2 Magnets Magnetic forces are known since 6th century B.C. Permanent magnets made from: Co, Ni,Feeach magnet has two poles: (south and north)Like poles repel; unlike poles attractDifference with electric phenomena: the poles of a magnet cannot be separated (no magnetic monopole)Similarity with electric phenomena: magnets are surrounded by magnetic fields- the magnetic field can be detected using a small compass- the compass points in the direction of the field lines- iron filings line up also in the direction of the magnetic field- strength of the magnetic field ~ density of field lines
3 Electric magnetismConnection between electric and magnetic phenomena (1820 Hans Christian Oersted)- current-carrying wire produces a magnetic field- stationary charges do not produce magnetic fieldDirection of the magnetic field produced bycurrents: right-hand ruleDifferent field patterns can be produced by bending the wire,making loopesAll magnetic fields originate from current loops (Andre Ampere)- the currents can be at microscopic level, originating from therotation of elementary charges- the macroscopic magnetic properties of an object are determinedby the combined effects of these atomic level magnets- if the orientation of the atomic magnets is random --> the object has no net magnetization (the same if no atomic magnet exist)- if the atomic magnets are aligned --> the object is magnetized
4 Electromagnetic interactions Interaction between a current-carrying wire and permanent magnetsInteraction between two current carrying wires- the source of the force on each wire is the magnetic field produced by the other- parallel wires attract if current directions are the same, and repel if current directions are opposite- this effect is used to define the unit of current ( Ampere) in the metric system: consider two long parallel wires separated by 1m distance and carrying the same current. If the force between these wires is 2 x 10-7 N on each meter of wire, the current is defined as 1 Ampere.- strength of the magnetic field, units in T (Tesla) or G (gauss)the strength of the magnetic field at a distance of 1m froma long straight wire carrying a current of 1A is 2 x 10-7 T.Interaction between magnetic field and moving charged particlesmagnetic field interacts only with moving charged particlesforce: depends on the velocity of the particle (v), intensity of`the magnetic field field (B), charge of the particle (q), andangle () between field and velocity:F=q v B sin()direction: perpendicular on the plane formed by v and B
5 Magnetic field of the Earth Earth is a giant magnet with its south magnetic pole in the Northern Hemisphereone pole (south) located north of Hudson Bay (Canada)the other pole (north) located directly on the other side of Earthstrength of the magnetic field at the surface of the Earth ~ 0.5 Gmagnetic axes tilted with 12 0 relative to the rotational axisorigin of the Earth’s magnetic field --> still a mystery (modern theories presume that is the result of the convection currents in melted metals in the core, but no accepted theories)Earth’s magnetic field is a shield against high energy and charged cosmic particlesthe strength of the magnetic field is varying--> momentary: decreasingThe Earth switched the poles 171 times during last 17 million yearsDuring switching: long periods without any magnetic field--> danger to living organism.Next disappearance of the magnetic field (prognoses) after 4000 years.
6 Magnetic electricity Electric current produces magnetic field QUESTION: Does magnetic field produce electric current?ANSWER: YES, but not by putting simply the wires in magnetic field!(Michael Faraday, in 1983)In order to produce electric current:- the wire must move relative to the magnetic field- OR the intensity of the magnetic field has to change- OR the or the amount of magnetic field lines surrounded by the wire has to changeDirection of the current given by Lenz’s law:The induced current always produces a magnetic field to oppose the change!
7 Transformers Used to change voltage of alternating currents The same amount of electric power can be delivered through wires at low voltage and high current as at high voltage and low currentUsing low electrical current for electric energy transportation is more efficient: loss of electric energy in the wires smallerTransformer: changes the voltage!Transformers made up by two coils on one iron corefirst coil: primary coil (N1 turns), connected to a source of alternating current (V1)second coil: secondary coil (N2 turns), produces an alternating voltage (V2)principle (the alternating magnetic field produced by the primary coil, induces an alternating voltage in the secondary coil)basic equation:
8 Generators and motorsPrinciple of a generator: rotating loop of wire in magnetic fieldWhile the loop is rotating the number of field lines within the loop changes --> induces voltage between the two ends of the wireThe voltage is an oscillating oneA simple change in the way the voltage is carried to the external circuits (with a commutator) converts the generator to produce pulsating direct currentElectric motor: basically a generator run backward (we apply voltage at the end of the wires, causing the loop to rotate!)
9 A question of symmetry…. Changing magnetic field produces electric current.Changing electric current can produce magnetic field? The answer is yes!!!Maxwell equations: describe mathematically the electromagnetic field as a whole, with connections between them. (James Clerk Maxwell, 1860)Changing magnetic field can produce electric field, and changing electric field can produce magnetic field EVEN in empty space!
10 Electromagnetic waves If the magnetic field changes at a constant rate, the electric field is constantIf the rate of change in the magnetic field varies, this produce a changing electric field, which can produce again a changing magnetic field and so on……--> possibility to propagate the electromagnetic field!!!!Propagating electromagnetic field --> electromagnetic waves (Heinrich Hertz, 1887)In order to generate electromagnetic waves, we need to change constantly the electric or magnetic field at one point of space with non-constant rate!Electromagnetic waves: transverse waves (electric and magnetic field oscillates perpendicularly to each other, and to the direction of the propagation)speed in vacuum: c=3 x 108 m/s;light, together with many other radiation are electromagnetic waves!As a function of the frequency many different waves!
11 Radio and TVRadio: coding electromagnetic waves with the useful information (sound), so they can be transmitted through space, intercepted and converted back into sound.Sound waves are transformed in electrical signals (oscillating electric current)This electrical signal is combined with a broadcast signal or carrying signal (much higher frequency), which is than is amplified and radiated through antennas as electromagnetic wavesElectromagnetic waves travel in vacuum and reaches the radio sets.the frequency of this electromagnetic waves (carrier frequency), depends on the broadcasting station.Combination of the useful signal with the carrying signal called: modulation!Two types of modulation: amplitude modulation (AM) and frequency modulation (FM)Radio set in order to receive themodulated signal, is tuned so it onlyresonates with on carrying frequencyat a given time.The radio filters out the carryingfrequency and retain the electricalversion of the sound informationamplified and reconverted to sound
12 Summarymagnets have two poles; like poles repel and unlike poles attract; poles cannot exist without each-other. Magnets attract some objects and have no effects on othersmagnets have no effect on stationary chargesfield lines can be used to represent magnetic field at all points in spacecurrent-carrying wire produces a magnetic field and interacts with other magnetsall magnetic fields originate from current loops.the Earth’s magnetism is still a mystery, its strength at the surface is ~ 5 x 10-5 Ta charged particle moving in magnetic field experiences force at right angles to its velocity and to the magnetic field.If the number of magnetic lines passing through a loop of wire changes for any reason, a current is produced in the loopConnections between electricity and magnetism are best expressed in terms of the fieldsChanging magnetic field can generate changing electric field, and changing electric field can generate a changing magnetic field, creating electromagnetic wavesElectromagnetic waves are transverse waves, and travel with the speed of lightHome-work: Part I. 542/1-5; 543/6-11,13-17,20; 544/21,22; 546/1-10Part II. 544/27-30,33-38; 545/39-43,45,46,49,50,52,55-57;546/58-62,11-15,19,20; 547/21,22