Transcript Electricity Ch. 18 Sect. 2

Magnetism
Section 2
Section 2: Magnetism from Electric Currents
Preview
• Key Ideas
• Bellringer
• Electromagnetism
• Electromagnetic Devices
Magnetism
Section 2
Key Ideas
〉What happens to a compass near a wire that
is carrying a current?
〉Why are electric motors useful?
Magnetism
Section 2
Bellringer
The temporary magnetic field created by current flowing
through a wire is used in many small appliances.
1. List as many things as possible that contain a small
electric motor.
2. What keeps the coil in an electric motor spinning?
3. A sewing machine needle can be magnetized by
gently stroking it with a magnet in one direction.
What is a possible explanation for this?
Magnetism
Section 2
Electromagnetism
〉What happens to a compass near a wire that is
carrying a current?
〉When the wire carries a strong, steady current,
all of the compass needles move to align with
the magnetic field created by the electric current.
• Hans Christian Oersted found that magnetism is
produced by moving electric charges.
– Electric currents produce magnetic fields.
Magnetism
Section 2
Electromagnetism, continued
• Use the right-hand rule to find the direction of the
magnetic field produced by a current.
• right-hand rule:
– If you hold a wire in your right hand and point your
thumb in the direction of the positive current, the
direction that your fingers curl is the direction of the
magnetic field.
Magnetism
Section 2
The Right-Hand Rule
• The thumb points in the
direction of the current.
• The fingertips point in
the direction of the
magnetic field.
• The magnetic field runs
counterclockwise.
Magnetism
Section 2
Magnetic Field of a Current-Carrying
Wire
Magnetism
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Electromagnetism, continued
• Solenoids and bar magnets have similar magnetic fields.
• solenoid: a coil of wire with an electric current in it
– In a solenoid, the magnetic field of each loop of wire
adds to the strength of the magnetic field of any
neighboring loops.
• The strength of a solenoid can be increased.
– More loops or more current can create a stronger
magnetic field.
• electromagnet: a coil that has a soft iron core and that
acts as a magnet when an electric current is in the coil
– The magnetic field of the rod adds to the coil’s field.
Magnetism
Visual Concept: Solenoid
Section 2
Magnetism
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Electromagnetism, continued
• Moving charges cause magnetism.
– Negatively charged electrons moving around the
nuclei of atoms make magnetic fields.
– Atomic nuclei also have magnetic fields because
protons move within the nucleus.
– Each electron has a property called electron spin,
which also produces a tiny magnetic field.
• The magnetism of the uncanceled fields in certain
materials combines to make the materials magnetic
overall.
Magnetism
Section 2
Electromagnetic Devices
〉Why are electric motors useful?
〉A motor can perform mechanical work when it is
attached to an external device.
• electric motor: a device that converts electrical energy to
mechanical energy
• Galvanometers detect current.
– galvanometer: an instrument that detects, measures,
and determines the direction of a small electric current
Magnetism
Section 2
Visual Concept: Galvanometer
Magnetism
Section 2
Electromagnetic Devices, continued
• ammeter: measures current
• voltmeter: measures voltage.
• Motors use a commutator to spin in one direction.
– commutator: a device used to make the current
change direction every time the flat coil makes a half
revolution.
– brushes: devices that connect the wires to the
commutator
Magnetism
Electric Motor
Section 2