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 1269. 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 • 1864- 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

Fmagnetic = q v B = q v sinθ B

B is a magnetic field vector The test object is taken to be a charged particle moving with a velocity v

**No Velocity = No Force **Units: B is measured in Tesla (T)

1T = Webers/m2 = 1Ns/Cm= 1x 104 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.

Fmagnetic = BIl = BsinθIl

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. 775 1,3,5

pg. 778 1,3

WKBK 21A 1. F = 5.4 x 10-11 N

2. F = 3.6 x 10-6 N 4. B = 2.6 T

21B 1. F= 0.23 N2. B = 7.4 x 10-5 T4. I = 1.34 A

Right Hand Rules: First 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.

Second 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.

Third Right Hand Rule• 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 90o from

the other.

Magnetic Field Definitions

Forces on Currents in Magnetic Fields• When you have a

current in a magnetic field it uses the third right-hand rule to show the direction of the force.

• The equation for this example is: F = BIL

Example of Current Carrying Wire• A good example of how we

utilize a current carrying wire is in a loudspeaker. As the levels of electrical signal changes, it causes moderating amounts of magnetic force that moves the speaker cone. The speaker cone compresses air into sound waves.

Torque on a Current Carrying Loop

• The torque due to a magnetic field B on a current carrying loop of area A is : = BIA sin

• This is the basis of volt and ammeters.

• If there are more than one loop of wire the equation becomes: = NIAB sin

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 = mv2 /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 = oI 2r

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 = oI

2d 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).