Physics Kenya
Electromagnetic Spectrum
Electromagnetic Spectrum
Electromagnetic Spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation.The "electromagnetic spectrum" of an object has a different meaning, and is instead the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object
The electromagnetic spectrum extends from below the low frequencies used for modern radio communication to gamma radiation at the short-wavelength (high-frequency) end, thereby covering wavelengths from thousands of kilometers down to a fraction of the size of an atom. The limit for long wavelengths is the size of the universe itself, while it is thought that the short wavelength limit is in the vicinity of the Planck length.
Properties of electromagnetic spectrum
- It is continuous i.e each band merges into the next and there are no gaps in the frequencies. The different kinds of radiation gradually change from one to another as their properties also gradually change.
- In some case, there is an overlap in the range of wavelength. This is because sometimes the name given to the wave(radiation) is determined by the source and not the wavelength(or frequency) for example x-rays and γ -rays.
Properties of electromagnetic waves
Detection of electromagnetic (e.m) radiations
Infrared waves
Infrared radiation is invisible to the human eye. However any nocturnal animal can “see” infrared radiations. We can sense infrared radiations as heat. Devices used to detect infrared radiations include black bulb thermometers, photographic films, thermistors and phototransistors.
Visible light
We see things because they either emit visible light or because they reflect visible light from another source.
Ultraviolet light.
Ultraviolet is light is detected by Photographic films; Fluorescent materials
Applications of e.m radiations ( include greenhouse effect)
Radio waves
- Fixed and mobile radio communication, broadcasting, radar and other navigation systems, communications satellites, computer networks and innumerable other applications.
- Astronomers use large radio telescopes to collect and study radio waves from distant stars and galaxies. This helps them to determine composition, structure and motion of the celestial bodies.
Microwaves
- In cooking
- Radar systems
- Long distance communication
Gamma rays
- Medical application the same as x-rays
- In agriculture
X-rays
- X-ray photography
- Diagnosis and treatment of cancer
Electromagnetic radiations are very useful in agriculture and climate as they support photosynthesis, the main source of oxygen in the atmosphere. Gamma rays are used for pest control by sterilising male pests with gamma radiation then releasing them into the wild, as is done to control tsetse flies. Gamma rays are used to obtain disease-resistant plants. Ultraviolet can purify water and air.
Problems involving c = fλ
Simple experiments to illustrate electromagnetic induction
Induced emf: Faradays' law
Lenz’s Law
This explains the direction of the induced e. m. f and it states that: “The direction of the induced e. m. f is such that the resulting induced current flows in such a direction that it opposes the change that causes it.”
Faraday’s Law.
It relates the magnitude of induced e. m. f and the rate of change of the magnetic flux linking the conductor. The magnitude of the induced e. m. f depends on:
- the strength of the magnetic field.
- the rate of change of the magnetic flux(speed of motion)
- the area of the conductor that is in the magnetic field.
Faraday’s law states that: “The e. m. f induced in the conductor in a magnetic field is proportional to the rate of change of magnetic flux linking the conductor.”
Induced emf: Lenz's law
Mutual Induction
When the current flowing through a conductor varies it creates a varying magnetic field that cuts across the conductor itself.
This results to self-induced e. m .f in the conductor that is opposite in direction to the original e. m. f. This voltage, usually referred to as back e. m .f, tends to limit or reverse the original current.
If the original current is increasing, then the induced current subtracts from it and then measured current is smaller than it would be if no self-induced magnetic field was produced in the conductor. If original current is decreasing, then the original current adds to it and the measured current is greater than it would be if self-induced magnetic field was produced in the conductor. This process is called self-induction since the changing current creates a back e. m. f in itself.
Consider a coil of wire wrapped around a cardboard tube. When the coil is connected to a battery a current flows in the coil producing a magnetic field as shown below.
If the current in the coil begins to increase, the magnetic flux increases. This induces an e. m. f that opposes the battery resulting in a back current that impedes the increase in current. If the current in the coil decreases, the magnetic flux decreases. This induces an e. m .f that adds to the battery resulting in an induced current that impedes the decrease in current.
If we place two coils near each other, a varying current in one coil will induce a current in the other. This is called mutual induction. The coil with a changing current is referred to as the primary coil while that in which a current is induced is the secondary coil.
The e.m.f induced in the secondary coil is proportional to the rate of change of the current in the primary coil.
Alternating current generator, direct current generator
A generator is a device which produces electricity on the basis of electromagnetic induction by the continuous motion of either a coil or a magnet.
A.C Generator or alternator
An arc generator utilizes Faraday’s law of induction, spinning a coil at a constant rate in a magnetic field to induce an oscillating a.m.
The arc generator consists of an armature made up of several turns of insulated wire wound on a soft-iron core. The armature revolves freely on an axis between the poles of a powerful magnet, which provides a strong magnetic field. Two slip rings are connected to the ends of the armature and two carbon brushes rest on the slip rings.
When the coil is vertical, no cutting of the magnetic lines of force takes place although the number of lines linking the coil is maximum. The rate of change magnetic flux is zero and as a result, no a.m. is induced in the coil.
When the armature is parallel to the magnetic field, the rate of change of magnetic flux is maximum and the motion of the coil is perpendicular to the magnetic field, hence an a.m. is induced along the sides of the coil.
After a 180° turn, starting from the vertical position, the sides of the loop interchange and the current in the loop is reversed. This means that the a.m. is positive for one half of the cycle and negative for the half. The maximum induced a.m. is at 90° rotation from the vertical position and the minimum is at 270° rotation. If there is an external circuit, the current through it would also have a maximum value at 90° and minimum at 270°.
This kind of current is called an alternating current and the corresponding a.m. is the alternating e.m.f.The number of cycles produced per second is called the frequency of the arc. The arc obtained is led to an external circuit through the slip rings and the carbon brushes.
D.C generator
It is made by replacing the slip rings in the arc generator with a commentator. Each half of the commentator ring is called a commentator segment and is insulated from the other half. Each end of the rotating loop of the wires connected to a commentator segment. Two carbon brushes connected to the outside circuit rest against the rotating commentator.
In the deck generator, the commentator rotates with the loop of wire, just as the slip rings do with the rotor of an arc generator.
When the loop is rotated in the magnetic field, the induced e.m.f is still in alternating form. However after rotation of 180° instead of the current reversing, the connections to the external circuit are reversed so that the current direction in the external circuit remains the same.
The output of a d.c generator is shown below.
Note:The lower half of the cycle is not cut off but is reversed.
Fleming's right hand rule
Transformers
A transformer is a device that uses mutual induction between two coils to convert an a.c across one coil to a larger or smaller a.c across the other coil.
A transformer is made up of two coils, each with a different number of loops linked by an iron core so that the magnetic flux from one passes through the other. When the flux generated by one coil changes the flux passing through the other will change, inducing a voltage in the second coil.
The coil that provides the flux that is the coil connected to the a.c power source is known as the primary coil while the coil in which the voltage is induced is known as the secondary coil.
When the number of turns in the primary coil (N) is lower than the number in the secondary coil (N),the secondary voltage will be lower than the secondary voltage. This is called the step-down transformer. The opposite of this is called the step-up transformer.
Activity 3
Construct a simple step-up and step-down transformer
Applications of electromagnetic induction: Induction coil
Is an electrical device consisting of two coils, the primary coil and the secondary coil, wound one over the other on an iron core.
It is used to produce high-voltage alternating current from low-voltage direct current. The primary coil is made up of tens or hundreds of turns of coarse wire while the secondary coil consists of thousands of turns of fine wire. The secondary coil is wound on top of the primary coil.
Mode of action
An induction coil produces high voltage in its secondary coil by electromagnetic induction. The direct current in the primary is switched on and off by a make-and-break mechanism. This produces change in current and magnetic field which is necessary for electromagnetic induction to occur in the secondary coil.
When the current in the primary coil is switched on, the induced magnetism in the iron core attracts the soft-iron armature.The moving iron armature opens a gap between the two contacts which breaks the primary coil circuit. This switches off the current. As the induced magnetism fades away, the armature springs back, closes the contacts and completes the circuit again. This allows the current to flow in the primary coil again. This cycle of events is repeated automatically.
The induced a.m. is very large, usually in the order of hundreds of kilovolts (kV).Such a high voltage is achieved because of two things:
- The secondary coil has a large number of turns compared to the primary coil.
- The rapid change in the primary current when it is switched on and off causes a rapid in the magnetic field through the secondary coil.
Applications of the induction coil
- It is used in the ignition system of internal combustion engines.
- It is used to trigger the flash tubes used in cameras and strobe lights.
- It is also used in wireless telegraphy.
Applications of electromagnetic induction: Moving coil loudspeaker
Problems on transformers
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