Английский язык: Аннотирование и реферирование текста

250 hand are about 1800 times the mass of electrons and are positively charged. Each are thought to have lines of forces (electric fields) sur- rounding them. In theory, negative lines of force will not join other negative lines of force. In fact they tend to repel each other. Similarly positive lines of force act in the same way. The fact that electrons repel electrons and protons repel protons, but electrons and protons attract one another follows the basic law of physics: Like forces repel and unlike forces attract. Sounds a bit like a teenage romance – opposites attract. When an electron and proton are brought in close proximity to one another it is the electron which moves because the proton is 1800 times heavier. It is the electron which moves in electricity. Even though the electron is much smaller, its field is quite strong negatively and is equal to the positive field of the proton. If the field strength around an electron at a distance of 1,000,000th of a centimetre was a certain amount, then the field strength around an electron at a distance of 2,000,000th of a centimetre will be 1/4 as much. This is because the field decreases inversely with the distance squared. If an increase in one thing causes an increase in something else, these two things are said to vary directly. 2,000,000 electrons on an object produce twice the negative charge than 1,000,000 electrons would. Since the electric-field strength of an electron varies inversely with the distance squared, the field strength a centimetre away would be quite weak. The fields surrounding protons and electrons are known as electrostatic fields. "Static" means stationary or not moving. When electrons are made to move, the result is dynamic electric- ity. "Dynamic" means movement. To produce a movement of an elec- tron it is necessary to either have a negatively charged field "push it", a positively charged field "pull it", or, as normally occurs in an electric circuit, a negative and positive charge (a pushing and pulling of forces).