Материал: Андрианова English Thesaurus of Student-Physicist 2013

At high frequency, there is less time available to charge the capacitor, so less charge and voltage accumulate on the capacitor, which translates into less opposition to the flow of charge and, consequently, a higher current. The analogy between capacitive reactance and resistance means that we can write an equation of the same form as Ohm's law to describe AC circuits containing capacitors.

Inductors in an AC circuit

Now consider an AC circuit consisting only of an inductor connected to the terminals of an AC source. The changing current output of the generator produces a back emf that impedes the current in the circuit. The effective resistance of the coil in an AC circuit is measured by a quantity called the inductive reactance. This occurs because back emfs always oppose the change in the current.

The RLC series circuit

We now consider when an inductor, a capacitor, and a resistor are combined. To account for the different phases of the voltage drops, we use a technique involving vectors. We represent the voltage across each element with a rotating vector. It's convenient to define a parameter called the impedance Z of the circuit. Both the impedance and, therefore, the current in an AC circuit depend on the resistance, the inductance, the capacitance, and the frequency (because the reactances are frequency dependent).

Power in an AC circuit

No power losses are associated with pure capacitors and pure inductors in an AC circuit. A pure capacitor, by definition, has no resistance or inductance, while a pure inductor has no resistance or capacitance. These are idealizations: in a real capacitor, for example, inductive effects could become important at high frequencies.

The power delivered by an AC source to any circuit depends on the phase difference between the source voltage and the resulting current. This fact has many interesting applications. For example, factories often use devices such as large motors in machines, generators, and transformers that have a large inductive load due to all the windings. To deliver greater power to such devices without using excessively high voltages, factory technicians introduce capacitance in the circuits to shift the phase.

Resonance in a series RLC circuit

The rms current in a series RLC has its maximum value when the impedance has its minimum value. In such a circumstance, the impedance of

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the circuit reduces to Z = R. The frequency at which this happens is called the resonance frequency of the circuit.

The tuning circuit of a radio is an important application of a series resonance circuit. The radio is tuned to a particular station (which transmits a specific radio-frequency signal) by varying a capacitor, which changes the resonance frequency of the tuning circuit. When this resonance frequency matches that of the incoming radio wave, the current in the tuning circuit increases.

The transformer

It's often necessary to change a small AC voltage to a larger one or vice versa. Such changes are effected with a device called a transformer. In its simplest form, the AC transformer consists of two coils of wire wound around a core of soft iron. The purpose of the common iron core is to increase the magnetic flux and to provide a medium in which nearly all the flux through one coil passes through the other.

Maxwell's predictions

During the early stages of their study and development, electric and magnetic phenomena were thought to be unrelated. In 1865, however, James Clerk Maxwell (1831-1879) provided a mathematical theory that showed a close relationship between all electric and magnetic phenomena. In addition to unifying the formerly separate fields of electricity and magnetism, his brilliant theory predicted that electric and magnetic fields can move through space as waves.

From Faraday's law and from Maxwell's own generalization of Ampere's law, Maxwell calculated the speed of the waves to be equal to the speed of light. He concluded that visible light and other electromagnetic waves consist of fluctuating electric and magnetic fields traveling through empty space, with each varying field inducing the other! This was truly one of the greatest discoveries of science, on a par with Newton's discovery of the laws of motion. Like Newton's laws, it had a profound influence on later scientific developments.

In 1887, after Maxwell's death, Heinrich Hertz (1857-1894) was the first to generate and detect electromagnetic waves in a laboratory setting, using LC circuits.

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The spectrum of electromagnetic waves

It is now known that other forms of electromagnetic waves exist that are distinguished by their frequencies and wavelengths. Brief descriptions of the wave types follow, in order of decreasing wavelength. There is no sharp division between one kind of electromagnetic wave and the next. All forms of electromagnetic radiation are produced by accelerating charges.

Radio waves are the result of charges accelerating through conducting wires. They are, of course, used in radio and television communication systems.

Microwaves (short-wavelength radio waves) are generated by electronic devices. Their short wavelengths make them well suited for the radar systems used in aircraft navigation and for the study of atomic and molecular properties of matter.

Infrared waves produced by hot objects and molecules. They are readily absorbed by most materials. Infrared radiation has many practical and scientific applications, including physical therapy, infrared photography, and the study of the vibrations of atoms.

Visible light, the most familiar form of electromagnetic waves, may be defined as the part of the spectrum that is detected by the human eye. Light is produced by the rearrangement of electrons in atoms and molecules.

Ultraviolet (UV) light covers wavelengths ranging from about 400 nm down 0.6 nm. The Sun is an important source of ultraviolet light. Most of the ultraviolet light from the Sun is absorbed by atoms in the upper atmosphere, or stratosphere. This is fortunate, because UV light in large quantities has harmful effects on humans.

X-rays are electromagnetic waves with wavelengths from about 10 nm down to 10~4nm. The most common source of x-rays is the acceleration of high-energy electrons bombarding a metal target. X-rays are used as a diagnostic tool in medicine and as a treatment for certain forms of cancer.

Gamma rays – electromagnetic waves emitted by radioactive nuclei – have wavelengths ranging from about 10~ 10m to less than 10~14m. They are highly penetrating and cause serious damage when absorbed by living tissues. Accordingly, those working near such radiation must be protected by garments.

1.Would an inductor and a capacitor used together in an AC circuit dissipate any energy?

2.What is the important quantity in an AC circuit?

3.Describe the role of capacitor in AC circuit.

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4.How a transformer is constructed?

5.What do you know about Maxwell’s predictions?

6.Who was the first to generate and detect electromagnetic waves in a laboratory setting, using LC circuits?

Exercises

3. Rearrange the letters in the anagrams to form equivalents for the Russian words:

2. Match the words in A with the words in B to form word combinations:

3.Fill in the gaps with the missing words from the list: electromagnetic wave, inductive coil, transformer, rms current, an-

tenna

1.The _____ is the square root of the average (mean) of the square of the current.

2.In any real circuit, there is some resistance in the wire forming the _____.

3.It may seem that a _____ is a device in which it is possible to get something for nothing.

4.An alternating voltage applied to the wires of an _____ forces electric charges in the antenna to oscillate.

5.A detailed analysis would show that the energy carried by an

_____ is shared equally by the electric and magnetic fields.

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