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

Magnetic Field of a Long Straight Wire The field is inversely proportional to the distance from the wire: The constant μ 0 is called the permeability of free space, and has the value:

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 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 l 2 of wire 2 is:

Force between Two Parallel Wires Parallel currents attract; antiparallel currents repel.

A loop of wire What if we bend our wire into a loop?

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

The Right-hand Rule

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 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 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 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 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.

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: Applications

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 Additional Links: All about how audio speakers work: