Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the questions from 71 to 80. The ability to conduct electricity is one of the key properties of a metal. Other solid material such as silicon can conduct electricity 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 conduct electricity is due to how their atoms bond together. In order to bond together the metal atoms lose at least one of their outermost electrons. This leaves the metal atoms with a positive charge and they are now strictly ions. The lost electrons are free to move in what are known as a sea of electrons. Since the electrons are negatively charged they attract the ions and this is what keeps the structure together. An electric current is a flow of charge and since the electrons in the sea of electrons are free to move they can be made to flow in one direction when a source of electrical energy such as a battery is connected to the metal. Hence we have an electric current flowing through the wire, and this is what makes metals such good conductors of electricity. The only other common solid conducting material that pencil users are likely to encounter is graphite (what the ‘lead’ of a pencil is made from). Graphite is a form 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 salt we can make the electrically charged ions flow to create a current but only when those ions are free to move, either when the substance is a liquid or dissolved in water. In its solid state an ionic substance like salt cannot conduct electricity as its charged ions cannot flow. Electrical insulators are substances that cannot conduct electricity well either, because they contain no charged particles or any charged particles they might contain do not flow easily. Water itself is a poor conductor or 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 than pure water. Many of the problems that occur when touching electrical devices with wet hands result from the ever-present salt that is left on our skin through perspiration and it dissolves in the water to make it more conductive. By Helena Gillespie and Rob Gillespie. Science for Primary School Teacher. OUP Question 71: Electrical conductivity is . A.one of the most important properties of metals B.one of the key properties of most solid materials C. impossible for any substance when it is dissolved in water D. completely impossible for silicon Question 72: According to the passage, a metal can conduct electricity due to . A. the absence of free electrons B. its atoms with a positive charge C. the way its atoms bond together D. the loss of one electron in the core of its atoms Question 73: The word “outermost” in paragraph 1 mostly means . A. the lightest B. nearest to the inside C. furthest from the inside D. the heaviest Question 74: The atoms of a metal can bond together because . A. the lost electrons cannot move freely in the sea of electrons B. electrons can flow in a single direction C. they lose all of electrons D. negatively charged electrons attract positive ions Question 75: Slat in its solid state is not able to conduct electricity because . A.it has free electrons B. its charged ions can flow easily C.it cannot create any charge ions D.it charged ions are not free to move Question 76: The word “they” in paragraph 3 refers to . A. charged ions B. electric currents C. charged particles D. electrical insulator Question 77: Water is a poor conductor because it contains . A. no positive or negative electric charge B. only a small amount of fully charged particles C. only a positive electric charge D. only a negative electric charge Question 78: We can have problems when touching electrical devices with wet hands because . A. the eater itself is a good conductor of electricity B. the water dissolves the salt on our skin and becomes more conductive C. the water contains too many neutral molecules D. the water containing no charged particles makes it more conductive Question 79: Which of the following is NOT true according to the passage? A. Pure water is much more conductive than most water we encounter every day. B. Graphite is a common solid substance that can conduct electricity. C. Salt can conduct electricity when it is molten or dissolved. D. Some materials are more conductive than others. Question 80: Which of the following could best serve as the title of the passage? A. Electrical Energy B. Electrical Devices C. Electrical Insulators D. Electrical Conductivity
Posted on: Sat, 05 Jul 2014 08:10:24 +0000
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