Magnetismx - Lawndale High School

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Nov 16, 2013 (3 years and 10 months ago)

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Magnetism

Physics

A little history…


Until the early 19
th

century, scientists thought electricity
and magnetism were unrelated


In 1820, Danish science professor Hans Christian
Oersted

was demonstrating electric currents in front of a class of
students


When electric current was passed in a wire near a magnetic
compass, the compass needle moved

Electricity and Magnetism are related!

Comparing E&M


Magnets exert forces on one another


Similar

to electric charges:


Can attract and repel without touching


Strength of interaction depends on the distance of
separation of the two magnets


Different

from electric charges:


Electric charges produce electrical forces


Regions called
magnetic poles
produce
magnetic forces

Magnetic Poles


All magnets have both a
north

and a
south

pole


Like poles repel


Opposite poles attract


Magnetic poles
always

exist in pairs

Magnetic Fields


Magnetic Field (B)
: The space around a magnet in which a
magnetic force is exerted [measured in Tesla


T].


Magnetic Field is a VECTOR (has magnitude and direction).


The direction of the magnetic field outside a magnet is
from the north to the south pole



N

S

B Field

Magnetic Domains


Magnetic Domain
=
large clusters of atoms
lined up with each
other


Domains start out
randomly

oriented in
this piece of iron


Domains align in the
direction of the
magnetic field as they
are brought closer to a
magnet

Permanent magnets are made by

placing
pieces of
iron or certain iron alloys in strong magnetic

fields

How are E&M related?


Charges in motion have both E (electric) fields and B
(magnetic) fields associated with them


In a bar magnet, electrons inside are constantly moving


Moving charge = current


浡杮整楣m晩敬e


Electrons also spin


Spinning charge =motion


浡杮整e挠晩敬e

Moving electric charges create magnetic fields

Electromagnets


If a current carrying wire is bent
into a loop, the magnetic field
lines become bunched up inside
the loop


A current
-
carrying coil of wire
with many loops is an
electromagnet

The Right Hand Rule


To find the direction of the magnetic field in a wire, point
the thumb/fingers of your right hand in the direction of
current flow.


Your fingers/thumb point in the direction of the magnetic
field.

Out of
the page

Into
the
page

The Right Hand Rule


What is the direction of the magnetic field in this wire?





What is the direction of the magnetic field in this coil or
wire?

ANSWERS


What is the direction of the magnetic field in this wire?





What is the direction of the magnetic field in this coil or
wire?

B Field

B Field







Force is
greatest

when the particle moves
perpendicular

to
the magnetic field


Force becomes less at angles less than 90 and
zero

when
the particle moves
parallel

to the field lines


When
MOVING

electric charges are placed in
magnetic fields, they feel a
FORCE.

Magnetic Force


The
force

that acts on a moving charged particle depends
on the particle’s
charge
, its
velocity
, and the strength of
the
magnetic field
.


B = magnetic field [T]


v = charge velocity [
m/s
]


F = force [N]


q

= charge [C]



F

=
qvB

The Right Hand Rule


To find the direction of the magnetic
force

on a charge


Take two pens


Hold them perpendicular to each other as in the picture


Take your RIGHT hand


Place your RIGHT hand at the point where the two pens meet


Push
v

towards B


The direction your thumb points is the direction of F


Out of the page

Into the page

Right Hand Rule Practice

v

v

v

B

B

B

Magnetic Force cont’d


Now we know:


a charged particle moving through a magnetic field
experiences a deflecting force


So…


a current of charged particles moving through a magnetic
field also experiences a deflecting force

F = force (N)

I = current (A)

L = length of wire (
m
)

B = magnetic field (T)

F

= ILB

Magnetic Force


When an electric charge moves in a magnetic field, it feels
a force.

Single charge:



Many charges (current):





F

=
qvB

F

= ILB

F = force (N)

I = current (A)

L = length of wire (
m
)

B = magnetic field (T)

F = force (N)

q

= charge (C)

v = velocity (
m/s
)

B = magnetic field (T)

Earth’s B Field


A compass points northward because Earth itself is a huge
magnet


The compass aligns with the magnetic field of the earth


Most geologists think that moving charges looping around
within Earth create its magnetic field


The magnetic field of Earth is not stable



It has flip
-

flopped throughout geologic time


Studies of deep
-
sea sediments indicate that the field was
virtually switched off for 10,000 to 20,000 years just over 1
million years ago


Electromagnetic
Induction

Physics

m
agnetism can produce electricity & electricity can produce magnetism

Electromagnetic Induction


Electric current can be produced in a wire by simply
moving a magnet into or out of a wire


Movement of the magnet
induces

a voltage, which causes
current flow





Voltage is induced whether the magnet is moved through
the wire or the wire is moved through the magnet

Flux


Magnetic Flux = the number of magnetic field lines passing
through a given area


Measured in
Webers

(
Wb
)


If a loop of wire lies perpendicular to a magnetic field, the
maximum possible number of lines of flux will pass
through the loop.


If the loop of wire lies parallel to the field, the flux through
the loop will be zero.

Φ

= AB

Φ

= flux (
Wb
)

A = area of loop (m2)

B = magnetic field (T)

Example


Eleanor is undergoing an MRI procedure and is placed
inside a chamber housing the coil of a large electromagnet
that has a radius of 25.0 cm. A flux of 0.290
Wb

passes
through the coil opening. What is the magnetic field inside
the coil?


greater number of loops of wire =
greater induced voltage =
greater current


Faraday’s Law
: Induced voltage in a coil is proportional to:


the product of the number of loops


the cross
-
sectional area of each loop


and the rate at which the magnetic field changes within those
loops



Example


The hood ornament on Abe’s sedan is shaped like a ring
8.00 cm in diameter. Abe is driving toward the west so that
Earth’s 5.00*10
-
5
T field provides no flux through the hood
ornament. What is the induced voltage in the metal ring as
Abe turns from this street onto one where he is traveling
north, if he takes 3.0
s

to make the turn?

Φ

= AB

Lenz’s Law


An induced voltage always produces a magnetic field that
opposes the field that originally produced it


In other words:


If the original magnetic field, and thus the flux, is going
toward the north, the induced voltage will produce an
opposing field and flux that goes toward the south

Generators & Motors


Generator = A machine that produces electric current by
rotating a coil within a stationary magnetic field







A

motor

converts electrical energy into mechanical energy.


A
generator
converts mechanical energy into electrical energy.

Transformers


A transformer works by inducing a changing magnetic
field in one coil, which induces an alternating current in a
nearby second coil


Voltages may be stepped up or stepped down with a
transformer

Power Transmission


Power is transmitted great distances at high voltages and
correspondingly low currents, a process that otherwise
would result in large energy losses owing to the heating of
the wires.


Power may be carried from power plants to cities at about
120,000 volts or more, stepped down to about 2400 volts
in the city, and finally stepped down again by a
transformer to provide the 120 volts in our houses