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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

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
slide5

Ampère’s Law

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

slide6

Ampère’s Law

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

slide7

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?
slide10

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:

slide11

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?
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.
slide20

Applications

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

slide21

Applications

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

slide22

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.

slide23

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:

slide24

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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