Transcript Magnetic Fields - Newman Physics

Bell Work: Magnetism
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When regions of iron atoms are aligned, a
magnetic ( block / domain / pole ) is created.
When a magnet attracts a paperclip, the clip is
( permanently / temporarily ) magnetized.
Name two metals that are not ferromagnetic.
A compass lines up ( parallel / perpendicular )
to the magnetic fields.
Magnetic fields are strongest near the ( poles /
center ) of the magnet.
Earth as a Magnet
► When
Electromagnetism
there is current
through a wire, the
filings will form a
pattern of concentric
circles, around the wire.
►A
wire with current will
create a magnetic field
perpendicular to the
current flow
Which way?
To determine the current and the magnetic
field, we use right hand rules.
Is the current flowing into or out of the
screen?
Is the magnetic
field rotating
clockwise or counterclockwise?
1st Right Hand Rule
Imagine holding a length of insulated wire with
your right hand. Keep your thumb pointed in
the direction of the conventional (positive)
current. The fingers of your hand circle the
wire and point in the direction of the magnetic
field.
Determines Direction of
the Magnetic Field relative
to the current
Determine the direction of the
current flow.
2nd Right Hand Rule
► Imagine
holding an insulated coil with your
right hand. If you then curl your fingers
around the loops in the direction of the
conventional (positive) current, your thumb
will point toward the north pole of the
electromagnet.
Determines Polarity
Which end is North?
N
Which end is North?
N
Which way is the current flowing?
Solenoid
► When
a wire is looped several times to form a coil
and a current is allowed to flow through the coil,
the field around all the loops is in the same
direction.
Solenoid
►A
long coil of wire
consisting of many
loops is called a
solenoid. The field
from each loop in a
solenoid adds to the
fields of the other
loops and creates a
greater total field
strength.
Electromagnets
When there is an electric current in a coil of wire, the coil
has a field similar to a permanent magnet.
► One end of the electromagnet will attract one end and
repel the other end of a magnet.
► Thus, the current-carrying coil has a north and a south
pole and is itself a magnet.
► This type of magnet, which is created when current flows
through a wire coil, is called an electromagnet.
►
► The
strength of the field is directly related to the
current in the coil.
► The strength of the field is directly related to the
number of coils.
 The magnetic field produced by each loop is the same.
Because these fields are in the same direction,
increasing the number of loops increases the strength of
the magnetic field.
► The
strength of an electromagnet also can be
increased by placing an iron rod or core inside the
coil.
 It increases the magnetic field because the field of the
solenoid creates a temporary magnetic field in the core,
just as a nearby permanent magnet does when brought
near a metal object.
Electromagnetic Induction
In 1822 Micheal Faraday wronte a goal in his
notebook, it was “Convert magnetism into
electricity”.
After nearly 10 years he found that he could induce
electric current by moving a wire through a
magnetic field.
The same year, Joseph Henry an American highschool teacher, also showed that a changing
magnetic field could produce electric current.
http://www.youtube.com/watch?v=WX1fkfJPWpY
Electromagnetic Induction
► When
the wire is moved parallel to the
magnetic field or held stationary, there is no
current, but when the wire moves up
through the field, the current is in one
direction.
► When
the wire is moves down through the
field, the current is in the opposite direction.
Electromagnetic Induction
An electric current is generated in a wire only when
the wire cuts magnetic field lines.
It is relative motion between the wire and the
magnetic field that produces a current.
Electromagnetic Induction
Electric Generator
► The
Electric Generator was invented by
Michael Faraday.
► It converts mechanical energy to electrical
energy.
Electric Generator
►
An electric generator consists of a number of wire
loops placed in a strong magnetic field. The wire is
wound around an iron core to increase the strength of
the magnetic field.
Electric Generator
►
The iron and wires are called the armature, which is
similar to that of an electric motor.
Electric Generator
 The armature is
mounted so that it can
rotate freely in the
magnetic field. As the
armature turns, the
wire loops cut through
the magnetic field
lines and induce a
potential difference.
Commonly called the
voltage.
Electric Generator
The current is greatest when the
motion of the loop is perpendicular to
the magnetic field (the loop is in the
horizontal position).
When the loop is in the vertical
position, the wire segments move
parallel to the field and the
current is zero.
Electric Generator
The current changes smoothly from zero to
some maximum value and back to zero
during each half-turn of the loop. Then it
reverses direction.
Electric Generators
► The
entire loop does not contribute to the induced
EMF?
► If the fourth right-hand rule is applied to segment
ab, the direction of the induced current is toward
the side of the wire. The same applies to segment
cd.
► No current is induced along the length of the wire
in ab or cd.
Generators and Motors
Generators and motors
are almost identical in
construction, but they
convert energy in
opposite directions.
Generators convert
mechanical energy to
electrical energy,
motors converts
electrical energy to
mechanical energy.
Transformers
► Transformers
are used o increase or
decrease AC voltages.
► Transformers change voltages with
relatively little loss of energy.
Transformers
There are two types of
transformers:
Step-Up Transformers the secondary voltage is
larger than the primary
voltage.
Step-Down Transformers the voltage coming out of
the transformer is smaller
than the voltage put in
the transformer.