the properties & effects of moving charges

THEPROPERTIES&EFFECTSOFMOVING CHARGES

What do we know?

• a magnet is NOT a charged rod• a magnet has two poles: north &

south• like poles repel•unlike poles attract• Can we ever find an isolated

pole?

Ferromagnetic Materials

–Iron, Nickel, and Cobalt

Magnetic Domains

A permanent magnet has iron (or nickel) atoms that have 2 unpaired electrons that have their spins rigidly fixed. If all the atoms are aligned in the same direction (called a DOMAIN) then all the little magnets line up to form a big magnetic effect (field)

Iron and some other substances that set up domains are called Ferromagnetic.

Paramagnetic substances (such as water) do not set up domains and are only weakly attracted to magnetic fields.

Diamagnetic substances (copper) have an exact internal magnetic balance that can be upset by a magnetic field so that it will be weakly repelled from a magnet.

Magnetic Field

• Operational Definition: A vector whose direction is indicated by a compass, which is really just a small magnet. The field is in the direction of the “north seeking” pole.

• Use symbol B for magnetic field.• Iron filings can act like a compass.

Which way will the magnet rotate?

N

S B Field

Which way will the magnet rotate?

B FieldNS

N

S

Magnetic Field Vectors Due to a Bar Magnet

N

S

N

S

N

S

N

S

N

S

N

S

N

S

N

S

N

S

Magnets have Magnetic Fields. Moving electrons produce magnetic fields.

Earth's Magnetic Field

Did you know?:

Magnetic north (actually the location of the south pole of the magnet) is in Hudson Bay some 1300 km from geographic north. It has moved about 700 km since 1904!!

Magnetic Field Lines

• Just like in electric fields, magnetic fields can be represented by field lines. Imagine that we take a really small compass (or magnet) and call it a test particle - then place it in various magnetic fields that we produce. The particle would move in a prescribed pattern called MAGNETIC FIELD LINES.

• The direction of the magnetic field at any point is tangent to the magnetic field line at that point.

These lines can also be called Lines of Force or FLUX LINES.

Magnetic fields can be represented by these flux lines and the density of lines represents the strength of the field.

These field lines NEVER cross and flux density is usually greatest near the poles of the magnet. The SI unit of flux is the weber.

N

S

N

S

For a charged particle to experiencea force due to a magnetic field

a) the charge must be moving, and

b) the velocity of the moving charge must have a component that is perpendicular to the direction of the magnetic field.

NOTE: the direction of the force is always perpendicular to both the velocity vector and the magnetic field B.

How Big is the Force?

• Force is proportional to: • perpendicular component of the velocity• charge (q)• size of the field (B)

F = q vperp B

Some typical fields:

surface of the Earth 10-4 T small bar magnet 10-2 T MRI 2-3 T magnet lab 30 T surface of neutron star 108 T

MAGNETISM/MAGNETISM/ELECTROMAGNETISMELECTROMAGNETISM

" Scientists in the 19th century inferred that Magnetism results from the motion of electric charges and should therefore be considered part of a larger phenomenon called ELECTROMAGNETISM”

A magnetic field (an area where magnetic force is detected) is produced by moving charges. Magnetism results when electrons spin on their axes or when electrons move through a conductor - no charge movement - NO MAGNETIC FIELD.

All matter has some magnetic properties - they all have electrons.

Oersted’s discovery

CURRENT CARRYING WIREA wire carrying current also has a

magnetic field. As the electrons move throughout the conductor, a magnetic field is set up around the wire.

x = field line into page x x x x x x x x x x x x

e- ---> • = field line out of page • • • • • • • • • • •

For this section, we need to know the exact direction of electron flow in the wire [not conventional current]and use the

LEFT-HAND RULE #1. If you were to grab (figuratively) a wire with electrons flowing (electron flow) through it with your left hand so that your thumb pointed in the direction of electron flow, your fingers would point in the direction of the magnetic field - N (flux or field lines).

Moving electrons create a magnetic field!

X

solenoid (or inductor)

An electromagnet

Magnetic fields that

move across a wire create

electricity!

An electric motor converts electricity to motion.

An electric generator converts motion to electricity.

Power plants create electrical

power using generators called

turbines.

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Electricity is not entirely efficient.