Magnetism

Historical

• Early magnets called lodestones, naturally occurring iron ore magnetite

• Named magnets by Greeks since they were found in the region Magnesia

• Important for early navigation

• Coulomb showed magnetic force has similar relationship to electric and gravitational forces

Historical

• Oersted showed electric current and magnetism related in 1820

• He found electric current created magnetic field which would deflect a compass needle

Magnetic Poles

• Like electric charges, there are two types

• Called north and south because of the way they react to the earth’s magnetic field

• Unlike electrical charges, can’t have isolated N or S pole; always have both

• like electrical charges, opposite poles attract, like poles repel

Magnetic Poles

• Forces between poles are directly proportional to strength of the poles, inversely proportional to the square of distance between them.

• Magnets can be large: a planet, or small: a spinning electron

Magnetic Fields

• Magnetic forces act over a distance by means of a field, like electric and gravitational forces

• Magnetic field is represented by lines of force like electric field

• Field lines can be seen by using bits of iron sprinkled on paper placed near magnet

Magnetic Fields

• Direction of field is direction of force on N pole placed in field

• Magnetic field lines shown as arrows going out of N poles, into S poles

Cause of Magnetism

• Magnetic fields created by moving electrical charges

• In common magnets, moving charges are spinning electrons

• Movement of charges in wires (electric current) creates electromagnets

Ferromagnetism

• In most materials, magnetic effects due to motion of electrons cancel out

• Some materials have several spinning electrons without an opposite spinning partner

• Called ferromagnetic from the Latin word for iron

• Fe, Co, Ni, Dy, Pa, Nd

Magnetic Domains

• Since each atom of ferromagnetic material is magnet, groups of atoms will line up together -- called domains

• In unmagnetized iron, domains are randomly oriented

• When magnet is brought near, domains become aligned together, iron becomes magnetized

Magnetic Domains

• Domains can be rearranged and magnetism lost by heating or vibration

Electric Current and Magnetism

• For straight wire with current, magnetic field lines form concentric circles around wire

• Changing direction of current changes direction of magnetic field

• If wire is bent into loop, magnetic field is concentrated inside loop

Electric Current and Magnetism

• If more loops are added, magnetic field strength increases with each loop in coil

• Long coil called a solenoid

• If iron rod is placed inside solenoid, magnetic field is concentrated inside iron and electromagnet results

Magnetic Field of Loop

Magnetism and Moving Charges

• Charged particle moving in magnetic field will experience deflecting force due to interaction of two magnetic fields

• Force is greatest when motion is perpendicular to external magnetic field, zero when parallel

• Direction of force is perpendicular to both velocity and external magnetic field

Magnetism and Moving Charges

• Electrons in TV sets are aimed to screen using magnetic fields

• Earth’s magnetic field deflects incoming charged particles from sun and outer space

• Current carrying wires in magnetic fields experience similar sideways force because of charges moving in wires

Magnetism and Moving Charges

• Two nearby current carrying wires will exert forces on each other because each creates a magnetic field

• So current carrying wire will deflect compass and will be deflected by strong external magnetic field

Meters and Motors

• Analog (not digital) meters use coil of wire with magnets on each side to measure current: galvanometer, ammeter, or voltmeter

• Current through coil creates magnetic field; coil will try to line up with field of magnets

• If coil is allowed to rotate, motor can be made

Galvanometer

Meters and Motors

• As coil becomes aligned with magnets, current direction changes through commutator and brushes

• In large motors, permanent magnets replace with electromagnets

• Armature is rotating coil in motor

Earth’s Magnetic Field

• Thought to be caused by convection currents in liquid interior of planet

• Geologic evidence shows magnetic field has moved around and reversed polarity

• Magnetic poles don’t line up with geographic poles

Earth’s Magnetic Field

• Difference between magnetic north and true north called magnetic declination

• Since north poles of magnets and compasses point to north, Earth’s north magnetic pole is really a south type pole; south pole is really north type pole

North Magnetic Pole