From the simplest touch to the most complex chemical reaction, electromagnetism is at work. From the most rudimentary compass to the most advanced supercomputer, electromagnetism is at work. From the magnetosphere's deflection of cosmic rays to the ionosphere's reflection of radio-waves, electromagnetism is at work.
Michael Faraday and Heinrich Henry performed lots of experiments to learn about the connection between electricity and magnetism. The results of these experiments have led to the life styles of today's human whose life is made easy by using lots of electrical applications , that use the basic principles of Electromagnetic Induction and Electromagnetic Radiation.
When current passes through a conductor, magnetic field will be generated around the conductor and the conductor become a magnet. The magnetic field disappears when the current ceases. This phenomenon is called electromagnetism. Since the magnet is produced by electric current, it is called the electromagnet. An electromagnet is a type of magnet in which the magnetic field is produced by a flow of electric current.
MAGNETIC FIELD PATTERN
The magnetic field forms by straight wire are concentric circles around the wire as shown in figure (a) above. When the direction of the current is inversed, the direction of the magnetic field line is also inversed
ELECTROMAGNETIC INDUCTION
Electromagnetic induction is the production of voltage across a conductor situated in changing magnetic field or a conductor moving through a stationary magnetic field.
FARADAY'S EXPERIMENT
Faraday took a ring of soft iron. On one side of it, an insulated conducting coil was connected with a battery. On the opposite side, another conducting coil was connected with a galvanometer. Faraday observed that passing a steady current through the left coil produced no effect on the galvanometer in the right coil. However, a momentary deflection of galvanometer was noticed whenever the battery was switched on or off. When a steady current is passed magnetic flux produced in the left coil passes through the right coil which does not produce any current in it. Whenever the battery is switched on or off, magnetic flux in the right coil changes from zero to maximum or maximum to zero respectively. This rate of change of magnetic flux in the right coil produces current in it.
EXPERIMENT - 2
Faraday arranged two bar magnets in the shape of V. At the open end of V, he kept one soft iron rod with an insulated copper wire wound around it to which galvanometer was connected. On moving the upper magnet up and down, galvanometer showed deflection. Magnetic flux through the coil increased when the magnet touched the iron rod and decreased when it moved away. Faraday concluded from these experiments that 'To produce electric field in a coil, the change in magnetic flux is important and not the flux itself.'
FARADAY'S CONCLUSIONS
More current is produced when the magnet is moved faster due to faster change of magnetic flux linked with the coil.
When a coil carrying electric current is placed above another coil and relative motion produced between the two coils, galvanometer shows deflection in the other coil.
If any of the two coils is rotated with respect to the other, then also galvanometer shows deflection
If the north pole of a bar magnet is moved towards a coil, the galvanometer shows deflection. Now if the magnet is moved away from the coil, the galvanometer shows deflection in the opposite direction. Similar results are obtained with the south pole of the magnet with deflections of galvanometer in opposite direction.
"The induced emf ε in a coil is proportional to the negative of the rate of change of magnetic flux."
DIRECTION OF INDUCED E.M.F - LENZ LAW
The direction of the induced current is determined by Lenz's law:
"The induced current produces magnetic fields which tend to oppose the change in magnetic flux that induces such currents."
Mathematical Interpretation of Faraday's Law and Lenz Law
MAXWELLS EQUATIONS AND HERTZ DISCOVERIES
In his unified theory of electromagnetism, Maxwell showed that electromagnetic waves are a natural consequence of the fundamental laws expressed in the following
four equations
He proposed that the phenomenon of light is therefore an electromagnetic phenomenon. Because charges can oscillate with any frequency, Maxwell concluded that visible light forms only a small part of the entire spectrum of possible electromagnetic-radiation.
The first clearly successful attempt was made by Heinrich Hertz in 1886 confirming that electromagnetic phenomena propagate through free space and dielectrics as waves travelling with the velocity of light (rather than instantaneously). The implications of this discovery have been awesome, both in science and in technology.
HERTZ EXPERIMENT
The experimental apparatus that Hertz used to generate and detect electromagnetic waves is shown below, an induction coil is connected to a transmitter made up of two spherical electrodes separated by a narrow gap. The coil provides short voltage surges to the electrodes, making one positive and the other negative. A spark is generated between the spheres when the electric field near either electrode surpasses the dielectric strength for air. In a strong electric field, the acceleration of free electrons provides them with enough energy to ionize any molecules they strike. This ionization provides more electrons, which can accelerate and cause further ionizations. As the air in the gap is ionized, it becomes a much better conductor, and the discharge between the electrodes exhibits an oscillatory behaviour at a very high frequency. From an electric-circuit viewpoint, this is equivalent to an LC circuit in which the inductance is that of the coil and the capacitance is due to the spherical electrodes.
Because L and C are quite small in Hertz's apparatus, the frequency of oscillation is very high, _100 MHz. Electromagnetic waves are radiated at this frequency as a result of the oscillation (and hence acceleration) of free charges in the transmitter circuit. Hertz was able to detect these waves by using a single loop of wire with its own spark gap (the receiver). Such a receiver loop, placed several meters from the transmitter, has its own effective inductance, capacitance, and natural frequency of oscillation.
In his experiment, sparks were induced across the gap of the receiving electrodes when the frequency of the receiver was adjusted to match that of the transmitter. Thus, Hertz demonstrated that the oscillating current induced in the receiver was produced by electromagnetic waves radiated by the transmitter. His experiment is analogous to the mechanical phenomenon in which a tuning fork responds to acoustic vibrations from an identical tuning fork that is oscillating.
Additionally, Hertz showed in a series of experiments that the radiation generated by his spark-gap device exhibited the wave properties of interference, diffraction, reflection, refraction, and polarization, all of which are properties exhibited by light. Thus, it became evident that the radio-frequency waves Hertz was generating had properties similar to those of light waves and differed only in frequency and wavelength. Perhaps his most convincing experiment was the measurement of the speed of this radiation. Radio-frequency waves of known frequency were reflected from a metal sheet and created a standing-wave interference pattern whose nodal points could be detected. The measured distance between the nodal points enabled determination of the wavelength. Hertz found that v was close to 3 _ 108 m/s, the known speed c of visible light.
A large oscillator (bottom) and circular, octagonal, and square receivers used by Heinrich Hertz
Features of Electromagnetic Waves
Any wave transfers energy from a point where there is a disturbance. Drop a stone into water, and you will see the waves moving away from where the stone fell in (the disturbance).
Electromagnetic waves are like water waves. They transfer energy from a source as waves. They have an electrical component and a magnetic component.
All electromagnetic waves travel at the speed of light.
Electromagnetic waves travel in straight lines.
Unlike other types of wave, electromagnetic materials do not need a material to travel through. They travel in a vacuum, which is why we see light from the Sun, but don't hear its roar.
All waves have a frequency. This means the number of waves per second. Frequency is measured in Hertz (Hz).
All waves have a wavelength.
UNDERSTANDING OUR PHYSICAL WORLD THROUGH ELECTROMAGNETISM
DYNAMO THEORY
The dynamo theory proposes a mechanism by which a celestial body such as the Earth or a star generates a magnetic field. The theory describes the process through which a rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time. This theory is used to explain the presence of anomalously long-lived magnetic fields in astrophysical bodies. Maxwell's equations are the basis of electromagnetic theory, and so they are the foundation of dynamo theory.
EARTH'S ROLE AS AN ELECTROMAGNET
Earth is largely protected from the solar wind, by its magnetic field, which deflects most of the charged particles. Some of the charged particles from the solar wind are trapped in the Van Allen radiation belt. A smaller number of particles from the solar wind manage to travel, as though on an electromagnetic energy transmission line, to the Earth's upper atmosphere and ionosphere in the auroral zone. The solar wind is responsible for the overall shape of Earth's magnetosphere, and fluctuations in its speed, density, direction, and entrained magnetic field strongly affect Earth's local space environment.
IONOSPHERE'S (EARTH'S ELECTROMAGNETIC MIRROR) REFLECTION OF RADIO WAVES
If we were limited to line-of-sight communications, long distance wireless communication, like ship-to-shore communication, would be impossible. At the turn of the century, Marconi, the inventor of wireless telegraphy, boldly tried such long distance communication without any evidence - either empirical or theoretical - that it was possible. When the experiment worked, but only at night, physicists scrambled to determine why (using Maxwell's equations, of course). It was Oliver Heaviside, a mathematical physicist with strong engineering interests, who hypothesized that an invisible electromagnetic "mirror" surrounded the earth. What he meant was that at optical frequencies, the mirror was transparent, but at the frequencies Marconi used, it reflected electromagnetic radiation back to earth. He had predicted the existence of the ionosphere, plasma that encompasses the earth at altitudes
between 80 and 180 km that reacts to solar radiation: It becomes transparent at Marconi's frequencies during the day, but becomes a mirror at night when solar radiation diminishes. The maximum distance along the earth's surface that can be reached by a single ionospheric reflection which ranges between 2,010 and 3,000 km when we substitute minimum and maximum ionospheric altitudes.
IMPORTANCE OF ELECTROMAGNETIC INDUCTION
Electromagnetic induction is an incredibly useful phenomenon with a wide variety of applications. Induction is used in power generation and power transmission,
ELECTRIC MOTOR - A electric motor is a device for transforming electrical energy into mechanical energy; an electric generator does the reverse, using mechanical energy to generate electricity. At the heart of both motors and generators is a wire coil in a magnetic field. In fact, the same device can be used as a motor or a generator. When the device is used as a motor, a current is passed through the coil. The interaction of the magnetic field with the current causes the coil to spin. To use the device as a generator, the coil can be spun, inducing a current in the coil.
A.C GENERATOR - An AC (alternating current) generator utilizes Faraday's law of induction, spinning a coil at a constant rate in a magnetic field to induce an oscillating emf. The coil area and the magnetic field are kept constant. In other words, a coil of wire spun in a magnetic field at a constant rate will produce AC electricity.
D.C GENERATOR - A DC generator uses the same kind of split-ring commutator used in a DC motor. Unlike the AC generator, the polarity of the voltage generated by a DC generator is always the same. In a very simple DC generator with a single rotating loop, the voltage level would constantly fluctuate. The voltage from many loops (out of synch with each other) is usually added together to obtain a relatively steady voltage. Rather than using a spinning coil in a constant magnetic field, another way to utilize electromagnetic induction is to keep the coil stationary and to spin permanent magnets (providing the magnetic field and flux) around the coil. A good example of this is the way power is generated, such as at a hydro-electric power plant. The energy of falling water is used to spin permanent magnets around a fixed loop, producing AC power.
MUTUAL INDUCTANCE - Faraday's law tells us that a changing magnetic flux will induce an emf in a coil. If the first coil has a current going through it, a magnetic field will be produced, and a magnetic flux will pass through the second coil. Changing the current in the first coil changes the flux through the second, inducing an emf in the second coil. This is known as mutual inductance, inducing an emf in one coil by changing the current through another. The induced emf is proportional to the change in flux, which is proportional to the change in current in the first coil.
TRANSFORMERS - Electricity is often generated a long way from where it is used, and is transmitted long distances through power lines. Although the resistance of a short length of power line is relatively low, over a long distance the resistance can become substantial. A power line of resistance R causes a power loss of I2R ; this is wasted as heat. By reducing the current, therefore, the I2R losses can be minimized. At the generating station, the power generated is given by P = VI. To reduce the current while keeping the power constant, the voltage can be increased. Using AC power, and Faraday's law of induction, there is a very simple way to increase voltage and decrease current (or vice versa), and that is to use a transformer.
A transformer is made up of two coils, each with a different number of loops, linked by an iron core so the magnetic flux from one passes through the other. When the flux generated by one coil changes (as it does continually if the coil is connected to an AC power source), the flux passing through the other will change, inducing a voltage in the second coil. With AC power, the voltage induced in the second coil will also be AC.
In a standard transformer, the two coils are usually wrapped around the same iron core, ensuring that the magnetic flux is the same through both coils. The coil that provides the flux (i.e., the coil connected to the AC power source) is known as the primary coil, while the coil in which voltage is induced is known as the secondary coil. If the primary coil sets up a changing flux, the voltage in the secondary coil depends on the number of turns in the secondary. A transformer in which the voltage is higher in the primary than the secondary is known as a step-down transformer. A transformer in which the secondary has more turns is known as a step-up transformer.
Power companies use step-up transformers to boost the voltage to hundreds of kV before it is transmitted down a power line, reducing the current and minimizing the power lost in transmission lines. Step-down transformers are used at the other end, to decrease the voltage to the 120 or 240 V used in household circuits.
IMPORTANCE OF ELECTROMAGNETIC RADIATION
ELECTROMAGNETIC SPECTROSCOPY - Electromagnetic spectroscopy a.k.a. spectrophotometry is the spectroscopy of electromagnetic spectra which arise out of atoms absorbing and emitting quanta of electromagnetic radiation.
MICROWAVES - Microwaves have wavelengths that can be measured in centimeters.The longer microwaves, those closer to a foot in length, are the waves which heat our food in a microwave oven. Microwaves are good for transmitting information from one place to another because microwave energy can penetrate haze, light rain and snow, clouds, and smoke. Shorter microwaves are used in remote sensing. These microwaves are used for radar like the Doppler radar used in weather forecasts. Microwaves, used for radar, are just a few inches long.
RADIO WAVES - Radio waves have the longest wavelengths in the electromagnetic spectrum. These waves can be longer than a football field or as short as a football. Radio waves do more than just bring music to your radio. They also carry signals for your television and cellular phones. The antennae on your television set receive the signal, in the form of electromagnetic waves which is broadcasted from the television station. It is displayed on your television screen.
X-RAYS IN MEDICINE -
1. X-rays are being widely used for detecting fractures tumors the presence of foreign matter like bullet etc., in the human body
2. X-rays are also used for the diagnosis of tuberculosis stones in kidneys gall bladder etc.
3. Many types of skin diseases malignant sores cancer and tumors have been cured by controlled exposure of X-rays of suitable quality
4. Hard X-rays are used to destroy tumors very deep inside the body
Industrial application of x-rays.
X-RAYS IN CHEMISTRY -
1. X-rays are used to detect the defects or flaws within a material.
2. X-rays can be used for testing the homogeneity of welded joints insulating material etc.
3. X-rays are used to analyse the structure of alloys and the other composite bodies.
4. X-.rays are also used to study the structure of materials like rubber cellulose plastic fibres etc.
GAMMA RAYS - These are the most energetic photons, having no defined lower limit to their wavelength. They are useful to astronomers in the study of high energy objects or regions, and find a use with physicists thanks to their penetrative ability and their production from radioisotopes. Gamma rays are also used for the irradiation of food and seed for sterilization, and in medicine they are used in radiation cancer therapy and some kinds of diagnostic imaging such as PET scan.
ULTRAVIOLET RAYS - Used in forensics, security, fluorescent lamps, Astronomy, Hunting, Biological surveys, Pest control etc.
Summing up Hertz's importance: his experiments would soon trigger the invention of the wireless telegraph and radio by Marconi and others and TV.
