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Creating Magnetic Fields

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Creating Magnetic Fields

Text: Ch. 20

M. Blachly, AP Physics

- Magnetic fields are created by moving charges.
- For a long, straight wire, the magnetic field circulates around the wire.

- Direction of B is given by right hand rule:
- Thumb in direction of current, fingers curl in the direction of B

Ampère’s Law

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

Ampère’s Law

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

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:

- A current moving in a wire produces a magnetic field
- A magnetic field produces a force on a wire that carries a current.

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

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:

Force between Two Parallel Wires

Parallel currents attract; antiparallel currents repel.

- What if we bend our wire into a loop?

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

The Right-hand Rule

- Electrons “orbit” the nucleus and also “spin”. This produces a magnetic field
- Electrons generally pair up
- most of the magnetic field cancels

- 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

- 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

- 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

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

Applications

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

Applications

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

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.

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:

Mass Spectrometer

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

- 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