Đề thi thử thpt quốc gia môn tiếng anh số 197

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Read the following passage và mark the letter A, B, C, or D on your answer sheet to indicate the correct answer khổng lồ each of the questions from 36 to 42

The ability to lớn conduct electrithành phố is one of the key properties of a metal. Other solid materials such as silibé can conduct electriđô thị but only effectively at certain temperatures. Also, some substances such as salt (sodium chloride) can conduct when molten or when dissolved in water. The ability of metals to lớn conduct electrithành phố is due khổng lồ how their atoms bond together. In order to lớn bond together the metal atoms thua trận at least one of theiroutermostelectrons. This leaves the metal atoms with a positive sầu charge và they are now strictly ions. The lost electrons are không tính tiền to move sầu in what is known as a sea of electrons. Since the electrons are negatively charged they attract the ions and this is what keeps the structure together.

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An electric current is a flow of charge & since the electrons in the sea of electrons are miễn phí to move sầu they can be made to lớn flow in one direction when a source of electrical energy such as a battery is connected to the metal.

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Hence we have sầu an electric current flowing through the wire, and this is what makes metals such good conductors of electriđô thị. The only other comtháng solid conducting material that pencil users are likely lớn encounter is graphite (what the „lead’ of a pencil is made from). Graphite is a size of carbon and again the carbon atoms bond in such a way that there is a sea of electrons that can be made to flow as an electric current. Likewise, if we have an ionic substance like salternative text we can make the electrically charged ions flow to create a current but only when those ions are miễn phí lớn move, either when the substance is a liquid or dissolved in water. In its solid state an ionic substance lượt thích salt cannot conduct electrithành phố as its charged ions cannot flow.

Electrical insulators are substances that cannot conduct electrithành phố well either, because they contain no charged particles or any charged particles they might contain vị not flow easily. Water itself is a poor conductor of electricity as it does not contain a significant amount of fully charged particles (the ends of a water molecule are partly charged but overall the molecule is neutral). However, most water we encounter does contain dissolved charged particles, so it will be more conductive sầu than pure water. Many of the problems that occur when touching electrical devices with wet hands result from the ever-present salternative text that is left on our skin through perspiration and it dissolves in the water khổng lồ make it more conductive sầu.