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Magnetism & Electromagnetic Induction. What causes magnetism?. Electrons in motion create a magnetic field. Electrons move in current, and also within individual atoms ALL magnetism comes originally from the movement of charged particles. Magnets. Magnets have “polarity”
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What causes magnetism? • Electrons in motion create a magnetic field. • Electrons move in current, and also within individual atoms • ALL magnetism comes originally from the movement of charged particles.
Magnets • Magnets have “polarity” • Always two poles: north and south • Comes from magnetic domains • Like poles repel and unlike poles attract. • There are two types of magnets: • Permanent magnets – domains are always aligned • Temporary magnets – formed when iron-bearing metals are brought near a magnetic field and their domains temporarily align
The Magnet Man… Nikola Tesla • Inventor and electrical engineer • Widely famous for his work on AC circuits • Pioneered wireless energy transfer to power electrical devices as early as 1873 • His unfinished “Wardenclyffe Tower” Project was meant to supply industrial power wirelessly on an international basis • Fought with Edison for years, over AC vs. DC • Tesla vs. Edison
Magnetic Fields Match the diagrams with the magnetic fields below.
Magnetic field - the space around a magnet over which the magnetic force exists • The more field lines, the stronger the magnetic field • Field lines ALWAYS point from north to south • Magnetic Field strength is measured in Teslas (T)
Direction of Magnetic Field • The direction is the same as where the north pole of a compass points when it is placed in the magnetic field. • So, field lines come out of the magnet at its north pole and enters at its south pole. • Magnetic field lines are most concentrated at the poles (the field is stronger at the poles) • Remember… opposites attract, likes repel!
Electromagnets • Electromagnets – devices that use electric current to produce a concentrated magnetic field • Iron in a coil of current carrying wire will become an electromagnet. • Temporary magnets • Some uses: telephone relays, levitation, magnetic cranes, sorting metal, MRI
Electromagnetism • Christian Oersted discovered that electric current in a wire creates a magnetic field. • Current, moving in a straight wire, produces a magnetic field surrounding the wire. • We can see this with a compass... • But, since the field goes around the wire, how do we know which way it goes?
The Right Hand Rules (RHR) • Rules to help you determine magnetic field direction, using your RIGHT hand. • First RHR – current in a straight wire • Grasp the wire with your right hand. • Thumb is in the direction of the positive current. • Fingers circle the wire and point in the direction of the magnetic field (circular).
We Use Two RHRs:(there are actually three) • First RHR: current in a straight wire • Grasp the wire with your right hand. • Point thumb in the direction of the current • Fingers circle the wire and point in the direction of the magnetic field (circular) • Second RHR: current in a coiled wire • Grasp the coil with your right hand. • Curl fingers around the loops of wire in the direction of the current • Thumb points toward the NORTH pole of the electromagnet
1st RHR Note: current traveling in opposite directions in a wire, will have magnetic fields in opposite directions
More on Electromagnetism • Loops of wire produce stronger electromagnets than straight wire • The more loops of wire, the stronger the field • Thicker wire produces a stronger magnet
Using the RHRs • Use right hand rules to draw the magnetic field lines. • The “dot” means current is coming out of the page • The “x” means current is going into the page
Electromagnetic Induction • Faraday’s Discovery: • Faraday found he could induce current by moving a wire in a magnetic field. • If the wire moves up through the field the current moves in one direction. • If the wire moves down through the field the current moves in the opposite direction. • If the wire moves parallel to the field lines no current is produced. • Electromagnetic Induction: the process of inducing current in a wire due to the relative motion of the wire and/or the magnetic field
Electromagnetic Induction Observing Faraday’s Experiment: Faraday’s Experiment 1 Faraday’s Experiement 2
Electric GeneratorsConvert mechanical energy to electrical energy • How it works: • Mechanical energy turns an armature • armature: wire wrapped around an iron core in a magnetic field • The induced voltage causes current to flow. • A generator at work
Electric Generator • The strength & direction of induced current changes as the armature rotates • Induced current is maximum when the wire loops move through the strongest areas of the magnetic field
Alternating Current (AC) Generators • The direction of the current switches back and forth at some frequency • The faster the coil turns, the faster the directions changes, so the higher the frequency. • Let’s Look
Is it AC or DC? • Wall plugs provide AC, batteries provide DC • Electricity transmission systems (grids) are AC, but operate on different frequencies • Five in the U.S. alone • Why you can’t plug your stuff directly into the wall in Europe • High-Voltage DC can connect AC systems running on different frequencies
Motor vs. Generator • What’s the difference? • Motor: Electrical to Mechanical Energy • Generator: Mechanical to Electrical Energy • Can sometimes be used interchangeably, like in a vehicle • Motor when accelerating – battery provides energy to run motor • Generator when braking – inertia used to recharge battery
Transformers • Used to change AC voltage • Very efficient (little energy lost to heat) • How they work: • two coupled coils are attached using a metal bar (usually a ring or “U”) • the coils have different numbers of loops • One is the primary coil • The other is the secondary coil
Types of Transformers:Step-Up or Step-Down • Step-Up: • More turns (loops) in the SECONDARY coil • The secondary voltage is greater than the primary voltage • Step-Down: • More turns (loops) in the PRIMARY coil • The primary voltage is greater than the secondary voltage