Magnetism CH 19

Magnetic Materials Materials that are attracted to magnets are called ferromagnetic Substances that are repelled by magnets are diamagnetic Substances that are slightly attracted by a strong magnet are called paramagnetic

The Domain Theory of Magnetism Magnetism is a property of a charge in motion Magnetism is caused by the movement of electrons –An electron revolving around the nucleus of an atom imparts a magnetic property to the atom structure –The electron spinning on its axis

Each spinning electron acts as a tiny magnet Opposite spins are designated + and – –Electrons spinning in opposite directions tend to form pairs and so neutralize their magnetic character Domains –Atoms are grouped in microscopic magnetic regions called domains –The atoms in each domain are magnetically polarized parallel to a crystal axis –The domains effectively cancel out one another –If domains rotate with a external field, the material becomes magnetized –Curie Point – The temperature at which the domain regions disappear and the material becomes paramagnetic

Force Between Magnet Poles Like poles repel Unlike poles attract Unit pole – a pole that repels an exactly similar pole, placed one centimeter away, with a force of N Coulomb’s Law of Magnetism – the force of attraction between two magnetic poles is directly proportional to the product of the strengths of the poles and inversely proportional to the square of the distance between them F = k M 1 M 2 d 2

Magnetic Fields of Force Magnetic fields can be represented by lines of flux A line of flux is drawn so that a tangent to it at any point indicates the direction of the magnetic field The unit of magnetic flux (  ) is the weber (Wb)

The magnetic flux density (B) is the number of flux lines per unit area that permeates the magnetic field B =  /A 1 weber/meter 2 = 1 tesla

Magnetic Permeability The property of a material by which it changes the flux density in a magnetic field from the value in air is called its permeability –All materials that are not ferromagnetic behave similar to air (they are magnetically inert) Magnetism produced in a ferromagnetic substance by the influence of a magnetic field is called induced magnetism

Terrestrial Magnetism The earth sort of behaves like it has a giant bar magnet inside of it The earth’s magnetic axis does not coincide with its geographic axis, it is inclined at a small angle –Magnetic N pole is located 1200 mi from N pole –So N pole of a compass needle does not point to the true geographic north

Declination At any surface location the angle between magnetic north and the true north is called the declination, or variation –LA = 15 degrees east –Cincinnati, OH = 0 degrees –Boston = 15 degrees west

Electromagnetism A magnetic field exists in the region near the conductor when a closed circuit exists The direction of the magnetic field is dependent on the direction of the current in the conductor –Example

Magnetic field and charge in motion Ampere’s Work –Two long, straight, parallel conductors of length l separated by distance d and carrying currents I 1 and I 2 will each experience a force F of magnitude F=2klI 1 I 2 /dk = N/A 2 –Ampere – the current in each of two long parallel conductors spaced one meter apart that causes a magnetic force of 2 x newton per meter length of conductor –Coulomb – the quantity of electric charge that passes a given point on a conductor in one second when the conductor carries a constant current of one ampere

Ampere’s Rule (Left hand rule) Grasp the conductor in the left hand with the thumb extended in the direction of the electron flow. The fingers will circle the conductor in the direction of the magnetic flux

The Galvanometer Used to detect the presence of an electric current or to determine its direction Made of a coil of wire wound on an iron core that is pivoted on jeweled bearings between the poles of a permanent horseshoe magnet

Other Instruments DC Voltmeter –The dc voltmeter is basically a high-resistance galvanometer, used to measure voltages DC Ammeter –An ammeter is basically a modified galvanometer used to measure current –Must be wired in series Ohmmeter –Measures the resistance of a circuit component –Must be used only on a completely de- energized circuit