or coils increases.


A coil takes advantage of these two techniques (bundling wires and making bundled wires into coils) for increasing field strength. Magnetic forces and electric currents Two wires carrying electric current exert force on each other, just like two magnets.

The forces can be attractive or repulsive depending on the direction of current in both wires.

Current in the SAME direction, wire are attracted to each other.

Current in the OPPOSITE direction, wires repel to each other. Things to note Basically, an electromagnet is a strong magnet that can be turned on or off.

You can increase the strength of an electromagnetic field…

Increase the current in the solenoid

Add more loops of wire to the solenoid

Wind the coils of the solenoid closer together

Using a stronger ferromagnetic material for the core The simplest electromagnet uses a coil of wire wrapped around some iron. In 1819, Hans Christian Oersted, a Danish physicist and chemist, and a professor.

He placed a compass needle near a wire through which he could make electric current flow.

When the switch was closed, the compass needle moved just as if the wire were a magnet. Electric current is made of moving charges (electrons), which creates the magnetic field around a current-carrying wire

Magnetism is created by these moving charges. The magnetic fields of straight wire Near a straight wire, the north pole of a compass needle feels a force in the direction of the field lines.

The south pole feels a force in the opposite direction.
As a result, the needle twists to align its north-south axis along the circular field lines. Common Electromagnets Speakers
Audiotapes
Videotapes
Computer hard drives
Credit cards
MRI
Radios
Refrigerators
Electromagnet Crane By changing the amount of current, you can easily change the strength of an electromagnet or even turn its magnetism on and off. Similarities in permanent and electromagnets The charged electrons in atoms behave like small loops of current.

Electric current through loops of wire creates an electromagnet.

Atomic-scale electric currents create a permanent magnet. Nonmagnetic materials Magnetic Materials Lets compare nonmagnetic vs magnetic Electromagnetic Induction If you move a magnet near a coil of wire, a current will be produced.

Electromagnetic induction occurs because a moving magnet induces electric current to flow.

Moving electric charge creates magnetism and conversely, changing magnetic fields also can cause electric charge to move.

Changing magnetic fields is what creates current.
If the magnet is stationary, the current is zero.
If the magnet is moving IN, the current is in one direction.
If the magnet is moving OUT, the current is in the opposite direction. Magnets Similar to electric and gravitational fields
Magnetic fields are produced by the MOTION of electric charges Revolving (spinning) electron produces field

In most atoms, combination of spins and orbits are in opposite directions

The fields cancel each other out
Some atoms, like iron, nickel, cobalt, aluminum don’t cancel

In iron, for example, field by an atom is so strong it causes neighboring atoms to arrange their spins and orbits to line up
These clusters form magnetic domains

Domains are randomly oriented in ordinary iron, nickel, etc.
Can force them to line up with external field Magnetic Fields Earth is a great big magnet.
Electrons are tiny ones.

Both of them have a north and a south pole
Both spin around an axis.

This spinning results in a very tiny but extremely significant magnetic field.

Every electron has one of two possible orientations for its axis. Web Demo: http://www.magnet.fsu.edu/education/tutorials/java/domains/index.html Web Demo: http://www.magnet.fsu.edu/education/tutorials/java/oersted/index.html Web Demo: http://micro.magnet.fsu.edu/electromag/java/compass/index.html Web Demo: http://www.magnet.fsu.edu/education/tutorials/java/solenoidfield/index.html Web Demo: http://www.magnet.fsu.edu/education/tutorials/java/magwire/index.html Lenz's Law In 1835 Heinrich Lenz put a definite direction to induced currents when the field was changing.

These opposing fields occupying the same space at the same time and result in a pair of forces. Web Demo: http://regentsprep.org/Regents/physics/phys08/clenslaw/default.htm Faraday's Experiment In 1831, Michael Faraday made his discovery of electromagnetic induction with an experiment using two coils of wire wound around opposite sides of a ring of soft iron

Faraday discovered that whenever the number of magnetic lines of forces in a circuit changes, a EMF (electromagnetic force) is known as electromagnetic induction.

If the circuit is closed one then a current flows through it which is known an induced current. This induced EMF and current lasts only for the time while magnetic flux is changing.

Faraday discovered that changes in a magnetic field could induce an electromotive force and current in a nearby circuit.

The generation of an electromotive force (EMF) and current by a changing magnetic field is called electromagnetic induction. Web Demo: http://micro.magnet.fsu.edu/electromag/java/faraday2/ Now draw the same picture with the current going in the opposite direction Draw this picture Web Demo: http://www.launc.tased.edu.au/online/sciences/physics/Lenz%27s.html An electromagnetic field interacting with a conductor will generate electrical current that induces a counter magnetic field that opposes the magnetic field generating the current. Can you name some electromagnets? Web Demo: http://www.pbs.org/wgbh/amex/edison/sfeature/acdc.html
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