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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” - PowerPoint PPT Presentation
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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 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