Transcript Document

Chapter 7 (3-5)
Magnetism
from
Electricity
Electricity
from
Magnetism
Electromagnetism- Using
electricity to produce magnetism
• Hans Christian Oersted (1820)
– discovered the relationship between electricity
an magnetism
– Experiments with compass and electric current
in a wire
– Results:
• Electric current produces a magnetic field and the
direction of the magnetic field depends on the
direction of the current
Oersted
Page 207
All of the compass
needles point north.
A current is
produced in the wire.
Electromagnetism
• The interaction between electricity
and magnetism
Figure 1 – pg 208
When there is no
current in the wire, the
compass needles
point north.
When current is present,
the compass needles
align with the magnetic
field produced by the
current in the wire.
Changing the Magnetic Fields
• It can be turned on by turning on or off
the current.
• The direction of the field can be
changed by reversing the current.
• Changing the strength:
– Increasing the amount of current.
– Increasing the amount of loops.
Figure 2 – pg 209
Reversing direction
Changing strength.
Turning current off
Solenoid
• A coil of wire that produces a
magnetic field when carrying an
electric current
– The strength of magnetic field increases
if you add more loops or increase the
current
Solenoid
Electromagnets
• A magnet that consists of a solenoid
wrapped around an iron core
• The magnetic field produced by the
solenoid causes the domains within
the iron to become aligned, making it
magnetized
Electromagnets
Electromagnets
• To increase the strength of an
electromagnet:
– Increase the number of loops in the
solenoid
– Increase the size of the iron core
– Increase the electric current in the
wire
– Wind the coils closer together
Uses of Electromagnetism
APPLY IT –pg 211
The steel bar will be attracted to the magnet. The
clapper will be pulled up with the steel bar and will
hit the bell.
The rising steel bar breaks the circuit.
Energy and Motion and
Energy Transformation
• Magnets can move a wire that has an
electric current – The direction of the
movement depends on the direction of
the current.
Types of Energy
• Energy: ability to move and object over
a distance
• Electrical Energy: energy associated
with electrical current
• Mechanical Energy: energy an object
has due to it’s movement or position
When a wire with a current is places in a
magnetic field, electrical energy is
transformed into mechanical energy.
Mechanical
Energy
Electrical
Energy
Measuring Electromagnetism
• Galvanometer
– A device used to measure current
through the interaction of an
electromagnet and a permanent magnet
– Pg 214
Galvanometer
APPLY IT – pg 215
Person’s skin would have more
sweat because it was
conducting more electricity. So
the person might be nervous.
• Electric motors
– A device that changes electrical energy into
kinetic energy
– Diagram of parts of a motor (p216)
•
•
•
•
Commutator
Brushes
Permanent magnets
Armature
Electromagnetic Induction
-Using magnets to produce electricity.
• Faraday and Henry (1831)
– Discovered a magnetic field can produce an
electric current
– Experiments with iron ring, 2 wires, and
galvanometer
– Results:
• Electric current was produced in the second wire only
when the magnetic field was changing (when the
battery was connected and disconnected)
Faraday
Electromagnetic Induction
• The process by which an electric
current is produced by a changing
magnetic field
Electromagnetic Induction
• Faraday’s experiments also showed:
– The magnetic field around a wire can be
changed by moving either the magnet or
the wire
– So…an electric current can be induced
by moving a magnet in a coil of wire or by
moving a wire between the poles of a
magnet
clockwise
counterclockwise
When part of the
circuit moves up,
the current flows
clockwise. When
parts move down,
the current is
counterclockwise.
clockwise
counterclockwise
Uses For EI
• Transformers
– A device that increases or decreases the
voltage of an alternating current
• Generators
– A device that uses electromagnetic induction to
change kinetic energy into electrical energy
Generators
The slip ring turns
with the armature
and transfer current
to the brushes.
The crank rotates the
armature.
The motion of the armature
in the magnetic field
induces a current
When the brushes are
connected to a circuit, the
generator can be used as
an energy source.
current
Magnetic field
Magnetic field
current
turn
Transformers
• Two separate coils of wire wrapped
around an iron core.
– Primary coil attached to a circuit with a
voltage and current
– Secondary coil is attached to a non voltage
current.
How does it work?
• The changing current in the first coil produces
a magnetic field
• The magnetic field induces a current in
the second coil.
– The change of voltage from the primary
coil to the secondary coil depends on the
amount of loops in each coil
Types of Transformers
• Step-up:
– Primary coils have smaller amount of loops
than the secondary coil
• Increases Voltage
• Step-down:
– Primary coils have more amount of loops
than the secondary coil
• Decreases Voltage
Transformer
How much increase or
decrease?
VOLTAGE (primary)
VOLTAGE (secondary)
Coils (pri)
Coils (sec)
The greater the difference between the
number of loops in the primary and
secondary coils in a transformer, the more
the voltage will change.

11,000
11

240,000 240
 .05
240,000 100

2,400
1
 100
step  up
step  down





2,400 20

120
1
 20
step  down