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”

– 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 FieldsMatch 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!

Draw the magnetic field lines below.

Draw the magnetic field lines below.

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

2nd RHR

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