Current flow
ELECTRIC CURRENT IN SOLIDS
If electrons are added to one end of a solid conductor, such as a piece of copper wire, while some of them are removed from the other end of that conductor, an electric field is set up between the two ends of the wire. This field tends to cause free electrons in the wire to move from the negative end to the positive one. As previously stated, this movement of electrons constitutes an electric current. The free electron moves comparatively slowly through the wire and travels but a short distance before it collides with an atom. This collision generally knocks an electron free from the atom. This new free electron travels a short distance toward the positive end of the wire before its collision with another atom. Thus, there is a flow of electrons from the negative to the positive end of the wire. Although the flow of electrons seems to be comparatively slow, the disturbance that causes this flow travels through the wire at a speed that approaches the speed of light (approximately 186,000 miles per second). The above action may be understood by visualizing a long hollow tube completely filled with balls. If a ball is added to one end of the tube, a ball at the other end is thrown out immediately. Thus, although each ball moves slowly and for only a short distance, the disturbance is transmitted almost instantaneously through the whole tube. The device that supplies the electromotive force is considered to be a sort of pump. This pump removes electrons from one end of the conductor, thus creating a deficiency of electrons or a positive charge at that end and crowds them up at the other end, creating an excess of electrons or a negative charge.
ELECTRIC CURRENT IN LIQUIDS
We know a molecule of ordinary table salt to consist of an atom of sodium and an atom of chlorine. When these two atoms combine to form a molecule of salt, the process is such that the sodium atom loses an electron and, hence, obtains a positive charge while the chlorine atom gains an electron, hence obtains a negative charge. A charged particle is called an ion. The molecule of salt contains a positive sodium ion and a negative chlorine ion. Since unlike charges attract, the two oppositely charged ions are held close together. However, the molecule of salt dissolving in water, the ions separate, or dissociate. If two metal plates (called electrodes) are set at opposite ends of the solution and a source of electromotive force is connected to these plates so that one becomes a positive (electron-deficient) electrode and the other a negative (electron- excess) electrode, an electric field is created between these two electrodes. Opposite charges attracting each other, the negative chlorine ion is attracted to the positive electrode and the sodium ion is attracted to the negative electrode. Upon reaching the positive electrode, the chlorineion surrenders its extra electron to the electrode and becomes a neutral chlorine atom. As the sodium ion reaches the negative electrode, it obtains an electron from the electrode and becomes a neutral sodium atom. The effect of the electromotive force, then, is to cause a movement of ions through the solution.
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для студентів III курсу, 5 семестр (ЕП, ЕС)
Підготував викладач: Кособокова А.В.
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