A. electromagnetic energyB. electromagnetic energy travels through spaceC. came from the s

对电磁兼容性 (Electromagnetic Compatibility, 简称 EMC) 标准的描述正确的是()

A. 同一个国家的是恒定不变的

B. 不是强制的

C. 各个国家不相同

D. 以上均错误

Electromagnetic Energy

White light seems to be a combination of all colors. The energy that comes from a source of light is not limited to the kind of energy you can see. Heat is given off by a flame or an electric light. On a cloudy day it is possible to get a sunburn even though you feel cool. Visible light and the kind of energy that produce warmth and sunburn are examples of electromagnetic energy.

The sun is 93 million miles from the earth. Yet we can use energy from the sun because electromagnetic energy travels through space.

Many other kinds of energy are also types of electromagnetic energy. Radio, television, and radar signals travel from transmitters to receivers as low-energy electromagnetic waves. Infrared (红外线的) radiation is an electromagnetic wave. When it is absorbed by matter, heat is produced. Waves of infrared and visible light have more energy than waves of radio, television, or radar. Ultraviolet rays (紫外线) and X-rays are electromagnetic waves with even greater amounts of energy. Infrared radiation is used in cooking food and heating buildings. Sunlight and electric lights are part of our requirements for normal living. Ultraviolet radiation is useful in killing certain disease organisms. X-rays and gamma rays have so mush energy that they travel right through solid objects. They can be used to detect and treat cancer. X-rays are used in industry to find hidden cracks in metal, and in medicine to reveal broken bones.

Usually we use electricity to generate electromagnetic energy. The source of most of our energy is the sun. Heat from the sun causes water to evaporate. When the water falls to the earth as rain, some of it is trapped behind dams and then used to operate electric generators. Other generators are powered by coal, but the energy stored in coal came from the sun, too.

Until recently, the source of the tremendous amount of energy given off by the sun was a puzzle. If the sun depended on chemical reactions, it would have used up all its energy long ago. Experiments with electromagnetic radiation led to the theory that mass can be converted into energy. About forty years after the theory was proposed, nuclear energy was harnessed (利用) by man. Chemical energy comes from electron (电子) rearrangement. Nuclear energy comes from a change in the nucleus of an atom. Compared with chemical reactions, nuclear reactions release millions of times more energy per pound of fuel. We now believe that the sun's energy comes from the nuclear reactions in which hydrogen is changed into helium(氦).

Nuclear energy is beginning to compete with coal as an economical source of power to generate electricity. It is also being used to operate engines in large ships. Scientists continue to seek new and better methods of obtaining and using energy.

A. Nuclear Reactions as the Lasting Source of the Sun's Energy

B. The Most Important Source of Energy

C. Types of Electromagnetic Energy

D. X-rays Are Used to Detect and Treat Cancer.

E. Seeking New Sources of Energy

F. Nuclear Energy is Beginning to Compete with Coal

Paragraph 3 ______

A.Electromagnetic Wave

B.Electricity Spectrum

C.Electromagnetic Spectrum

D.Electricity Wave

A.such fast as

B.many fast as

C.as fast as

D.the same fast as

Which of the following statements represents an advantage of an electromagnetic clutch？

A.Large misalignments can be tolerated between the shaft and engine coupling

B.Slip is held to a minimum when reversing shaft rotation

C.Engine torsional vibrations to the driven shaft are eliminated

D.It aids in maintaining power factor

A.value

B.opinion

C.fact

D.attitude

A.magnetically

B.electromagnetic

C.paramagnetic

D.thermomagnetic