magnetismsection 2 section 2: magnetism from electric currents preview key ideas bellringer...
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Magnetism Section 2
Section 2: Magnetism from Electric Currents
Preview• Key Ideas • Bellringer • Electromagnetism • Electromagnetic Devices
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Magnetism Section 2
Key Ideas
〉What happens to a compass near a wire that is carrying a current?
〉Why are electric motors useful?
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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?
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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.
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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.
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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.
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Magnetism Section 2
Magnetic Field of a Current-Carrying Wire
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Magnetism Section 2
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.
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Magnetism Section 2
Visual Concept: Solenoid
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Magnetism Section 2
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.
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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
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Magnetism Section 2
Visual Concept: Galvanometer
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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
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Magnetism Section 2
Electric Motor