Fields - Magnetic Fields

The Law of Magnetic Poles

• Opposite magnetic poles attract

• Like magnetic poles repel.

Magnetic Force Field

• The area around a magnet in which magnetic forces are exerted

• The magnetic field at any point is a vector quantity, represented by 𝛽 Ԧ.

Principle of Electromagnetism

Moving electric charges produce a magnetic field

• was an accidental discovery

Magnetic Fields Around Bar Magnets

Magnetic field lines are ALWAYS drawn from North Pole → South Pole.

Permanent Magnets

Magnetic field lines always form closed loops – you can never have just a south pole or just a north pole.

• the field is the strongest at the poles (the ends)

Domain Theory of Magnetism

tiny magnetically homogeneous regions or “domains” behave based on e- spin

• if all e in all the domains spins in the same dir in the structure of a material, the material is magnetic

Earth’s Magnetic Field

• The convention is to refer to the south magnetic pole as Earth’s “north magnetic pole.’

• Geological studies show that Earth’s magnetic field has completely reversed direction many times during the planet’s history.

right hand rule for solenoids and straight conductors

see notes/watch video

current

charges per time measured in amperes w base units of C/s

current convention

conventional current (flow of positive charges flow from pos to neg) based on orig definition of current b4 scientists det it was the flow of electrons

magnetic field of a current loop

the magnetic field is very strong inside the loop but weak outside.

solenoid

long conductor wound into a coil of many loops that produces a strong magnetic field inside the coil.

electromagnet

a magnet produced from electric current

electromagnets cont’d

• Centers of solenoids are often filled with iron to strengthen 𝛽

• Adding carbon to iron “locks” domains in place, to create permanent magnets.

• Electromagnetics can be turned ON/OFF using electricity.

2D diagrams of currents

dot → going out of page

x → going into page

applications of magn fields

an MRI machine uses magnetic fields that are 60,000x stronger than the Earth’s magnetic field to obtain images of the human body

magn field stren unit

tesla (T)

T = kg / C * s

magn force

• non-contact

• if a current (moving charges) produces a force on a magnet, then from Newton’s 3rd Law the magnet must produce an equal but opposite force on the current.

• act on e and protons and ions

• only CHARGED particles moving thru a mag field experiences a mag force

comparing magnetic force w grav force and elec force

• Fm is dependent on velocity of the charge, grav and elec are independent of the v of the mass or charge

magn force is non-zero only if

the charge is moving w velocity

when the angle between magn field stren and the v = 90

Fm is a max

when the angle between magn field stren and the v = 0 or 180

Fm = 0 bc the particle is moving in the dir of the magn field

right hand rule for a moving charge in a magn field

If you point your thumb in the direction of the velocity of the charge v, and your straight fingers in the direction of the magnetic field 𝛽, then your palm will point in the direction of the resulting magnetic force Fm.

• when q is pos, Fm pts out of palm

• when q is neg, Fm pts into palm

motor principle

a magn field can exert a force on a MOVING CHARGE or CURRENT CARRYING CONDUCTOR

• this force is known as the lorentz force

magn of Fm depends on

the field stren

I =

current, which is charge over time

I is perpen to field and perpen to force

electric and magn fields can

induce each other

reversing either the current dir or magn field reverses the

direction of the force

a magn field does not

exert a force on a current moving || to the dir of the B

a mass spectrometer is used to

accelerate particles

when launched into a magn field, a moving particle starts undergoing

uniform circ motion

faraday’s iron ring

• To improve his results, Faraday built an “iron ring” to increase the magnetic field inside the solenoids

• Current still occurred only when current was switched ON/OFF

Law of Electromagnetic Induction

A magnetic field that is moving or changing in intensity in the region around a conductor causes or induces charge to flow in the conductor.

when a current is steady, the magn field is

const; so no current is induced in secondary circuit

the greater the change in magn field,

the larger the induced current.

A constant magnetic field ______ induce a current.

does not; A changing magnetic field induces electric current.

faraday’s principle involves

Moving magnetic field around a conductor

permanent magnet

pnj made from a material that is magnetized and creates its own persistent magnetic field

how whow to tell when an obj is magnetized

domains are lined up (unmagnetized is when they are not aligned)

how earth’s magnetic field contributes to aurora borealis

energetic electrically charged particles (mostly electrons) accelerate along the magnetic field lines into the upper atmosphere, where they collide with gas atoms, causing the atoms to give off light.

magnetic field lines in a solenoid

MAGLEV TRAIN

magnetic forces to lift, propel, and guide a vehicle over a track, extremely fast due to the lack of friction, currents are turned on and off

current is measured in

amperes (C/s)

current is a

vector

V increases if

work is done to move a charge against its natural tendency

Ee / q