conceptual physics chapter 24:...

Conceptual Physics

Chapter 24:

MAGNETISM

Magnetism

• The term magnetism comes from the name Magnesia, a coastal district of ancient Thessaly, Greece.

• Unusual stones, called lodestones, were found by the Greeks more than 2000 years ago. They had the intriguing property of attracting pieces of iron.

• Magnets were first fashioned into compasses and used for navigation by the Chinese in the 12th century.

Magnetic Forces

• The force between any two charged particles is described in Coulomb’s law:

But Coulomb’s law is not the whole story!

• When charged particles are moving with respect to each other, there is a force due to the motion of the charged particles that we call the magnetic force.

Magnetic Poles

All magnets have a North and South pole

•North pole (north-seeking pole)

•South pole (south-seeking pole)

Rule for magnetic forces between magnetic poles:

• Like poles repel; opposite poles attract.

Magnetic Poles

• In all magnets—can’t have one pole without the other

•No single pole known to exist

Example: • simple bar magnet—poles at

the two ends• horseshoe magnet: bent

U shape—poles at ends

Magnetic Fields

•Direction (by convention) is North to South

•Strength indicated by closeness of the lines

• lines close together; strong magnetic field• lines farther apart; weak

magnetic field

Magnetic Fields

The letter B represents the vector for magnetic field, which is measured in teslas (T).

Magnetic Fields

Magnetic Fields

Uniform Magnetic Fields

Magnetic Fields

Magnetic Fields

•Produced by two kinds of electron motion• Electron Spin• main contributor to magnetism• pair of electrons spinning in same direction creates a

stronger magnet• pair of electrons spinning in opposite direction

cancels magnetic field of the other• Electron Revolution

Magnetic Domains

Permanent magnets are made by:

• placing pieces of iron or similar magnetic materials in a strong magnetic field.

• stroking material with a magnet to align the domains.

Magnetic domains are magnetized clusters of aligned

magnetic atoms

Magnetic Domains

Difference between permanent magnet and temporary magnet:

•Permanent magnet–Alignment of domains remains once external

magnetic field is removed

•Temporary magnet–Alignment of domains returns to random

arrangement once external magnetic field is removed

Magnetic Domains

Electric Currents & Magnetic Fields

•Magnetic field forms a pattern of concentric circles around a current-carrying wire.

•When current reverses direction,

the direction of the field lines

reverse.

Connection between electricity and magnetism


We can calculate the magnitude of the magnetic field, B, of a

long, straight, current-carrying wire using the following

equation:

𝐵 =𝜇02𝜋

𝐼

𝑟

Where 𝜇0 is known as the permeability of free space and

has the value 𝜇0 = 4𝜋 x 10-7 T·m/A


Magnetic field intensity

• Increases as the number of loops increase in a current-carrying coil temporary magnet.

Electric Currents and Magnetic Fields

Electromagnet

• Iron bar placed in a current-carrying coil

•Most powerful—employs superconducting coils that eliminate the core

•Applications • control charged-particle beams in high-energy

accelerators• lift automobiles and other

iron objects• levitate and propel

high-speed trains

Electromagnets

•An electromagnet is simply a current-carrying coil of wire.

•The strength of an electromagnet is increased by• increasing the current through the coil and• increasing the number of turns in the coil. • having a piece of iron within the coil.

•Magnetic domains in the iron core are induced into alignment, adding to the field.

Electromagnets

• Electromagnets that utilize superconducting coils produce extremely strong magnetic fields—and they do so very economically because there are no heat losses.

The Large Hadron Collider (LHC)

uses more than 50 types of

electromagnets to accelerate

sub-atomic particles to 99.9% the

speed of light.

Giant electromagnets

are used to move

scrap metal at a

construction site.

Magnetic Forces on Moving Charges

•Greatest force – when particle movement in direction perpendicular to the magnetic field lines

•Least force - particle movement other than perpendicular to the magnetic field lines

•No force - particle movement parallel to the magnetic field lines

Moving charges in a magnetic field experience a deflecting force.


Moving charges in a magnetic field experience a deflecting force. (continued)


We use a different right hand rule to determine the Magnetic Force on a positive charge moving perpendicular to a magnetic field.


The following formula can be used to calculate the magnetic force on a charge moving in an external magnetic field.

𝐹𝐵 = 𝑞 Ԧ𝑣𝐵 sin 𝜃

𝐹𝐵 = 𝑞 Ԧ𝑣𝐵 Reduced to this form when charges move perpendicularly to the magnetic field. (sin Ɵ = 1 when Ɵ = 90 degrees.)


When the velocity of a charged particle is perpendicular to a uniform magnetic field, the particle moves in a circular path in a plane perpendicular to B.

FB acts as a centripetal force.

𝐹𝐶 = 𝐹𝐵

𝑚𝑣2

𝑟= 𝑞𝑣𝐵


A charge, q, moving at a speed of v enters a uniform magnetic field, B. I. Determine the radius of the circular path in terms

of the given variables.II. Determine whether the charge shown in the

diagram is positive or negative.

B


How much work is done by the magnetic force?

None!

The force of magnetism ALWAYS acts perpendicularly to the motion of charges. Therefore, there is no force in the direction of motion and so no work is done.

Magnetism CANNOT change the kinetic energy or speedof a charged particle.

It CAN however, accelerate it by changing its directiononly.

Magnetic Force on Current- Carrying Wires

•Current of charged particles moving through a magnetic field experiences a deflecting force.

• Direction is perpendicular to both magnetic field lines and current (perpendicular to wire).• Strongest when current is perpendicular to the magnetic

field lines.


Ԧ𝐹𝐵 = 𝐼𝐿𝐵 sin 𝜃* L is the length of the wire inside the field


II I I

FB FB FBFB

Currents in the SAME direction attract; Currents in the OPPOSITE direction repel.


Electric meters detect electric current

Example: •magnetic compass•compass in a coil of wires


Galvanometer

•Current-indicating device named after Luigi Galvani•Called ammeter when calibrated to measure current•Called voltmeter when calibrated to measure electric

potential


Electric motor

Earth’s Magnetic Field

• Earth is itself a huge magnet.

• The magnetic poles of Earth are widely separated from the geographic poles.

• The magnetic field of Earth is notdue to a giant magnet in its interior—it is due to electric currents.

• Earth’s magnetic field reverses direction: 20 reversals in last 5 million years.


•Universe is a shooting gallery of charged particles called cosmic rays.

•Cosmic rays are deflected away from Earth by Earth’s magnetic field.

•Some of them are trapped in the outer reaches of Earth’s magnetic field and make up the Van Allen radiation belts


•Disturbances in Earth’s field often allow the ions to dip into the atmosphere, causing it to glow like a fluorescent lamp. Hence the aurora borealis or aurora australis.

conceptual physics chapter 24:...

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