Assignment 7- Current & Resistance - Introduction...

Assignment 7- Current & Resistance
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Introduction to Electric Current Learning Goal: To understand the nature of electric current and the conditions under which it exists. Electric current is defined as the motion of electric charge through a conductor . Conductors are materials that contain movable charged particles. In metals, the most commonly used conductors, such charged particles are electrons. The more electrons that pass through a cross section of a conductor per second, the greater the current. The conventional definition of current is , where is the current in a conductor and is the total charge passing through a cross section of the conductor during the time interval . The motion of free electrons in metals not subjected to an electric field is random : Even though the electrons move fairly rapidly, the net result of such motion is that (i.e., equal numbers of electrons pass through the cross section in opposite directions). However, when an electric field is imposed, the electrons continue in their random motion, but in addition, they tend to move in the direction of the force applied by the electric field. In summary, the two conditions for electric current in a material are the presence of movable charged particles in the material and the presence of an electric field . Quantitatively, the motion of electrons under the influence of an electric field is described by the drift speed , which tends to be much smaller than the speed of the random motion of the electrons. The number of electrons passing through a cross section of a conductor depends on the drift speed (which, in turn, is determined by both the microscopic structure of the material and the electric field) and the cross- sectional area of the conductor. In this problem, you will be offered several conceptual questions that will help you gain an understanding of electric current in metals. Part A You are presented with several long cylinders made of different materials. Which of them are likely to be good conductors of electric current? Check all that apply. ANSWER:
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Picture 4268 Picture 4269 Picture 4270 Picture 4271 Picture 4272 Picture 4273 Picture 4274 Picture 4275 Introduction to Electric Current Part A copper aluminum glass quartz cork plywood table salt gold Correct Part B Metals are good conductors of electric current for which of the following reasons?
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Picture 4276 Picture 427 Picture 4278 Picture 4279 Picture 4280 Picture 4282 Introduction to Electric Current Part A ANSWER: Correct Part C Which of the following is the most likely drift speed of the electrons in the filament of a light bulb? ANSWER:
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Picture 4284 Picture 4286 Picture 428 Introduction to Electric Current Part C Correct Part D You are presented with several wires made of the same conducting material. The radius and drift speed are given for each wire in terms of some unknown units and . Rank the wires in order of decreasing electron current. Hint D.1
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