MagnetismMagnetism

Lodestone : Naturally Magnetic Rock

The early Greeks found natural rock magnets in an area of Greece called Magnesia. They called this magnetic stone, lodestone. Our term, magnet derives from Magnesia.

Lodestone Used For Navigation From 1200s

Lodestone needles were placed on cork which was floated in a water dish. The needle always pointed in a north/south direction which helped ships to navigate their way.

William Gilbert 1600s

William Gilbert, Queen Elizabeth’s physician, experimented with lodestone and concluded that the earth itself is a lodestone to be able to cause compasses to align themselves along a north/south axis.

The Poles of A Magnet

A magnet, when suspended from a string will align itself with one pole pointing north and the other pole pointing south. The north-seeking pole is labelled N while the south-seeking pole is labelled S.

The Poles of a Magnet Can Not be Separated

Unlike electric charges which can be separated, magnetic poles can not be separated. When a magnet is broken, each piece is found to have a north and south pole.

The Laws of Magnetic Forces

Like poles of magnets repel each other while opposite poles attract each other.

The Nature of Earth’s North and South Magnetic Poles

Since a magnet’s north pole is attracted to Earth’s Magnetic North Pole, the Magnetic North Pole must actually be a south pole, since only opposite poles attract each other. Likewise the Magnetic South Pole must be an actual north pole.

Earth’s Geographic and Magnetic Poles Differ

Earth’s Magnetic poles are aligned on an angle to Earth’s Geographic poles (true north and south pole).

Earth’s Wandering Magnetic PolesThe magnetic poles of the Earth are created by its moving

molten iron and nickel core. The internal fluid nature of the Earth causes its magnetic poles to move yearly. Over the past 100 years the pole has moved over 1000 km. Presently it is moving at 40 km per year.

Adjusting Compass ReadingsSince the Magnetic pole is not aligned with the Geographic

pole, compass readings must be adjusted in most regions. These adjustments are referred to as declinations and are printed on maps of various regions in Canada.

Magnetic Field

The region surrounding a magnet in which magnetic forces can be exerted is called a magnetic field. Iron filing sprinkled around a magnet reveal the nature of the invisible magnetic field.

The Vector Nature of a Magnetic Field

When compasses are set in a magnetic field, they show a direction for the magnetic field. Magnetic fields like electric fields are vector quantities, showing both magnitude and direction.

Magnetic Field Line Direction

Magnetic field lines are drawn from north to south poles in the same direction as a compass would point in the region outside the magnet. (The field lines continue inside the magnet from from south to north).

Fields Between Attracting And Repelling Poles

Field lines between attracting poles link together while field lines between repelling poles stay apart.

Comparing Electric and Magnetic Fields

Electric and Magnetic Fields show similar field lines for attracting and repelling fields.

Uniform Magnetic Fields

The region between two opposite poles of two bar magnets is uniform except that it “fringes” out at the edges. In Fig. 12.4 below, The magnetic field lines in (a) are from left to right. The magnetic field lines in (b) are perpendicular into the page, away from you. The magnetic field lines in (c) are perpendicular and out of the page, towards you.

Magnetic Field StrengthThe number of magnetic field lines, referred to as magnetic

flux, indicates the strength of the magnetic field and is measured in teslas (T - SI unit). A 1 tesla magnetic field exerts a force of 1 N on a charge of 1 coulomb moving at 1 m/s. Earth’s magnetic field is 5 x 10-5 T, a fridge magnet produces 5 mT. Strong neodymium magnets produce 1-2 T. Bar magnets produce from .001 to .01 T.

Strengths of Some Magnetic Materials

  Smallest value in a magnetically shielded room

10^-14 Tesla   10^-10 Gauss

    Interstellar space 10^-10 Tesla   10^-6 Gauss

    Earth's magnetic field 0.00005 Tesla   0.5 Gauss

    Small bar magnet 0.01 Tesla   100 Gauss

    Within a sunspot 0.15 Tesla   1500 Gauss

    Small NIB magnet 0.2 Tesla   2000 Gauss

    Big electromagnet 1.5 Tesla   15,000 Gauss

    Strong lab magnet 10 Tesla   100,000 Gauss

    Surface of neutron star 100,000,000 Tesla   10^12 Gauss

    Magstar 100,000,000,000 Tesla   10^15 Gauss

The Origin of Magnetic Fields

The spin and angular momentum of an electron generate a magnetic field. In most atoms, reverse spins and motions cancel the magnetic fields generated.

Magnetic Classification of Materials

In terms of magnetism, materials are classified as paramagnetic, diamagnetic or ferromagnetic.

Paramagnetic materials are weakly attracted to magnetic fields. Diamagnetic materials are weakly repelled by magnetic fields.

Ferromagnetic materials (like iron and nickel) respond strongly to magnetic fields.

Ferromagnetic Materials: DomainsIn ferromagnetic substances like iron and nickel, their atoms

have a number of unpaired electrons whose magnetic fields are NOT cancelled by opposing motions. Atoms in ferromagnetic substances cooperate with 1015 – 1020 nearby atoms to create small microscopic regions (10-6 m) called domains in which the atoms’ magnetic fields are all aligned. Each domain acts as if it were a small magnet with a north and south pole.

Temporary and Permanent Magnets

Ferromagnetic materials can be made into permanent or temporary magnets. If, as liquid iron or nickel is cooled, a magnetic field lines up the domains while the metal solidifies, the domains may remain more or less lined up throughout the substance, forming a permanent magnet. If the domains are not lined up in the solid state, an external magnetic field may be applied to line up the domains, forming a temporary magnet that demagnitizes when the magnetic field is removed.

Magnetic InductionWhen a magnetic field is applied to a non-magnetic,

ferromagnetic substance, the magnetic field causes or “induces” the ferromagnetic substance to become a temporary magnet by aligning its domains.

Electric Current and MagnetismHans Christian Oersted in 1820 accidentally discovered that an

electric current produces a magnetic field. He noticed that a magnet near a wire moved when an electric current was passed through the wire. This suggested that the current in the wire was producing a magnetic field that affected the compass.

Attracting and Repelling Magnetic Fields

Magnetic fields in the same direction repel each other while magnetic fields in the opposite direction attract each other.

The Magnetic Field Around A Current-Carrying Wire

The current in a wire creates a circular magnetic field around a wire.

Right Hand Rule for a Current-Carrying Wire

Using conventional current flow, grasp the wire with the thumb of the right hand in the direction of the current flow (+ to -). The direction of the magnetic field is shown by the direction fo the right hand fingers.

Right and Left Hand Rules

When working with conventional current, the right hand wire rule gives the correct relation of current and magnetic field directions. When working with actual flow, the left hand rule gives the correct relation of current and field directions.

Diagram Conventions for Wire On-End Views

In on-end views of wires, an X denotes current flowing into the page while a dot denotes current flowing out of the page. Use the right hand rule with the thumb in the direction of current flow to find the direction of the magnetic field (direction of right hand fingers).

Magnetic Field Around a Wire Loop

Current flowing through a loop generates a somewhat linear magnetic field inside the loop.

Electromagnets or Solenoids

When a wire is coiled with many loops, the magnetic field within and around the coil is intensified and resembles the field of a bar magnet.

Electromagnets or Solenoids

If the core of the coil (space inside) is occupied by a ferromagnetic material, the electromagnet thus formed becomes very strong.

Right Hand Coil Rule

If a coil is grasped with a right hand with the fingers pointing in the direction of the conventional current flow, the thumb will point in the direction of the magnetic field or in the direction of the north pole.

Finding Magnetic Strength Inside an Electromagnet

Inside a coil or solenoid with an air core, the Magnetic Field Strength, B, (in teslas –T) is found by the formula, B = µ0In , where µ0 is a constant, I = the current in amperes and n = the number of loops per metre. µ0 = 4π x 10-7 Tm/A . µ0 is a

constant called the permeability of free space .

Interactions of Magnetic Fields 1

Magnetic fields in the same direction repel each other while magnetic fields in the opposite direction attract each other.

Interactions of Magnetic Fields 2

Parallel current-carrying wires close together, experience a force between them of repulsion or attraction, depending on the direction of the current in the two wires.

no current

current in

opposite

direction

current in

same

direction

Predicting Compass Positioning

A compass needle (itself a magnet with a field) will move itself to form an attraction between its field and any nearby field.

X

N

CompassNeedle

Wire withcurrent into the page

Predicting Compass PositioningIn the diagram below, the compass is placed above the wire.

In (A), a larger current is produced (with lower resistor). In (B) the current is reduced by ½ (2X resistor) so that the magnetic field in the wire is only slightly larger than the Earth’s field, causing an intermediate movement in which the Earth’s field (dashed arrow) opposes a full movement.

Predict the Compass or Current Direction

Predicting Coil Current and Poles

Below, which side is the north pole and what is the field direction if current flows from A to B?

Below, which side is the north pole and in what direction is the current flowing?

Uses for Solenoids or Electromagnets

Electric meters make use of electromagnets placed between permanent magnets.

Uses for Solenoids or Electromagnets

Speakers for stereos, radios and broadcast systems use electromagnets placed within a permanent magnetic field. As the electromagnet receives pulses of current, it sets up a changing magnetic field that pushes it back and forth within the permanent magnetic field.

Uses for Solenoids or Electromagnets

Electromagnets are used to lift and move ferromagnetic materials.

Uses for Solenoids or Electromagnets

A relay switch uses an electromagnet in one circuit (usually a low current circuit) to open and close a second circuit (usually a high current circuit).

Uses for Solenoids or Electromagnets

Bells and buzzers use electromagnets. In the circuit below, a single tone is made when the switch is pressed. In other circuits, the hammer can be made to hit continuously.