Transcript 12.4 Solenoids

12.4 Solenoids
p. 559-562
https://www.youtube.com/watch?v
=EsJXZLwSCdA
Ampere’s Experiment
Investigated further aspects of electricity and
magnetism
 Took two parallel wires to see if they would
attract or repel one another when opposing
currents were sent through them
 Two magnetic fields interact with one another
to cause a force; force can be attractive or
repulsive depending on the direction of the two
fields

If two field lines are in same directionrepulsion force applied
 If two field line are in opposite directionattractive force applied

Solenoid
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A wire that has been bent into a loop has a stronger
magnetic field than a straight wire conductor.
We can represent the magnetic field lines created by a
looped wire in two ways.
Figure (a) shows the field lines all pointing in the same
direction (into the page) on the inside of the loop, and
out of the page outside the loop.
We can also represent the magnetic field lines
in a loop of wire in three-dimensional form as
in Figure (b).
 An × represents a field line going into the
page.
 A • represents a field line coming out of the
page.
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

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If several loops of wire are used, the intensity of
the magnetic field through the loop increases.
A wire that has been looped many times to
increase the intensity of the magnetic field when a
current is applied is called a solenoid.
A current passing through a solenoid produces a
straight magnetic field that is similar to the
magnetic field of a bar magnet.
Discussion: Solenoids
Section 12.4
Looking at the diagrams, how is a solenoid like a bar magnet?
How are they unlike each other?
Answers for Discussion Questions:
Section 12.4
A solenoid and a bar magnet both
have similar magnetic fields.
However, the field of a bar magnet
is always present, whereas a
solenoid’s magnetic field can be
turned on or off as part of an
electric circuit.
Usually an iron core is used in a solenoid since
the magnetic fields pass through iron better
than through air because the domains in iron
align with the field, which strengthens the field.
 A solenoid is often referred to as an
electromagnet because the magnetic field is
generated by a current that flows through the
wire.
 The magnet can be turned on or off by turning
the current on or off.
 As the number of loops increases, the intensity
of the magnetic field inside the loop also
increases.

Conventional current
Use the RHR to determine the direction of the
magnetic field in a solenoid for conventional
current.
 If you grasp the solenoid with your right hand
and wrap your fingers around the solenoid in
the direction of conventional current, then your
thumb indicates the direction of the magnetic
field lines inside the core toward the north pole
of the core magnet.

Right hand rule for a solenoid
Electron Flow
Use the LHR to determine the direction of the
magnetic field in a solenoid if the moving
charge is negative.
 If you grasp the solenoid with your left hand and
wrap your fingers around the solenoid coil in the
direction of electron flow, then your thumb
indicates the direction of the magnetic field lines
inside the core.
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p. 561 Applications of Solenoids

Subwoofer
p. 561 An electric bell

When switch is closed
current is directed into the
solenoid. The solenoids
produce a magnetic field
that is amplified by the soft
iron cores. The soft iron
armature is attracted to the
core and the bell rings
once. Now armature pulls
away from the contact so
the circuit is interrupted.
Armature is on a spring and
makes contact again…until
switch opened
Electromagnets
An electromagnet uses a current-carrying
solenoid to generate a magnetic field that is
easy to switch on and off.
The strength of an electromagnet can be
increased by:
 • increasing the current through the wire.
 • increasing the number of loops in the solenoid
 • decreasing the diameter of the loops in the
solenoid
 • changing the core of the solenoid

Section 12.4
Solenoids
Match each term to the appropriate description.
Name
• produces a magnetic field
whose strengths can change
depending on the value for
the current, the number of
turns in the wire, and the
spacing of wire, etc.
Description
• amplifies a
• produces a
• solenoid
magnetic field all magnetic field
produced by
the time
an electric
• soft iron core
current
• bar magnet
Section 12.4
Solenoids
Match each term to the appropriate description.
Name
Description
• soft iron core
• amplifies a magnetic field produced by an
electric current
• solenoid
• produces a magnetic field whose strengths
can change depending on the value for the
current, the number of turns in the wire, and the
spacing of wire, etc.
• bar magnet
• produces a magnetic field all the time
Section 12.4
Solenoids in Action
1. Which features do the devices below have in common?
spring
battery contact point soft-iron bell
armature
switch
light bulb soft-iron
core
In
common:
2. Label the diagram using the features listed in Question 1.
Section 12.4
Solenoids in Action
1. Which features do the devices below have in common?
spring
battery contact point soft-iron bell
armature
switch
light bulb soft-iron
core
soft-iron
spring
contact point
switch
In
armature
common:
2. Label the diagram using the features listed in Question 1.
switch
spring
battery spring
soft-iron
armature
light bulb
contact point
soft-iron
core
bell
switch
soft-iron
core
soft-iron
armature
contact point
soft-iron
core
soft-iron
core
Homework
• 12.4 P. 562 questions #1,4,7