Monday, February 14, 2011

Electromagnetic Radiation For standard 10 CBSE Course

Review
You have learnt in your previous classes about energy, heat, waves and light. Let us recall some of those concepts.
Energy is the capacity to do work.
 Energy exists in various forms.
 Energy can be transformed from one form to another.
 Wave is a disturbance setup in a medium.
 Energy can be transmitted by wave motion. Wave velocity is the product of frequency and wave length.
 Radiation is a mode of emission of energy.
 Some waves like sound waves require a material medium for their propagation.
 Some waves like light waves do not require any material medium for their propagation.

2.1 INTRODUCTION
The term radiation refers to energy in motion also. The type of radiation for which light, heat, X-rays are examples, is characterized by velocity equal to that of light. This type of radiation is referred to as electromagnetic radiation. It is different from  a-rays and b-rays which are material particles. These have speeds lesser than the velocity of light. We will study about electromagnetic radiations.

2.2 ELECTROMAGNETIC RADIATION
You know that a current (change in motion) produces a magnetic field and a changing magnetic field produces current.
A changing electric field can produce magnetic field and a changing magnetic field can produce electric field. Therefore it should be possible to sustain the electric and magnetic fields jointly.
An alternating electric field produces an alternating magnetic field; this alternating magnetic field would produce and electric field and so on.
We may say that, once started, the two fields sustain each other and a disturbance, that is a wave propagates through the medium. The wave is called electro-magnetic wave. Electromagnetic wave consists of varying electric and magnetic fields in mutually perpendicular planes. The direction of wave propagation is perpendicular to the planes of electric and magnetic fields. Hence electromagnetic wave is a transverse wave (electromagnetic waves can also be considered as waves produced by oscillating or accelerating charges). As characteristic of wave motion, electromagnetic energy called electromagnetic radiation is transmitted in the direction of wave propagation of an electromagnetic wave.

Note
Just as an magnet produces a magnetic field, an electric charge produces an electric field. Motion of charge, that is current, results in a changing electric field and this gives rise to a magnetic field.
James Clerk Maxwell, a Scottish physicist, summed up the researches of Michael Faraday in electricity and magnetism, in a set of equations called Maxwell’s equations. Maxwell showed that these equations represent the propagation of transverse electromagnetic waves and these waves travel with the speed of light.
Heinrich Hertz(1857-1994), a German physicist, succeeded in 1887, in producing electromagnetic wave (wavelength about 10 meters). He showed that these waves travel with the velocity of light.

Varying electric and magnetic fields are represented in the XY plane and XZ plane respectively. X axis represents direction of propagation.
Electromagnetic Wave

 2.3 PROPERTIES OF ELECTROMAGNETIC WAVES

Electromagnetic waves are transverse waves.
 They have electric and magnetic fields as components which vary periodically with time and in space.
 The fields are perpendicular to each other and to the direction of propagation.
 Electromagnetic waves exhibit properties such as reflection, refraction interference and diffraction in common with other forms of waves.
 they have a wide range of frequencies or wavelengths (The velocity c of the waves is given by c= fl, where f= frequency and l= wavelength)
Electromagnetic radiation has particle like properties in addition to those associated with wave motion.
 Radiations of different frequencies react differently with matter.
 Vacuum is the only perfect transparent medium for electromagnetic waves and all other materials media absorb energy strongly from some regions of the entire range of electromagnetic radiations.
 The differences in the properties of the different electromagnetic radiations, are a consequence of their different frequencies.

2.4 ELECTROMAGNETIC SPECTRUM

Light is not he only kind of electromagnetic radiation. Ultra violet rays, infrared rays, X-rays, g-rays and radio waves are also electromagnetic radiations. We are surrounded by a sea of electromagnetic radiations. The entire group is designated as electromagnetic spectrum . They travel through space with the velocity of light(3 X 108 m/s).
Electromagnetic Spectrum








The range of wavelengths of electromagnetic waves varies from about 10-15m to about 100km. the entire range of electromagnetic spectrum is divided into different regions depending on their effects. Each region of the spectrum overlaps the adjoining regions at both ends.
The range of visible light is very narrow, extending from about 400 nm to about 750 nm (4 X 10-7 to 7.5 X 10-7 m)
Beyond the red on the longer wavelength side, are the infrared rays extending from 750 nm to about 0.4 nm
The waves having wavelengths from about 0.4 nm to about 10 cm are called microwaves. The waves with wave lengths greater than 0.1 m are called radio waves and they are classified as short, medium and long waves.
Just beyond the violet on the shorter wavelength side, there are ultraviolet rays, extending  upto 4nm. X-rays occur beyond the ultraviolet and extend upto 0.1 Å . waves in the region with wavelengths from 0.1Å  to 10-2 Å  are called g - rays

Remember
Wavelengths are expressed in submultiples and multiples of meter.
1 nanometer = 1nm = 10-9 m
1 mm = 1 micrometer = 10-6 m
1Å = 10-8 cm = 10-10 m   Å is read as angstrom
Frequencies (cycles/s) are expressed in hertz(Hz)

The approximate range of wavelength and order of frequency of different regions of electromagnetic spectrum are given in the following table.
Wavelength Range













2.5 USES OF ELECTROMAGNETIC RADIATIONS

Light :
Light plays a major role in life. It is very difficult to imagine life and other activities in the absence of light. Although light has a very narrow range of wave length its importance is very high. Almost every one is familiar with the uses of light.

Infrared radiations :
infrared radiation was first detected in 1800 by W. Herschel, by the heating effect of the radiation.
Activity
List the uses of light in daily life and in various devices.
Infrared  rays find various applications.
Infrared spectrum of a compound may be used for identification and in the determination of molecular structure.
 They are found to be most suitable for long distance photography.
 Infrared photography is used in detecting enemy concentration, in examining old paintings and in the detection of forgery of old paintings.
 Infrared search lights and telescopes were used during the second world war.
 In medical field, they are useful in the diagnosis of superficial tumors, dislocations of bones and in the treatment of sprains.
 It stimulates blood circulation.
 The remote and set of a Tv uses infrared radiation to control different settings.
 Infrared radiations from the sun are used in solar energy devices.
Ultraviolet radiation :
Ultraviolet radiation was discovered by J. W. Ritter, in 1801, by its photographic action. Short ultraviolet radiations are harmful to living tissues; these harmful radiations are absorbed by the ozone layer surrounding the earth.
Ultraviolet rays have a verity of applications.
Ultraviolet radiations activate some chemical reactions.
 They excite fluorescence in many substances which led to the development of fluorescent tubes.
 Uv-radiations are used to distinguish between real gems and artificial gems.
 They are used in the treatment of rickets, diseases of the bone and skin diseases.
 Uv-radiations of lower frequencies are useful in the synthesis of vitamin D in our bodies.
They are useful in the operation of photoelectric alarms.

X–rays :
X-rays were discovered by a German scientist William Rontgen in 1985. He as awarded Nobel prize in 1901. X-rays find extensive applications in medicine, industry and scientific research.

Fracture of bones can easily be located by X-ray photograph.
 X-rays are used to locate foreign bodies such as bullets, coins, pins etc., in human body.
 X-rays are used for the treatment of cancer and some skin diseases.
 They are used to detects like cavities in castings and cracks in welding and in locating flaws in the parts of machines and all kinds of transport vehicles. This technique is called radiography.
They are used in the study of crystal structure.

Gamma rays (g-rays) :
Gamma rays were first discovered among the radiations emitted by radioactive nuclei. g-rays like X-rays, find extensive applications.
Gamma rays are used in the treatment of cancer.
 They act as catalyst in the manufacture of some chemicals.
 They are used in g -ray microscopes.
 Gamma rays are used to produce photoelectric effect.
They are used in radiography.
Microwaves :
Microwaves find applications in Radar, Satellite communication and microwave ovens. They are used for experimental purposes.
Radio waves :
As these waves are used in radio and television, they are called radio waves. Radio waves of short wavelength are used in communication systems including satellite systems, in Radars and TV broadcasting. Radio waves of longer wavelengths are used in radio broadcasting.
Do you know
Dr. J.C. Bose (1858-1937) an Indian physicist, had demonstrated the possibility of radio communication. In 1901 Guglielmo Marconi, succeeded in long distance communication across the Atlantic ocean.

2.6 PHOTOELECTRIC EFFECT
Introduction :-
Max Plank’s quantum theory of black body radiation - 1900
The atoms of the black body absorb or emit radiation.
 the atoms absorb or emit radiation as if they are oscillators.
                 (oscillator is a particle which is oscillation)
 Oscillators cannot have any arbitrary energy.
 They can have energies which are integral multiples of hv where v is the natural frequency of the oscillator, h is Planck’s constant, h= 6.63 X 10-27 erg s or 6.63 X 10-34 Js
                i.e.,E = nhv            where n = { 1,2,3,4……….}

Note this
A black body is one which absorbs all the radiation falling on it; radiation emitted by a black body, when hot is called black body radiation.

Dual nature
Radiation sometimes manifests itself as particles and sometimes as waves. This holds good for matter also.


Note:-
According to Planck, radiation is ‘wave like’ in nature & the black body can have only energies that are integral multiples of hv & not fractions of it.
According to Niels Bhor, a system (oscillator) emits energy when it falls from the higher energy state to the lower energy state & absorbs energy when it goes from the lower energy state to the higher energy state. This process of absorption of emission will be through electromagnetic waves or electromagnetic radiation.

Do you know
Photoelectric effect was discovered by  H. Hhertz in 1887. Detailed experimental investigation was done by German physicists W Hallwachs and P. Lenard.

Einstein’s quantum theory of light- 1905
(Albert Einstein 14-3-1879 to 12-4-1955; German physicist; Nobel prize 1921)

Light behaves like a stream of particles.
These particles are called QUQNTA.
Each quantum of light has an energy given by E=Cv,
                where C is a constant & v is the frequency of the radiation.
If Planck’s theory is invoked, then the constant C turns out to be equal to the Planck’s constant h.
Thus Einstein quantized light.
In 1926, American physicist Lewis Gilbert named the ‘light quanta’ as photons.

Properties of Photons
Photons travel at the speed of light in vacuum. i.e., 3 X 108 ms-1.
Photons travel in straight lines(only in a homogeneous medium)
Energy of a photon depends on its frequency. Therefore energy of the photons does not change when it travels from one medium to another.
Photons do not have any change. Therefore they are electrically neutral.

Phenomenon :
Several experiments towards the end of the last century indicated that certain materials(usually metallic surfaces) emit electrons, when exposed to light. This phenomenon of emission of electrons by materials under the action of light is called photoelectric effect. The electrons so emitted are called photoelectrons. Ultraviolet rays, X-rays and g-rays also produce this effect to certain materials.

Following are the experimental facts regarding photoelectric effect.
The phenomenon is instantaneous.
 There is a certain frequency called threshold frequency for radiation, below which no photoelectric effect takes place. Threshold frequency is different for different materials.
 For radiation of a given frequency, the number of photoelectrons released in proportional to the intensity of the radiation. But the kinetic energy of photoelectrons remains the same.
 When radiations of different frequencies are used, the velocity (energy) of photoelectron increases with increasing frequency.
EINSTEIN’S EXPLANATION OF PHOTOELECTRIC EFFECT.
According to Einstein, each quantum of light, i.e., a photon has an energy equal to hv.
It penetrates the metal surface & transfers all its energy to an electron.
Electrons are held inside the metal by internal attractions.
He made the reasonable assumption that the electron must perform some work W to come out of the metal.
If the transferred energy is more than the internal attractions, the electron gets liberated.
Thus one electron is released per photon.
The kinetic energy of the liberated electron is given by
                       mv2 = hv - W, where m is the mass of the electron, v is the velocity of the liberated electron, W is the constant for a given metal. This is the famous Einstein's equation for the photoelectric effect.
We can see clearly that when the frequency v increases, the kinetic energy of the electron increases.
The above equation implies that when hv<W, the kinetic energy of the electron becomes negative which is not physically possible. For frequencies below a threshold equal to        there will be no photoelectric effect.

In the Einstein’s quantum theory, the intensity of a light beam is equal to the number of photons in it. Thus , as the intensity of light increases, the number of photons increases & this in turn increases the number of liberated electrons.
These features are borne out by experiments.
Quantum :    Latin – Quantus
Discrete amount of energy.
Hypothesis : It is an educated guess based on observation.
It is a rational explanation of a single event or phenomenon based on what is observed, but which has not been proved.         
Law :  It is a statement of fact to explain in concise terms, an action or set of actions.                      
Theory :  It is an explanation of set of related observations or events based upon proven hypothesis & verified multiple times by detached groups of researchers.
Arbitrary :   Random, any value, not fixed.
n : Greek alphabet, to be pronounced as nv.

Applications :
The principle of photoelectric effect is used in photoelectric cells to convert light energy into electrical energy. The number of applications is enormous.

They are used in reproduction of sound in cinematography.
They are used in exposure meters.
Photoelectric cells are used for automatic switching on and off of street lights.
They are used in automatic control of traffic signals.
They are used in counting machines.
Photoelectric cells are used in the operation of burglar alarm.
They are used in television transmission.

2.7 LASER
The acronym LASER stands for Light Amplification by Stimulated Emission of Radiation. Laser is a device for producing a highly intense narrow beam of nearly monochromatic light. Laser light can travel large distances without spreading and is capable of being focused to give enormous power density as high as 108 Watt/cm2. power density is the energy incident on unit area in one second.
  When  an electron from an orbit of higher energy (E2) jumps to an orbit of lower energy (E1), a photon of energy = (E2 – E1)= hv is emitted, where v is the frequency of the photon emitted. As this takes place spontaneously, it is called spontaneous emission.
If a photon of proper energy falls on an atom, it may be absorbed completely and an electron of the atom in a lower energy state may be raised to a higher energy state. This process is called excitation.
Electron at initial energy level

Photon
In addition to the above two processes there is another process. An electron in a higher energy level may remain in that level for sometime. If another photon of energy hv=(E2 – E1) is incident on it, then the electron in the higher energy level is made to jump to the lower energy level emitting a photon of exactly the same frequency as the incident photon (figure). This type of emission is called stimulated emission. Incident photon is the stimulating photon and the emitted one due to this, is the stimulated photon. The emitted photon is exactly in phase with the stimulating photon. This makes the laser action possible. Thus, two identical photons are produced in stimulated emission. This process is called light amplification by stimulated emission and laser works on this principle.
Usually, most of the atoms in a system remain in the lowest energy state. If light amplification has to take place, it is necessary to raise a larger proportion of the atoms to higher energy levels. The process of raising atoms from lower energy levels to higher energy levels is called population inversion. Population inversion is achieved by supplying energy from external sources. The process of supplying energy from an external source, to achieve population inversion in a system, is called optical pumping.
Once population inversion is attained to sufficient extent, laser process is started by a stray photon of suitable energy. Photon amplification takes place by stimulated emission. These photons are made to come out in the same direction as a narrow beam.
                A mixture of helium and neon in a definite proportion is filled in a narrow cylindrical glass tube. Two mirrors, one perfectly reflecting and the other partially reflecting, are fixed one at each end of the tube such that their surfaces are perfectly parallel to one another. The mixture of helium-neon in the cylinder is ionized by passing direct current. This provides the energy required for optical pumping and the resulting population inversion.(Figure)
Helium-Neon Gas Laser
  
During stimulated emissions. Photons which travel perpendicular to the mirror surfaces, move to and fro after being reflected by the mirrors. Due to multiple reflections, intensity of light increases. When it reaches a certain level, light comes out continuously through the partially reflecting mirror as a strong narrow beam of monochromatic light.

Characteristics of laser radiation
Laser light is highly directional. It is emitted in a single direction as a narrow beam where as ordinary light spreads.
It is highly monochromatic(It has almost a single wavelength) whereas ordinary light is polychromatic.
Laser light is coherent. The emitted photons are in phase with each other in other words the photons are identical. Ordinary light is incoherent.
Laser light has high intensity than ordinary light.

Uses :
Lasers have a very large  number of applications. Some of them are given below.
       (i)            The distance between two objects can be found accurately using laser reflectors. This technique is known as ‘laser ranging’.
     (ii)            Laser is used in laser Raman spectroscopy to understand the molecular structure of a material.
    (iii)             It is used in laser optical surgery in welding back detached retina into proper position and save eye sight.
   (iv)             It is used in the treatment of dental decay and skin diseases.
     (v)             Laser cutting, drilling and welding have wide ranging industrial applications.
   (vi)             One of the most useful applications of laser is optical communication using optical fibers. This technology has revolutionized the modern communication system.
  (vii)              Laser is used in the measurement of pollutants in the atmosphere.
(viii)             Laser are extensively used in holography and its applications.

Do you know
The distance between the earth and the moon has been calculated using laser ranging technique. In 1969 astronauts of Apollo-11 had left behind laser reflector on moon. A laser light is sent from the earth to the moon. Light reflected by the reflector on the moon is received. Distance to moon can we calculated knowing the time taken for to and fro journey. The distance can be measured to an accuracy of 5 cm in 4 lakh km.
Holography
Holography is a technique which helps in taking complete three dimensional images of a given object or scene. The word ‘holography’ is derived from the Greek words ‘holos’ (the whole or complete) and graphos (writing).

POINTS TO REMEMBER
A varying electric field produces a varying magnetic field and vice-versa. Such a disturbance propagates in a medium in the form of electromagnetic wave.
Electric field and magnetic field are perpendicular to each other and together perpendicular to the direction of propagation.
Electromagnetic wave is transverse in nature.
Electromagnetic radiation is electromagnetic energy.
Electromagnetic spectrum is an arrangement of electromagnetic radiations.
Electromagnetic radiation are used in molecular study, radiography, treatment of cancer etc.,
The phenomenon of emission of electrons by materials under the action of light is called photo electric effect.
Number of photo electrons is directly proportional to the intensity of the radiation.
Velocity of photo electrons increases with increasing frequency.
Photo electric effect was explained by Einstein on the basis of Planck’s quantum hypothesis.
LASER – Light Amplification of Stimulated Emission of Radiation
Laser production involves the following stages.
(a)     Optical pumping
(b)     Population inversion
(c)     Electron cascade process
(d)     Stimulated emission
(e)     Amplification
Laser is used in laser Raman Spectroscopy, optical surgery, optical fibers.

Tag: Electromagnetic, Radiation, Waves, Spectrum, IInfrared, Ultraviolet, Photoelectric, X-rays, Gama-rays, Photons, Laser, 

More Details: Magnetism and Electricity for standard 10 GSEB Course

Saturday, February 12, 2011

Magnetism and Electricity for standard 10 GSEB Course

Review
An electric current passing through a conductor produces a magnetic field around it. This effect of electric current is called magnetic effect.

When a cell is not connected to a load, the potential difference between the poles of the cell is called its electro motive force (emf).

The S,I. unit of potential difference and ‘electro motive’ force is ‘volt’ and the device used to measure them is ‘voltmeter’.

1.1 INTRODUCTION
As you know, a current flowing through a conductor produces a magnetic field around it.
Can we have reverse effect of this?
Can a magnetic field produce an electric field?
Michael Faraday, a British scientist, answered this question through a series of experiments.

1.2 FARADAY’S EXPERIMENTS
Faraday wound a long copper wire, on a cardboard cylinder. Between the turns, he wound twine and between the layers he placed calico-cloth. The ends of the wire were connected to a galvanometer.
Faraday thrust a pole of a bar magnet quickly into the coil. The galvanometer showed the presence of an electric current. He pulled the magnet out of coil. The pointer of the galvanometer deflected, showing the presence of an electric current; but this time the pointer moved in the opposite direction. The amount of deflection was found to increase with the increase in speed of the magnet. He found that when the magnet was at rest inside the coil, no electricity was produced. He repeated the experiment in a different way, moving the coil and keeping the magnet still. The result was the same. Thus, he discovered how magnetism could produce electricity. Relative motion between the conductor and the magnet produces electricity in the conductor.

Food for thought
1.                 Why did Faraday wind twine in between the turns of copper wire?
2.                 Why did he place calico-cloth in between layers?
Faraday's Experiment














Faraday's Experiment Presentation.

When south pole of a magnet approaches the coil the direction of induced current in the coil is clockwise.
When the south pole of a magnet recedes from the coil, the direction of induced  current in the coil is anticlockwise.

MICHAEL FARADAY
(1791 – 1867)
Faraday was born in a poor blacksmith’s family. He has made remarkable discoveries in both physics and chemistry. He was an assistant to Sir Humphery Davy. In 1831, he liquified chlorine and he discovered banzene in 1825. in 1831, he conducted his experiments on electro-magnetic induction. He has formulated the laws of electrolysis also.

Activity
i.Conduct the above experiment by using magnets of different strengths and a coil of insulated copper wire.
ii.Keep the magnet stationary and move the coil.
iii.Increase the number of turns of the coil and repeat the experiment.
iv.Observe the deflections for different speeds of the magnet.
Record your observations in all the situations. What conclusion can you draw out of your observation?

Recall
What is galvanometer?
For what purpose can it be used?

Electromagnetic induction – Meaning :

“When the magnetic field linked with a circuit changes, an electromotive force (emf) will be induced in the circuit. This phenomenon is called electromagnetic induction”.
In a closed circuit an electric current will be produced.
It is called induced current.

 1.3 FACTORS INFLUENCING INDUCED EMF.

Experiments show that the induced e.m.f. in a coil increases with the increase in
       i.            The number of turns of the coil and
     ii.            The rate of change of magnetic field linking with the coil.

1.4 FARADAY’S LAWS OF ELECTROMAGNETIC INDUCTION.

Faraday enunciated the following laws on the basis of his experiments carried out by him.
       i.             First law : -  A changing magnetic field linking a conductor induces an electro motive force in the conductor.
     ii.            Second law : - The induced electromotive force is proportional to the rate of change of magnetic field linking the conductor.

1.5 FLEMING’S RIGHT HAND RULE (DYNAMO RULE)

Fleming’s right hand rule gives the relationship between the directions of magnetic field, induced current and the direction of motion of the coil.
“Arrange the main finger(thumb), the fore finger and the centre finger of the right hand at right angles to each other such that the fore finger (2) indicates the magnetic field and the main finger (1) indicates the direction of motion of the conductor, then the centre finger (3) indicates the current induced.”
FLEMING’S RIGHT HAND RULE (DYNAMO RULE)
1.6 AC DYNAMO

A device that converts mechanical energy into electrical energy using the principle of electromagnetic induction is called a dynamo.

John Ambrose Fleming
(1849 -  1945)
Fleming was a British electrical engineer. He invented diode, which was used as a rectifier in Radio, TV, Telegraph, Radar and other devices. This paved the way for a new branch of physics. He worked as a professor in a university college in London. He is remembered for his contribution in the field of electronics.

A schematic representation of a simple dynamo is shown. A dynamo consists of  a rectangular coil of insulated copper wire ABCD mounted between the poles N and S of  a powerful magnet. The free ends of the copper wire are connected to two copper rings R1 and R2. Two carbon brushes B1 and B2 are touching the rings R1 and R2 respectively.
These brushes are connected to load L in the external circuit. The coil along with the rings is called armature.
A C Dynamo











When the armature is made to rotate, say in clockwise direction, the magnetic field linked with the coil changes. This induces an electric current in the coil ABCD. During the first half of the rotation, the current flows along ABCD. D is connected to R1 which is in contact with B1 . Therefore the current flows in the external circuits from B2 to B1.
A C Dynamo Graph








AC Dynamo Graph full








During  the second half of the rotation of the armature, current is induced in the coil along DCBA. Therefore, in the external circuit current flows from B1 to B2 . In all these cases the direction of induced current is found by applying Fleming’s right hand rule. It is found that the direction of induced current in the external circuit keeps reversing every half a cycle. Such a current is called ‘alternating current’ or ‘AC’. A dynamo that produces an alternating current is called ‘alternating current dynamo’ or ‘AC dynamo’.

1.7 DC DYNAMO

The construction of a DC dynamo is similar to that of an AC dynamo. But instead of full rings as in AC dynamo, two halves S1 and S2 of a copper ring, are used in DC dynamo. This is called ‘split ring’
When the armature is made to rotate in clock wise direction, the magnetic field linked with the coil changes. This induces an electric current in the coil ABCD. During the first half of the rotation of the
armature, the current flows along ABCD. As D is connected to S2 and S2 to B2, the current flows through the load L in the external circuit from B2 to B1.
D C Dynamo










During the second half of the rotation of the coil, the current is induced in the coil in the direction DCBA. As the split half S2 now comes in contact with B1, the current flows in the external circuit in the same direction. This type of current is called ‘direct current’ or DC and the dynamo is called ‘direct current dynamo’ or ‘DC dynamo’.
D C Dynamo Graph








Do you know
The electric current supplied to homes and industries is alternating current.

Discuss with your teacher.
Observe the structure of a dynamo used in a bicycle with the help of a mechanic. There instead of a single coil, a number of coils are used. They are arranged in different planes, but in series, why? What is the reason ? Discuss with your teacher.
  
1.8 FLEMING’S LEFT HAND RULE (MOTOR RULE)

A conductor carrying current when kept in a magnetic field experiences a mechanical force. As a result, the conductor moves or tends to move in the direction of the mechanical force. Mechanical force is highest when the direction of current is at right angles to the direction of magnetic field. Fleming’s left hand rule gives the relation between the directions of electric current, magnetic field and the mechanical force acting on the conductor.
FLEMING’S LEFT HAND RULE (MOTOR RULE)














“Arrange the main finger (Thumb), forefinger and the centre finger of the left hand in such a way that they are perpendicular to one another. If the forefinger indicates the magnetic field and the centre finger indicates the direction of electric current, then the main finger indicates the direction of mechanical force acting on the conductor. (and direction of motion, if the conductor is free to move)”.

Do it your self
Arrange an aluminum rod AB of about 1 or 2 mm thick in between the two poles N and S of a powerful magnet as shown in the fig, with a switch and a DC source connected in the circuit. What do you notice when you switch on? Note the directions of magnetic field, electric current and the mechanical force on th rod. Apply Fleming’s left hand rule to find the relation between these directions. If the current is reversed, the deflection of the rod is also reversed.
Magnetic field direction










1.9 ELECTRIC MOTOR – DC MOTOR.
Electric motor is a device that converts electrical energy into mechanical energy. A motor works on the principle that a conductor carrying current in a magnetic field experiences a mechanical force.
A simple DC motor consists of a rectangular coil of insulted copper wire(ABCD) mounted in between the poles N and S of a powerful magnet. The free ends of the coil are connected to two halves S1 an S2
of a split ring,. The conducting brushes B1 and B2 are in contact with S1 and S2 respectively. These brushes are connected to poles of a battery.

D C Motor










Let an electric current be passed through the coil, say, in the direction ABCD. Mechanical force acts on its limbs in opposite directions. By applying motor rule, the direction of mechanical force can be found. The forces acting on AB and CD, form a couple. Therefore, the coil begins to rotate about its axis. When the coil completes the first half of its rotation, S1 comes in contact with B2 and S2 comes in contact with B1.
Now, electric current flows in the coil in the direction DCBA. Due to the effect of the coupled formed, the coil continues to rotate in the same direction. In the same way, a motor converts electrical energy into mechanical energy. Since it works on direct current, it is called ‘DC motor’.
Motor is used in electric fans, cranes, electric trains, textile machines, and in many other devices.

Remember
Two equal and parallel forces acting on a body at two different points in opposite directions, constitute a couple.

A couple acting on a body tends to rotate the body or actually rotates the body.
In a motor, the coil of wire together with split rings is called ‘armature’. Brushes and the connecting wires together with load form ‘external circuit’.

Activity
Can a DC motor work on AC?
What changes are to be made in it so that it works on AC?
Discuss this with  your teacher


POINTS TO REMEMBER

When the magnetic field linked with a circuit changes, an e.m.f. will be induced in the circuit. This phenomenon is called electromagnetic induction.

The induced emf in a coil increases with the increase in the number of turns of the coil and the rate of change of magnetic field linking with the coil.

A changing magnetic field linking a conductor induces an e.m.f. in the conductor.
The induced emf is proportional to the rate of change of magnetic field linking the conductor.

Dynamo Rule:
Arrange the main finger (thumb), the fore finger and the centre finger of the right hand at right angles to each other such that the fore finger indicates the magnetic field and main finger indicates the direction of motion of the conductor, then th centre finger indicates the current induced.

Dynamo:
A device that converts mechanical energy into electrical energy using the principle of electromagnetic induction is called a dynamo.

Motor Rule:
Arrange the main finger(thumb), fore finger and the centre finger in such a way that they are perpendicular to one another. If the fore finger indicates the magnetic field and the centre finger indicates the electric current, then th main finger indicates the direction of mechanical force acting on the conductor.

Electric motor (DC motor) is a device that converts electrical energy into mechanical energy.

Thursday, February 10, 2011

Magnetic Substances and molecular theory


Magnetic substances
The substances that are attracted by a magnet are called magnetic substances. Example : Iron, nickel, cobalt, steel etc.

Nonmagnetic substances :
The substances that are not attracted by a magnet are called nonmagnetic substances. Example : Paper, glass, water, common salt, plastic etc.
In a sense, all substances have magnetic properties. Howerer the degree of magnetism exhibited by them differs. Based on their properties when brought under the influence of powerful magnetic field, They are classfied into three groups –
Diamagnetic,
Paramagnetic and
Ferromagnetic substances.

Do You Know ?
Experiments have revealed that the ability of paramagnetic substance to get magnetised goes on increasing as we go on cooling it. This abillity goes on decreasing as its temperature increases and at one specific temperature it altogether vanishes, Can you guess why ?

A diamagnetic material shows no magnetic peoperty. For example, bismath. Antimony. Lead, gold and water, Diamagnetic materials show feeble magnetism when placed in a strong magnetic field.
Paramagnetic materials show some magnetism in a magnetic field. Examples are aluminium, platinum. Chromium, copper and sulphate.
Ferromagnetic materials have good magnetic peoperty and are easily magnetized by external magnetic fields Examples are iron, nickel and cobalt.

Molecular theory
Molecular theory is the first theory which made an attempt ot explain the phenomenon of magnetism. It attempted to find the reason for the magnetic property of substances at the molecular level. According to this theory, every molecule of a magnet behaves like amagnet. Every molecular magnet has two poles, namely, north pole and south pole, Molecular magnets have the ability to rotate around their centres. All substances, even the unmagnestised ones are made up of such molecules.
How does molecular theory explain the properties of a magnet ?

Observation : Certain substances do not exhibit magnetic peoperties whereas some other substances do.

Explanation : All substances are made up of molecules that behave like magnets. In nonmagnetic substances, the molecules are scattered rendomly in all directions. Hence the effect of poles of molecular magnets cancel each other. Hence they do not exhibit magnetic properties. In a magnet, all ther molecular magnets are arranged in the same direction parallel to each other. Hence the effect of poels of molecular magnets add up. Therefore magnets exhibit magnetism.
Molecular Theory
 Explanation : Whenever a magnet a dropped, the uni-directional arrangement of molecular magnets gets disturbed. Kinetic energy of molecular magnets increases when heated. Therefore, when heated, the molecular magnets attain a disorderly arrangement.
The knowledge of magnetism has gone beyond the framework provided by molecular theory. Now we know that it is more complicated than what it was thought to be. It is now established that each and every proton and electron of an atom has magnetic property. The molecular magnetism is a resultant of all these magnetic properties. Yet, the molecular theory has not lost its importance. Why ? It is because,it interprets most of the phenomena that are associated with magnetism.