Magnetism

A Strangely Attractive Topic

History #1

Term from the ancient Greek city of Magnesia, Many natural magnets found

We now refer to these natural magnets as lodestones

contain magnetite, a naturally magnetic material Fe3O4.

(Pliny the Elder (23-79 AD Roman) wrote of a hill near the river Indus that was made entirely of a stone that attracted iron.)

History #2

121 AD Chinese scholars knew that an iron rod which had been brought near one of these natural magnets acquired and retained the magnetic property that such a rod when suspended from a string would align itself in a north-south direction.

Use of magnets to aid in navigation can be traced back to at least the eleventh century.

(1819) a connection between electrical and magnetic phenomena is shown.

Danish scientist Hans Christian Oersted observed that a compass needle in the vicinity of a wire carrying electrical current was deflected!

(1831), Michael Faraday discovered that a momentary current existed in a circuit when the current in a nearby circuit was started or stopped

Shortly after, he discovered that motion of a magnet toward or away from a circuit could produce the same effect.

(Let This Be a Lesson!)

(Joseph Henry (first Director of the Smithsonian Institution) failed to publish what he had discovered 6-12 months before Faraday)

SUMMARY:

Oersted showed that magnetic effects could be produced by moving electrical charges;

Faraday and Henry showed that electric currents could be produced by moving magnets

*All magnetic phenomena result from forces between electric charges in motion.

Looking in More Detail Andre Ampere first suggested in 1820 that magnetic properties of matter were due to tiny atomic currents

All atoms exhibit magnetic effects

Medium in which charges are moving has profound effects on observed magnetic forces

1. There are North Poles and South Poles.

2. Like poles repel, unlike poles attract.

3. Magnetic forces attract only magnetic materials.

4. Magnetic forces act at a distance.

5. While magnetized, temporary magnets act like permanent magnets.

What We Will Learn About Magnetism

6. A coil of wire with an electric current flowing through it becomes a magnet.

7. Putting iron inside a current-carrying coil increases the strength of the electromagnet.

8. A changing magnetic field induces an electric current in a conductor.

9. A charged particle experiences no magnetic force when moving parallel to a magnetic field, but when it is moving perpendicular to the field it experiences a force perpendicular to both the field and the direction of motion.

10. A current-carrying wire in a perpendicular magnetic field experiences a force in a direction perpendicular to both the wire and the field.

For Every North, There is a South

Every magnet has at least one north pole and one south pole. 

Field lines leave the North end of a magnet and enter the South end of a magnet. 

If you take a bar magnet and break it into two pieces, each piece will again have a North pole and a South pole.  No matter how many times.

S N S N S N

(No Monopoles Allowed)

(It has not been shown to be possible to end up with a single North pole or a single South pole, which is a monopole.

Note: Some theorists believe that magnetic monopoles may have been made in the early Universe. So far, none have been detected.

S N

Magnets Have Magnetic Fields

We will say that a moving charge sets up in the space around it a magnetic field,

andit is the magnetic field which exerts a force on any other charge moving through it.

Magnetic fields are vector quantities….that is, they have a

magnitude and a direction!

Defining Magnetic Field DirectionMagnetic Field vectors are written as B

Magnitude of the B-vector is proportional to the force acting on the moving charge, magnitude of the moving charge, the magnitude of its velocity, and the angle between v and the B-field. Unit is the Tesla or the Gauss (1 T = 10,000 G).

F = qvBsin θ

Magnetic Field Lines

Magnetic field lines describe the structure of magnetic fields in three dimensions.

If at any point on such a line we place an ideal compass needle, free to turn in any direction (unlike the usual compass needle, which stays in 2 dimensions) then the needle will always point along the field line.

Field lines are closer together where the field is the strongest, and spread out when the field is weak.

Field Lines Around a Doughnut Magnet

Field Lines Around a Bar Magnet

Field Lines Around a Magnetic Sphere

Field Lines of Repelling Bars

Field Lines of Attracting Bars

Showing the Direction of Magnetic Field in a wire

FIRST RIGHT-HAND RULE

Hold wire in your right hand with your thumb pointing in the direction of current. ( + to - )

The magnetic field of the wire wraps around it in the direction of your fingers. (At 90 degrees to the wire)

Finding Poles of an Electromagnet

SECOND RIGHT-HAND RULE:

Hold an insulated coil of wire in your right hand. Wrap your fingers in the direction of current.

Your thumb points toward the north pole of the electromagnet.

Force on a Current Carrying Wire

THIRD RIGHT-HAND RULE

Point your thumb in the direction of current. Point your pointer finger in the direction of the magnetic field. (N to S)Point your middle finger perpendicular to your pointer finger.That is the direction of the force on the wire.

Cyclotron

Developed in 1931 by E. O. Lawrence and M. S. Livingston at UC Berkeley

Uses electric fields to accelerate and magnetic fields to guide particles at very high speeds

How a Cyclotron Works Pair of metal chambers shaped like a pillbox cut along one of its diameters (cleverly referred to as “D”s) and slightly separated

Ds connected to alternating current

Ions injected near gap

Ions are accelerated as long as they remain “in step” with alternating electric field

Magnetic Force on Current-Carrying Wire

Since moving charges experience a force in a magnetic field, a current-carrying wire will experience such a force, since a current consists of moving charges. This property is at the heart of a number of devices.

Force on a current carrying wire.

FORCE on a current carrying wire

F = ILB Sin θ

F =(current)(Length)(strength of Field in Tesla)

So Field strength = 1N/(1A(1m))

Force on a Charged Moving Particle

F = qvBSin θ

A beam of electrons travels at 3 x 106 m/s through a field of 4.0 x 10-2 T at right angles to the field. How strong is the force on each electron?

F = 1.6 x 10-19 C(3 x 106 m/s )(4.0 x 10-2 T )

Electric MotorAn electric motor, is a machine which converts electrical energy into mechanical (rotational or kinetic) energy.   

A current is passed through a loop which is immersed in a magnetic field. A force exists on the top leg of the loop which pulls the loop out of the paper, while a force on the bottom leg of the loop pushes the loop into the paper.

The net effect of these forces is to rotate the loop.

Electromagnet (Magnetism from Electricity)

An electromagnet is simply a coil of wires which, when a current is passed through, generate a magnetic field, as below.

Magnetic Properties of MatterIn other words….materials which produce magnetic fields with no apparent circulation of charge.

All substances - solid, gas, and liquid - react to the presence of a magnetic field on some level. Remember why?

How much they react causes them to be put into several material “types”.

Magnet - isms Ferromagnetism - Iron, cobalt, nickel, gadolinium, dysprosium and alloys containing these elements exhibit ferromagnetism

The direction the electron spins within one atom interacts with those of nearby atoms.  

They will align, creating magnetic domains forming a permanent magnet.  

If a piece of iron is placed within a strong magnetic field, the domains in line with the field will grow in size as the domains perpendicular to the field will shrink in size. 

Making a Magnet from a Ferromagnetic Material

• domains in which the magnetic fields of individual atoms align

• orientation of the magnetic fields of the domains is random

• no net magnetic field.

• when an external magnetic field is applied, the magnetic fields of the individual domains line up in the direction of the external field

• this causes the external magnetic field to be enhanced

A Ferromagnet in the Middle

If we look at a solenoid (a coil), but rather than air, wrap it around a nice iron core. What happens to the change in flux for a given current?

Can you see why ferromagnetic materials are often put in the middle of current-carrying coils?

We can build one!

Well, a simpler one…