Bell Ringer • Give an example of something that you observed over break which involved physics. • Be sure to thoroughly explain your answer. Magnetism 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 • Magnetic poles behave similarly to electric charges EXCEPT: – Electric charges can be isolated – Magnetic poles cannot 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 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 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 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 180o 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 Electromagnets 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 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 electromagnet will rotate one half turn – Its south pole will align with the north pole of the permanent magnet Motors 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