Eleanor Roosevelt High School Chin-Sung Lin

Eleanor Roosevelt High School Chin-Sung Lin
Lesson Magnetism Eleanor Roosevelt High School Chin-Sung Lin

History of Magnetism

History The lodestone, which contains iron ore, was found more than 2000 years ago in the region of Magnesia in Greece

History The earliest Chinese literature reference to magnetism lies in the 4th century BC writings Guiguzi (鬼谷子): "The lodestone attracts iron”

History Zheng He used the Chinese compass as a navigational aid in his voyage between 1405 and 1433

History In the 18th century, the French physicist Charles Coulomb studied the force between lodestones

History In 1820 Danish physicist and chemist Hans Christian Ørsted who discovered that electric currents create magnetic fields

Magnetic Poles

Magnetic Poles Magnets attract and repel without touching
The interaction depends on the distance Magnetic poles produce magnetic forces

Magnetic Poles Magnet can act as a compass
The end that points northward is called north pole, and the end that points south is call the south pole

Magnetic Poles All magnets have north and south poles
They can never be separated from each other If you break the magnet in half, what will happen?

Magnetic Poles Each half will become a complete magnet
Unlike electric charge, you cannot have north or south pole alone

Like poles repels; opposite poles attract
Magnetic Poles Like poles repels; opposite poles attract

Magnetic Fields

Magnetic Fields The space around the magnet is filled with a magnetic field

Magnetic Fields The magnetic field lines spread from the north pole to the south pole Where the lines are closer (at the poles), the field strength is stronger

Magnetic Fields The magnetic field unit: Units: tesla (T) or gauss (G)
1 tesla = 10,000 gauss

Magnetic Fields What will happen If we place a compass in the field?

Magnetic Fields A magnet or small compass in the field will line up with the field

Magnetic Fields Electric charge is surrounded by an electric filed
The same charge is surrounded by a magnetic field if it is moving Which types of electron motion exist in magnetic materials?

Magnetic Fields Electrons are in constant motion about atomic nuclei
This moving charge constitutes a tiny current and produces a magnetic field

Magnetic Fields Electrons spinning about their own axes constitute a charge in motion and thus creates another magnetic field Every spinning electron is a tiny magnet

Magnetic Fields Electrons spinning in the same direction makes up a stronger magnet Spinning in opposite directions cancels out The field due to spinning is larger than the one due to orbital motion

Magnetic Fields For ferromagnetic elements: iron, nickel, and cobalt, the fields do not cancel one another entirely Each iron atom is a tiny magnet

Magnetic Domain

Magnetic Domain Interactions among iron atoms cause large clusters of them to line up with one another These cluster of aligned atoms are called magnetic domains

Magnetic Domain There are many magnetic domains in a crystal iron
The difference between a piece of ordinary iron and an iron magnet is the alignment of domains

Magnetic Domain Iron in a magnetic field:
A growth in the size of the domains that is oriented in the direction of the magnetic field A rotation of domains as they are brought into alignment

Magnetic Domain Permanent magnets:
Place pieces of iron or certain iron alloys in strong magnetic fields Stroke a piece of iron with a magnet

Electric Currents & Magnetic Fields

Current-Carrying Wire: A moving electron produces a magnetic field Electric current also produces magnetic field A current-carrying conductor is surrounded by a magnetic field

Right-hand rule: Grasp a current-carrying wire with your right hand Your thumb pointing to the direction of the current Your fingers would curl around the wire in the direction of the magnetic field (from N to S)

What will happen to the compasses if the current is upward? ?

The current-carrying wire deflects a magnetic compass

Current-Carrying Loop: A wire loop with current produces a magnetic field

Coiled wire— Solenoid: A solenoid can be made of many wire loops

Coiled wire— Solenoid: A current-carrying coil of wire with many loops The magnetic field lines bunch inside the loop

Coiled wire— Solenoid: A coil wound into a tightly packed helix which produces a magnetic field when an electric current is passed through it Solenoids can create controlled magnetic fields and can be used as electromagnets

Intensity of Magnetic Field of Electromagnet (B): Increased as the number of loops increased (B ~ N) Increased as the Current increased (B ~ I) Intensity is enhanced by the iron core (B ~ μ) N B I

Permeability: The measure of the ability of a material to support the formation of a magnetic field within itself. Magnetic permeability is typically represented by the Greek letter μ B μ

Permeability: Medium Permeability μ [H/m] Relative Permeability μ/μ0 Mu-metal (nickel-iron alloy) 2.5×10−2 20,000 Ferrite (nickel zinc) 2.0×10−5 – 8.0×10−4 16 – 640 Steel 8.75×10−4 100 Vacuum ×10−6 (μ0) 1 Water ×10−6 Superconductors

Direction of magnetic field of electromagnet follows the Right-hand Rule: Your fingers indicate the direction of the current (I) your thumb points the direction of the field (B) B I

Magnetic Forces on Moving Charged Particles

When a charged particle moves in a magnetic field, it will experience a deflecting force (FB) + I

When a charged particle moves in a magnetic field, it will experience a deflecting force (FB) FB = qvB FB magnetic force [N] q electric charge [C] v velocity perpendicular to the field [m/s] B magnetic field strength [T, Teslas] I

The magnetic field unit: Units: tesla (T) or gauss (G) 1 tesla = 10,000 gauss tesla = (newton × second)/(coulomb × meter) T = Ns / (Cm)

Direction of the magnetic force (FB) follows the Fleming’s Left Hand Motor Rule I

What will happen to the positively charged particle? +

The positively charged particle will experience a force always perpendicular to the motion The particle will have a circular motion

The magnetic field has been used to detect particles in the cloud chamber What will happen to the different radiation?

The magnetic field has been used to detect particles in the cloud chamber α He2+ helium nucleus (+) β e– electron (–) γ uncharged EM ray

The magnetic field has been used to deflect the electron beam. Where will the electron beam hit the screen? electron beam S N screen magnet A B C D

Mass spectrometry: To determine masses of particles, for determining the elemental composition of a molecule

Mass spectrometry: magnetic force = centripetal force FB = FC qvB = mv2/r r = (mv)/(qB)

Mass spectrometry: r = (mv)/(qB) the faster it is travelling the bigger the circles the bigger its mass is the bigger the circles the bigger its momentum the larger the circles the stronger the magnetic field the smaller the circles the larger the charge the smaller the circles

A positively charged particle moving along a spiral path inside a uniform magnetic field

Magnetic Force on Current-Carrying Wires

What will happen to the current carrying wires? I I

The current-carrying wire also follows Fleming’s left hand motor rule

The current-carrying wire deflects a magnetic compass and a magnet deflects a current-carrying wire are different effect of the same phenomena

Magnetic Force Between Wires: What will happen to the parallel wires if both current are in the same direction? I2 I1

Magnetic Force Between Wires: Parallel wires carrying currents will exert forces on each other When the current goes the same way in the two wires, the force is attractive When the currents go opposite ways, the force is repulsive

Magnetic Force Between Wires: What will happen to the parallel wires if the current are in the opposite direction? I2 I1

Galvanometers & Motors

Galvanometer A sensitive current-indicating instrument
The coil turns against a spring, so the greater the current, the greater its deflection

Galvanometer A galvanometer may be calibrated to measure current— an ammeter A galvanometer may be calibrated to measure voltage— a voltmeter

Motor Converts electrical energy into mechanical energy
Motors operate through interacting magnetic fields and current-carrying conductors to generate force

DC Motor The current-carrying wire of the motor coil follows Fleming’s left hand motor rule

DC Motor

AC Motor

Earth’s Magnetic Field

Earth itself is a huge magnet The magnetic poles of Earth do not coincide with the geographic North pole – magnetic declination

Magnetic Pole Shift: The magnetic poles of Earth keep changing The pole kept going north at an average speed of 10 km per year, lately accelerating to 40 km per year

Magnetic Pole Weakening: The strength of the magnetic field of Earth keep decreasing The magnetic field has weakened 10% since the 19th century

A geomagnetic reversal is a change in the Earth's magnetic field such that the positions of magnetic north and magnetic south are interchanged

A positively charged particle moving along a spiral path inside a uniform magnetic field

Earth’s magnetic field will deflect the charged particles from outer space to reduce the cosmic rays striking Earth’s surface

Van Allen radiation belt: is a torus of energetic charged particles around Earth, which is held in place by Earth's magnetic field

Van Allen radiation belt: energetic electrons forming the outer belt and a combination of protons and electrons creating the inner belt

Aurora: a natural light display in the sky, particularly in the polar regions, caused by the collision of charged particles directed by the Earth's magnetic field

The End

Eleanor Roosevelt High School Chin-Sung Lin

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