Bell Ringer Give an example of something that you observed over break which involved physics. Be sure to thoroughly explain your answer.

Magnetism

Force of attraction or repulsion cause by the movement of charged participles –It is not charge –It is caused by the movement of charge

Magnetic Poles Magnetic Poles: A region on a magnet which produces magnetic forces The poles of a suspended magnet will align themselves to the poles of the Earth Fundamental Rule: Like poles repel; opposite poles attract

Magnetic Poles –Electric charges can be isolated –Magnetic poles cannot Magnetic poles behave similarly to electric charges EXCEPT:

Magnetic Fields Magnetic Field: The space around a magnet in which a magnetic force is exerted –The shape of a magnetic field is revealed by magnetic field lines Directed away from north poles and toward south poles The strength of magnetic fields are measured in units of Tesla (T)

Magnetic Fields

Magnetic fields are caused by the motion of electric charges –Since motion is relative, so are magnetic fields Magnets at rest consist of charges in motion –Every spinning electron is a tiny magnet –Electrons spinning in the same direction produce a stronger magnet

Magnetic Domains Magnetic Domain: Clusters of aligned atoms

Magnetic Domains

Bell Ringer 1.) Draw the magnetic field lines for a bar magnet with its North Pole on the bottom and its south pole on top. 2.) How are the domains arranged in a permanent magnet?

Magnetic Domains If the magnetic domains of a material can align, this material is called ferromagnetic.

Magnetic Domains Permanent magnets are made by placing materials in strong magnetic fields –Aligns the domains It is possible to weaken a magnet by dropping or heating it –Pushes the domains out of alignment

Earth’s Magnetic Field Currents in the molten part of Earth beneath the crust create the Earth’s magnetic field –Moving charges loop within the Earth Earth’s magnetic field is not stable –Magnetic pole and geographic pole are offset

Earth’s Magnetic Field Magnetic poles are located: –North: Hudson Bay region of northern Canada (1800 km from geographic pole) –South: Just south of Australia Magnetic Declination: Difference between geographic and magnetic poles

Bell Ringer 5/13 From what was discussed in yesterday’s class…what do you believe the true name of our school should be?

Earth’s Magnetic Field

The magnetic field of Earth reverses on an irregular cycle 1 million years ago, the field completely “shut off” for 10,000-20,000 years Its strength has decreased by 5% over the last 100 years –Indicates a reversal possibly in the next 200 years

Earth’s Magnetic Field

Courtesy of NASA, JPL and GSFC

Electric Current Recall: Electric current is the movement of electric charges –Current produces a magnetic field When current is carried through a conductor, the magnetic field produces a pattern of concentric circles

Electric Current This concept is known as Oersted’s Principle If the current switches direction, the compass needles will turn 180 o

Electromagnets If a current carrying wire is bent into a loop: –Field lines become bunched inside the loop Two overlapping loops: –Twice the concentration of magnetic field lines

Electromagnets

Electromagnet: A current-carrying coil of wire with many loops –The magnetic domains of a piece of iron placed in the coil will be aligned –The will intensify the magnetic field –The strongest electromagnets use superconductors as a core

Electromagnet Applications An alarm bell uses an electromagnet with a changing magnetic field to move a plunger

Junkyardpickerupper

Bell Ringer How does a speaker work?

Moving Particles A moving charged particle could be deflected by a magnetic field For particles moving parallel to the magnetic field, no deflection occurs For particles moving perpendicular to the magnetic field, maximum deflection occurs

Moving Particles

A current carrying wire will also be deflected when it encounters a magnetic field –The particles trapped inside the wire are deflected, therefore pushing the wire itself If the direction of the current is reversed, the deflecting force acts in the opposite direction

Moving Particles

Applications The follow items use the idea TVs MRIs Aurora Borealis

Bell Ringer When will a charged particle experience the maximum amount of force when entering a magnetic field?

Motors A simple DC motor uses a permanent magnet to establish a field where an electromagnet is made to rotate –When connected to a battery, the electro- magnet will rotate one half turn –Its south pole will align with the north pole of the permanent magnet

Motors

Brushes Commutator These two devices change the direction of the current at exactly the correct moment so that the poles of the electromagnet reverse This causes the electromagnet to keep spinning

Motors As the armature passes through the horizontal position, the poles of the electromagnet reverse due to the commutator

Bell Ringer Where the motors that you made last week operating on alternating or direct current? What would happen with your motor from yesterday if you took off all the insulation off of each end?

Meters Galvanometer: A sensitive instrument used to detect electric current –Consists of a magnetic needle centered under loops of insulated wire –Electric current will cause the needle to pivot, detecting even very small currents –May be calibrated to measure current (ammeter) or voltage (voltmeter)

Meters