Creating magnetic fields
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Creating Magnetic Fields. Text: Ch. 20 M. Blachly, AP Physics. Magnetic Field. Magnetic fields are created by moving charges. For a long, straight wire, the magnetic field circulates around the wire. Magnetic Field. Direction of B is given by right hand rule:

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Creating Magnetic Fields

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Creating magnetic fields

Creating Magnetic Fields

Text: Ch. 20

M. Blachly, AP Physics


Magnetic field

Magnetic Field

  • Magnetic fields are created by moving charges.

  • For a long, straight wire, the magnetic field circulates around the wire.


Magnetic field1

Magnetic Field

  • Direction of B is given by right hand rule:

  •  Thumb in direction of current, fingers curl in the direction of B


Creating magnetic fields

Ampère’s Law

Ampère’s law relates the magnetic field around a closed loop to the total current flowing through the loop.


Creating magnetic fields

Ampère’s Law

Ampère’s law can be used to calculate the magnetic field in situations with a high degree of symmetry.


Creating magnetic fields

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:


Summary

Summary

  • A current moving in a wire produces a magnetic field

  • A magnetic field produces a force on a wire that carries a current.


Two wires

Two Wires

  • What will happen if there are two long, parallel wires that each carry a current?


Creating magnetic fields

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:


Creating magnetic fields

Force between Two Parallel Wires

Parallel currents attract; antiparallel currents repel.


A loop of wire

A loop of wire

  • What if we bend our wire into a loop?


Solenoid

Solenoid

  • What if we bend our wire into lots of loops?


Creating magnetic fields

The Right-hand Rule


Magnetic materials

Magnetic Materials

  • Electrons “orbit” the nucleus and also “spin”. This produces a magnetic field

  • Electrons generally pair up

    • most of the magnetic field cancels


Magnetic materials1

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


Paramagnetic material

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


Nonmagnetic material

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


Cores

Cores

  • Adding a core to a solenoid can greatly increase the magnetic field strength.


Creating magnetic fields

Applications

A galvanometer takes advantage of the torque on a current loop to measure current.


Creating magnetic fields

Applications

An electric motor also takes advantage of the torque on a current loop, to change electrical energy to mechanical energy.


Creating magnetic fields

Applications

Loudspeakers use the principle that a magnet exerts a force on a current-carrying wire to convert electrical signals into mechanical vibrations, producing sound.


Creating magnetic fields

Applications

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:


Creating magnetic fields

Mass Spectrometer

All the atoms reaching the second magnetic field will have the same speed; their radius of curvature will depend on their mass.


Links

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


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