Magnetism, electromagnetic induction, alternate current

Talián Csaba Gábor PTE Med. School Inst. of Biophysics 06.11.2014.

Empirical knowledge •

Some iron ores attract other metal pieces

•

China – –

e g. magnetite (Fe2+Fe23+O4) –

–

Earth-divining (→ Feng-shui) How many needles can it lift → quality categories Moving magnets always take one direction: navigation, difference of magnetic and astronomic Earth poles VII-X. century: compass

Magnesia, Greek city in Asia Minor

•

England, ~ 1600

Earth is a big magnet

William Gilbert (1544-1603, ENG)

•

Natural magnetic property can be transmitted to another steel object by touch (artificial magnet)

•

Magnet rod attracts other objects with its ends the most, barely with its middle part magnetic poles (pointlike impact centers), magnetic axis (Poles from the end of the rod ~ 1/12 distance)

•

Magnet freely rotating around an axis takes always the same direction → the poles are unlike („positive” and „negative”)

•

Unlike poles attract, like poles repel one another repulsion

attraction

•

Two magnets positioned with their unlike poles cancel each other

repulsion

Magnetic influence •

Softh iron

temporary magnet

•

Steel

permanent magnet

•

Magnet is like electric dipoles magnetic momentum:

m = p*l

(p: pole strength; an analogon to electric charge, a quantity proportional to the force/effect exerted by the magnet)

• •

By division new dipoles can be made The strength of the new pole equals the original one

Elementary, separable magnetic monopoles, „charges” do not exist (theoretically predicted, but not found)

•

A metal rod contains many unoriented elementary magnets that cancel the effect of the others they can become oriented by an outer magnetic impact like a chain of electric dipoles orientation can remain or leave off

+

-

+

-

+

-

Magnetic force •

Model of the „magnetic charge”: one end of a very long and thin magnet rod

~ Coulomb’s law:


=  ∙

 ∙  

 =  ∙

 ∙  ∙ 

p1, p2: magnetic pole strength, measure: V*s r: distance(vector)

10  ∙  1 = = (4 ∙ )  ∙  4 ∙  ∙ 

 = 1,257 ∙ 10

∙ ∙

(induction constant, permeability)

Magnetic field •

Every moving charged particle creates a magnetic field In an unorganized physical body they cancel each other

•

Magnetic field (force) exerts torque on other magnets in it: If a magnetic dipole stands in the direction of the outer magnetic then Mf = 0, so it gives the direction of the field at that place

•

Magnetic field lines run from the northern to the southern pole Magnetic field strength points in a tangential direction of the field lines

•

Magnetic field strength I.



!" = #

H=

!$ %

(A/m)

Magnetic field strength II. (magnetic inducion vector ) (induction density):

B

Measure: Tesla (Vs/m2 = N/Am = 104 Gauss)

Nicola Tesla (1856-1943, SRB-USA)

Magnetic field of the electric current •

Strong direct current through a conductor makes the compass needle turn (Ørsted, 1820) – – – –

Electric current creates a magnetic field (Ampere’s law) Every moving charge in general Every substance has its magnetic field Reverting the direction of the current the shift of the pointer is opposite

Hans Christian Ørsted (1777-1851, DEN)

Nearly homogenous magnetic fields

& = 

' 2π

)' &   *

' &   2

&  

)' 2

North északi pole pólus

I

Righ hand rule

Effect of the magnetic field on conductors • •

On conductors carrying current a force is acting in a magnetic field Proportional to the angle between the direction of the current and the magnetic field strength (ϕ)

 '*&+,-

./  '01&+,-

1. EXERCISE: MAGNETIC FIELD OF THE ELECTRIC CURRENT In a coil the number of turns is 13, the distance betwen them is 0,4 cm. A wire frame of 8 cm edge length is located in the coil, parallel to the longitudinal axis. If 1,5 A current is coupled on both the coil and the frame, what is the torque acting on the frame?

)' &   * 

&  1,257 ∙ 10

 ∙  13 ∙ 1,5 ∙  5,1 ∙ 104 5  ∙  0,048

./  '01&+,-

./  1,5 ∙ 0,08



∙ 5,1 ∙ 104 5  4,9 ∙ 10 ) ∙ 

& = 

' 2 

  ' *&

Modern definition of current(1948)

' ' *

   2 

Effect of the magnetic field on moving charges •

Lorentz’s force:


= 7[9:]

An opposite direction on negative charges!

Hendrik Lorentz (1853-1928, NED) B and v perpendicular

not perpendicular

Devices working by the magnetic effect of current •

Galvanometer

DC

Ammeter with a turning coil

also AC

Electrodynameter

also AC

•

Electromagnet

•

Electric bell

Loudspeaker

Relay and interrupter

Cathode ray tube, television

Mass spectrometer

Electronmicroscope

2. EXERCISE: LORENTZ’S FORCE What is the radius of orbit of an electron accelerated to 1 eV kinetic energy in a magnetic field perpendicular to its direction of movement, if the magnetic field strength is 0,5 T? The mass of the electron is 9,11·10-31 kg.  ∙