TAILIEUCHUNG - English for students of Physics_Unit 14

Electric charge and a measure for the quantity of charge Any household electrical appliance – be it a light bulb, a motor, or a television set – has two contacts which have to be plugged in to get the device to operate. This common characteristic, that an electrical apparatus must have two wires connecting it to a source of electricity, gave the rise in the eighteenth century to the idea that when an electrical device is working, something is moving through it. That something is called electric charge. When you pull out a plug, turn off a switch, or disconnect. | 144 Unit Fourteen ELECTRIC CHARGE READING PASSAGE Electric charge and a measure for the quantity of charge Any household electrical appliance - be it a light bulb a motor or a television set - has two contacts which have to be plugged in to get the device to operate. This common characteristic that an electrical apparatus must have two wires connecting it to a source of electricity gave the rise in the eighteenth century to the idea that when an electrical device is working something is moving through it. That something is called electric charge. When you pull out a plug turn off a switch or disconnect a battery the flow of electric charge stops and with it the operation of the apparatus. The idea of a flowing electric charge is quite attractive because it permits us to draw in our minds a mental picture which may eventually lead to a useful model. To develop the intuitive idea of a flowing charge into a model we must find a way to measure the quantity of electric charge that flows through a bulb a motor or any other device. We have to look for an effect produced by moving charge which can be measured quantitatively. You have used such indirect methods many times before probably without noticing it. For example you can not see temperature directly. To measure temperature we use the fact that substances expand when heated and we can construct various kinds of thermometers using thermal expansion. We shall use a similar method to build a charge meter. In the experiment on the decomposition of water the longer the time the electrodes are connected to the battery the greater the volume of both gases produced. This suggests that more charge must have flowed through the apparatus when it was connected for a longer time. Thus it seems reasonable to use the quantity of either gas produced in the reaction as a measure of the quantity of electric charge that passes through the water. We shall choose the quantity of hydrogen since we get twice as much of this gas as we do of

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