Chp. 21 Magnetism

MAGNETS  Magnets are pieces of metal (iron, nickel and steel) that work according to rules similar to electric charges.  All magnets have 2 poles, north (north seeking), and south poles.  Like electrostatics: similar poles repel and dissimilar poles attract.

History of Magnetism –1  Pierre de Maricourt mapped out and found "poles" on a spherical magnet in This was the first encounter with the well known electrostatic principals of like charges (poles) repel each other and opposite charges (poles) attract.

Magnetic History - 2  In 1600 William Gilbert extended these experiments to a variety of materials. He even found that the earth was a permanent magnet with a magnetic force field. He concluded that poles always appear in pairs and that magnet poles cannot be isolated.

Magnetic History -3  In 1819 Hans Oersted found that an electric current in a wire deflected a nearby compass needle.  Andre Ampere deduced the quantitative laws of magnetic force between current carrying conductors.

Magnetic History - 4  In the 1820's, Joseph Henry and Michael Faraday showed that an electric current could be produced in a circuit by either moving a magnet near the circuit or by changing the current in another nearby circuit. These observations demonstrated that a changing magnetic field produces an electric field. *However there was no quantitative explanation until Maxwell’s Equations.

Magnetic History –5  James Clerk Maxwell was able to show that electricity and magnetism are two perpendicular aspects of the same thing in his unified theory of electromagnetism. He published his 4 mathematical equations that related all of electricity and magnetism through calculus.

Different Magnetic Materials  Materials that are not affected by magnetic forces (non-magnetic) are called diamagnetic.  Materials that are affected by a magnetic field (temporary magnets) are called paramagnetic.  Materials that produce or retain their magnetism (permanent magnets) are called ferromagnetic.

Temporary Magnetic Materials- Paramagnetism  Paramagnetism occurs in substances in which the atoms contain unpaired electrons.  This is common in most metals that are not permanent magnets. Example: paper clips.

Permanent Magnetic Materials - Ferromagnetism  Ferromagnetic materials contain clusters of atoms that all have their unpaired electrons aligned (domains) and produce a magnetic field. These are permanent magnets.

Magnetic Fields  Magnetic Fields are like electrical and gravitational fields, they produce forces on the surrounding area that drops off as you move away from the magnet.  The vector arrows move out of the north end and curl around to the south end. The biggest magnet in the world is the Earth itself.

The Magnetic Force  The MAGNETIC Force acting on a charge q moving with a velocity v in an external magnetic field B is given by F magnetic = q v B = q v sin θ B **No Velocity = No Force ** Units: B is measured in Tesla (T) 1T = Webers/m 2 = 1Ns/Cm= 1x 10 4 Gauss (cgs unit)

Magnetic Force on a Current Carrying Conductor  For a current in a conductor, we have charges in motion.  The force of a magnetic field on a wire is a summation of the forces on the individual charges moving through the wire. F magnetic = B I l = B(sin θ ) I l I is the current l is the length of the wire

Strength of the Magnetic Field Plus Examples: 21A and 21 B pg. 774 & 778

Force on a Current Carrying Wire

Force on a Charged Particle

 Hmwk. Chp. 21 BK and WKBK (11) Book pg ,3,5 pg ,3 WKBK 21A 1. F = 5.4 x N 2. F = 3.6 x N 4. B = 2.6 T 21B 1. F= 0.23 N 2. B = 7.4 x T 4. I = 1.34 A

Right Hand Rules: 1 st Right hand Rule: Current produced Magnetic Field  A series of right hand visualizations are possible to help you understand magnetism.  The first one is to describe the direction of magnetic field lines around a current carrying wire.

2 nd Right Hand Rule- Electromagnet Polarity  The direction of the field produced by an electromagnet can be found by using the Second Right-Hand Rule.  Curl your fingers around the loops in the direction of the conventional (positive) current flow. Your thumb points toward the North pole.

3 rd Right Hand Rule Finding Magnetic Flux  The easy way to “see” this 3 way mutually perpendicular component is the second right hand rule. The velocity of charges, magnetic flux (B) and the force are each 90 o from the other.

Magnetic Field Definitions

Induced EMF

Motion of a Charged Particle in a Magnetic Field  The force of a charged particle is perpendicular to both the field and velocity and therefore a center seeking circle force (centripetal) equal to F = qvB = mv 2 /r and thus r = mv/qB showing the radius is proportional to the momentum mv.

Magnetic Field of a Long Straight Wire  The direction of B around a wire is consistent with the first right-hand rule: grasp the wire with the right hand and the thumb pointing in the direction of the current; the fingers will point in the direction of the magnetic field lines.  The strength is found with: B =  o I 2  r where r is perpendicular distance from the wire to the point and  o is the permeability of free space (4  x10 -7 (Tm/A).

Magnetic Force Between Two Parallel Conductors  The magnitude of the magnetic field around a long straight wire is determined to be B =  o I 2  d where d is distance.

Magnetic Field of a Current Loop  The magnetic field produced by a single, circular loop of wire looks similar to that produced by a short dipole magnet

Magnetic Field of a Solenoid  A solenoid is a long wire wound in the form of a helix. Tightly wound solenoids produce a very strong magnetic field inside of the loops. The strength depends on the number of loops of wire. Solenoids are used widely in switches.

Magnetic Fields in a Solenoid

Induced Electrical Current  Just like moving charges produce a magnetic field…. A moving magnetic field can produce an Induced Electrical Current.  Faraday’s Law of induction related magnetic flux change to the electromotive force (emf) or potential electrical change (voltage).