- •Міністерство освіти і науки україни Запорізький національний технічний університет методичні вказівки
- •Part I electricity and magnetism unit 1 nature of electricity dialogue
- •Exercises
- •Unit 2 electric current dialogue
- •Unit 3. Electromotive force
- •Unit 4. Electricity in motion
- •Unit 5. Electric circuits
- •Unit 6 ohm’s law
- •Text 3 inductance
- •Importance of Inductance in a. C. Circuits.— Inductance is a property of a circuit, just as is resistance, and is therefore possessed by d.C.
- •Unit 9 lenz’s law
- •Unit 10 self-induction
- •Text 5 electromagnetic induction
- •In general, any movement of an electrically charged particle, or any electric current, creates a magnetic force, and conversely any movement of a magnetic pole creates an electric force.
- •The electromagnetic field
- •Unit 11 condensers and dielectric materials
- •Unit 12 some facts about magnets
- •Magnetic fields
- •Unit 13. Electromagnets and their uses
- •Electromagnetic waves
- •If c is measured in metres per second and X in metres, the time to complete one cycle, X/c, will be in seconds.
Electromagnetic waves
The two chief characteristics of a wave are its amplitude and its wavelength.
The amplitude, or height, is a measure of the amount of energy it contains (the energy is actually proportional to the square of the amplitude) while its wavelength determines its nature. Very long electromagnetic waves (several hundreds of metres) are those used in ordinary sound broadcasting; very short waves (millionths of a metre) transfer energy to which our eyes are sensitive — that is to say they are light waves.
The speed at which electromagnetic waves travel through a vacuum is finite — it does not depend on the wavelength, and is the same for all electromagnetic radiations. For historical reasons it is usually referred to as the "velocity of light"', although it would be more correct to speak of the "velocity of electromagnetic radiation".
The velocity of light has a special significance in the Universe, its value is 300 million metres per second in a vacuum. When traveling through matter, rather than empty space, this velocity is slightly reduced — the extent of the reduction depending on the nature of the matter and the wavelength of the radiation.
The time that a wave takes to complete one cycle will obviously depend on how fast it is traveling. If c is the velocity of light in a vacuum and X is its wavelength, the time taken to complete one cycle will be X/c.
If c is measured in metres per second and X in metres, the time to complete one cycle, X/c, will be in seconds.
Therefore the number of cycles per second will be c/ X: this quantity is known as the frequency and it is measured in units called hertz, one hertz being equal to one cycle per second. Frequency can vary from about 20 hertz to about 1024 hertz. Frequencies and wavelengths are commonly used when tuning a radio.
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