creating magnetic fields text: ch. 20 m. blachly, ap physics
TRANSCRIPT
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Creating Magnetic Fields
Text: Ch. 20 M. Blachly, AP Physics
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Magnetic Field
Magnetic fields are created by moving charges.
For a long, straight wire, the magnetic field circulates around the wire.
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Magnetic Field
Direction of B is given by right hand rule: Thumb in direction of current, fingers curl in the direction of B
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Ampère’s Law
Ampère’s law relates the magnetic field around a closed loop to the total current flowing through the loop.
0 encB dl I
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Ampère’s law can be used to calculate the magnetic field in situations with a high degree of symmetry.
Ampère’s Law
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Magnetic Field of a Long Straight Wire
The field is inversely proportional to the distance from the wire:
The constant μ0 is called the permeability of free space, and has the value:
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Summary
A current moving in a wire produces a magnetic field
A magnetic field produces a force on a wire that carries a current.
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Two Wires
What will happen if there are two long, parallel wires that each carry a current?
I1
I2
d
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Force between Two Parallel Wires
The magnetic field produced at the position of wire 2 due to the current in wire 1 is:
The force this field exerts on a length l2 of wire 2 is:
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Force between Two Parallel Wires
Parallel currents attract; antiparallel currents repel.
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A loop of wire
What if we bend our wire into a loop?
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Solenoid
What if we bend our wire into lots of loops?
oB nI
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The Right-hand Rule
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Magnetic Materials
Electrons “orbit” the nucleus and also “spin”. This produces a magnetic field
Electrons generally pair up
• most of the magnetic field cancels
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Magnetic Materials
In some materials, the magnetic fields do not cancel
iron, cobalt and nickel
The atoms “align” in a small region and create a domain.
The domains persist when the external magnetic field is removed
Also called “hard” magnets or permanent
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Paramagnetic material
The magnetic spins align in small regions forming a domain.
Domains can align with an external magnetic field
The domains do not persist when the external magnetic field is removed
Also called “soft” magnets
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Nonmagnetic material
Atom has no net magnetic moment so there can be no domain
Exhibits no magnetic effects: magnets cannot “stick” to nonmagnetic metals
Example: copper, stainless steel
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Cores
Adding a core to a solenoid can greatly increase the magnetic field strength.
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Applications
A galvanometer takes advantage of the torque on a current loop to measure current.
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An electric motor also takes advantage of the torque on a current loop, to change electrical energy to mechanical energy.
Applications
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Loudspeakers use the principle that a magnet exerts a force on a current-carrying wire to convert electrical signals into mechanical vibrations, producing sound.
Applications
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A mass spectrometer measures the masses of atoms. If a charged particle is moving through perpendicular electric and magnetic fields, there is a particular speed at which it will not be deflected:
Applications
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Mass Spectrometer
All the atoms reaching the second magnetic field will have the same speed; their radius of curvature will depend on their mass.
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Links
Additional Links:
http://www.physics.sjsu.edu/becker/physics51/induction.htm
http://hyperphysics.phy-astr.gsu.edu/HBASE/magnetic/magcon.html#c1
All about how audio speakers work: http://electronics.howstuffworks.com/speaker6.htm