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18 Οκτ 2013 (πριν από 4 χρόνια και 7 μήνες)

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How to Use This Presentation

Section 1
Magnets and Magnetic Fields

Section 2
Magnetism from Electric Currents

Section 3
Electric Currents from Magnetism

Objectives

Recognize
that like magnetic poles repel and unlike poles attract.

Describe
the magnetic field around a permanent magnet.

Explain
how compasses work.

Describe
the orientation of Earth’s magnetic field.

Bellringer

1.

Magnets are a part of daily life. Most of us use them without thinking about it.
Name five places that yo
u use magnets.

2.

If you had two bar magnets and you touched the end of one bar magnet to
the end of the other bar magnet, what are the two possible outcomes? If you
rotated one of the bar magnets 180°, what are the two possible outcomes?

3.

Explain why a
compass is a useful tool for navigation.

Bellringer,
continued

4.

On Earth, there is the magnetic N pole and the geographic North Pole.
According to the picture above, are they located at the same geographic
location? Where do you think the magnetic S pole

is located?

Magnets

Some materials can be made into permanent magnets.

Although a magnetized piece of iron is called a “permanent” magnet, its
magnetism can be weakened or even removed.

Iron is a
soft

magnetic material.

It is easily magnetized.

It tends
to lose its magnetic properties easily.

Cobalt is a
hard

magnetic material.

It more difficult to magnetize.

Once magnetized, it doesn’t lose its magnetism easily.

Magnetic Materials

Magnets,
continued

Magnets exert magnetic forces on each other.

Like pol
es repel, and opposite poles attract.

A

magnetic pole
is one of two points, such as the ends of a magnet, that
have opposing magnetic qualities.

Magnets have a pair of poles, a north pole and a south pole.

It is impossible to isolate a south magnetic pole
from a north magnetic pole.

Magnetic Poles

Magnetic Fields

Magnets are sources of magnetic fields.

Magnetic force is a field force.

When magnets repel or attract each other, it is due to the interaction of
their.

A

magnetic field
is a region where a magnet
ic force can be detected.

Magnetic field lines are used to represent a magnetic field.

The magnetic field gets weaker with distance from the magnet.

Magnetic Fields

Magnetic Field

Magnetic Fields,
continued

Compasses can track magnetic fields.

A compass is

a magnet suspended on top of a pivot so that the magnet can
rotate freely.

A compass aligns with Earth’s magnetic field.

Earth’s magnetic field is like that of a bar magnet.

Earth’s magnetic field has changed direction throughout geologic time.

Earth’s ma
gnetic poles are not the same as its geographic poles.

Earth’s Magnetic Field

Earth’s Magnetic Field

Objectives

Describe

how magnetism is produced by electric currents.

Interpret

the magnetic field of a solenoid and of an electromagnet.

Explain

the magne
tic properties of a material in terms of magnetic domains.

Explain
how galvanometers and electric motors work.

Bellringer

The temporary magnetic field created by current flowing through a wire is used in
many small appliances.

Make as long a list as possi
ble of items that contain a small electric motor.

How can temporary electromagnets be used to turn a shaft in an electric
motor?

A sewing machine needle can be magnetized by gently stroking it with a
magnet in one direction. What is a possible explanation
for this?

Bellringer

List as many items as you can that use DC current. (
Hint:
Batteries supply DC
current.)

List as many items as you can that use AC current. (
Hint:
Standard wall
outlets supply AC current.)

If you want to plug a CD player that normally u
ses batteries into a wall socket,
an AC adapter is required. What is the function of the AC adapter?

Electric power that goes into a neighborhood must be stepped down, or
decreased, in voltage before it goes into a home. Explain why this is
necessary.

Magn
etism from Electric Currents

Hans Christian Oersted found that magnetism is produced by moving electric
charges.

Electric currents produce magnetic fields.

Use the right
-
hand rule to find the direction of the magnetic field produced by a
current.

The
right
-
hand rule:

If you imagine holding the wire in your right hand with your thumb pointing
in the direction of the positive current, the direction your fingers would curl
is in the direction of the magnetic field.

The Right
-
Hand Rule

Right
-
Hand Rule for a Cur
rent
-
Carrying Wire

Magnetic Field of a Current
-
Carrying Wire

Magnetism from Electric Currents,
continued

The magnetic field of a coil of wire resembles that of a bar magnet.

A

solenoid

is a coil of wire with an electric current in it.

In a solenoid, the ma
gnetic field of each loop of wire adds to the strength of
the magnetic field of the loop next to it.

More loops or more current can create a stronger magnetic field.

An

electromagnet
is a coil that has a soft iron core and that acts as a magnet
when an ele
ctric current is in the coil.

Solenoid

Magnetism from Electric Currents,
continued

Magnetism can be caused by moving charges.

Negatively charged electrons moving around the nuclei of all atoms make
magnetic fields.

Atomic nuclei also have magnetic fields b
ecause protons move within the
nucleus.

Each electron has a property called
electron spin,
which also produces a
tiny magnetic field.

Magnetic atoms rotate to align with the magnetic fields of nearby atoms
creating small regions within the material called

domains.

Magnetic Domain

Electromagnetic Devices

Galvanometers detect current.

A
galvanometer
is an instrument that detects, measures, and determines
the direction of a small electric current.

Electric motors

convert electrical energy to mechanical energy
.

A device called a
commutator
is used to make the current change direction
every time the flat coil makes a half revolution.

Devices called
brushes
connect the wires to the commutator.

Galvanometer

Electric Motor

Electromagnetic Devices,
continued

Stereo
speakers use magnetic force to produce sound.

When the direction of the current in the coil of wire changes, the paper cone
attached to the coil moves, producing sound waves.

Objectives

Describe
the conditions required for electromagnetic induction.

Appl
y
the concept of electromagnetic induction to generators.

Explain
how transformers increase or decrease voltage across power lines.

Electromagnetic induction
is the process of creating a current in a circuit by
changing a magnetic field.

states the following:

An electric current can be produced in a

circuit by a changing
magnetic field.

As the loop moves in and out of the magnetic field of the magnet, a current
is
induced
in the circuit.

Rotating

the circuit or changing the strength of the magnetic field will also
induce a current in the circuit.

Electromagnetic Induction

Ways of Inducing a Current in a Circuit

continued

Electromagnetic induction does n
ot violate the law of conservation of energy.

Moving electric charges experience a magnetic force when in a magnetic field.

The force is at its maximum value when the charge moves perpendicular to
the field.

As the angle between the charge’s direction and
the direction of the
magnetic field decreases, the force on the charge decreases.

Electromagnetic Induction

continued

A

generator
is a machine that converts mechanical energy to electrical
energy.

An

alternating

current (AC)
is an electric current that changes direction at
regular intervals.

For each half rotation of the loop in an AC generator, the current produced
by the generator reverses direction.

Generators produce the electrical energy you use in your home
.

AC Generator

Induced Current

Function of a Generator

continued

Electricity and magnetism are two aspects of a single electromagnetic force.

The energy that results from these two forces is called electromagne
tic
(EM) energy.

Light is a form of electromagnetic energy.

EM waves are made up of oscillating electric and magnetic fields that are
perpendicular to each other.

EM waves are also called
EMF
(electromagnetic frequency)

Electromagnetic Wave

Electromagnetic

Waves

Transformers

A

transformer
is a device that increases or decreases the voltage of alternating
current.

Transformers can increase or decrease voltage.

The voltage induced in the secondary coil of a transformer depends on the
number of loops, or
turns
,
in the coil.

In a
step
-
up transformer
the voltage across the secondary coil is greater than
the voltage across the primary coil.

In a

step
-
down transformer,
the secondary coil has fewer loops than the
primary coil and the voltage is lowered by the transf
ormer.

Transformers

Transformer

Concept Mapping

Understanding Concepts

1.

How many coil turns are needed on the secondary coil of a step
-
down
transformer that reduces voltage from 2 400 volts to 120 volts if the primary
coil has 1,000 turns?

A.

1

B.

2
0

C.

50

D.

120

Understanding Concepts,
continued

1.

How many coil turns are needed on the secondary coil of a step
-
down
transformer that reduces voltage from 2 400 volts to 120 volts if the primary
coil has 1,000 turns?

A.

1

B.

20

C.

50

D.

120

Un
derstanding Concepts,
continued

2.

What conditions are necessary to induce an electric current?

F.

A conductor must move past a stationary

magnetic field.

G.

A magnetic field must move past a stationary

conductor.

H.

A conductor and a magnetic field
must move

relative to one another.

I.

A magnetic field and a conductor must move

together relative to a
stationary point.

Understanding Concepts,
continued

2.

What conditions are necessary to induce an electric current?

F.

A conductor must move past a

stationary

magnetic field.

G.

A magnetic field must move past a stationary

conductor.

H.

A conductor and a magnetic field must move

relative to one another.

I.

A magnetic field and a conductor must move

together relative to a
stationary point.

Unde
rstanding Concepts,
continued

3.

What is the result of cutting a bar magnet in half?

A.

two unmagnetized bars

B.

two magnets with north poles only

C.

two smaller magnets with both north and south

poles

D.

one magnet with north poles only and one

mag
net with south poles
only

Understanding Concepts,
continued

3.

What is the result of cutting a bar magnet in half?

A.

two unmagnetized bars

B.

two magnets with north poles only

C.

two smaller magnets with both north and south

poles

D.

one magnet with

north poles only and one

magnet with south poles
only

Understanding Concepts,
continued

4.

Light is a form of electromagnetic energy. Explain how the effect of electric
and magnetic fields on each other produces a light wave.

Understanding Concepts,
con
tinued

4.

Light is a form of electromagnetic energy. Explain how the effect of electric
and magnetic fields on each other produces a light wave.

Answer: The moving electric field generates a magnetic field and the moving
magnetic field generates an ele
ctric field. The light wave is a combination of
the two fields.

Understanding Concepts,
continued

5.

Differentiate between an alternating electric current and a direct electric
current.

Understanding Concepts,
continued

5.

Differentiate between an alter
nating electric current and a direct electric
current.

Answer: In a direct current, the electrons always flow in one direction, but in an
alternating current, the direction of flow reverses periodically.

A type of train that is in developm
ent uses the force of magnetism to propel it forward. It
is known as a magnetic levitation or maglev train, because magnetic forces are also
used to reduce friction. Instead of wheels, the train has large magnets that float above
magnets in the track. Alte
rnating electromagnetic fields drive the train forward or slow it
down using magnetic attraction and repulsion. Because there is very little friction to
overcome, maglev trains move rapidly while consuming less energy than traditional
vehicles.

lls,
continued

6.

How does the design of the maglev train reduce friction and consume less
fuel?

Answer: Because the train "floats" on a magnetic field, there is no contact with
the ground. Other kinds of railcars have wheels that turn on rails and creat
e
friction that slows the train and wastes fuel.

Interpreting Graphics

7.

What is the purpose of the commutator in this electric motor?

F.

constant direction of

electron flow

G.

alternation of the coil

magnetic field

H.

production of

mechanical ener
gy

I.

production of a

magnetic field

Interpreting Graphics

7.

What is the purpose of the commutator in this electric motor?

F.

constant direction of

electron flow

G.

alternation of the coil

magnetic field

H.

production of

mechanical energy

I.

pro
duction of a

magnetic field