# hw4sols - MasteringPhysics 10/19/08 6:50 PM Assignment...

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MasteringPhysics Page 1 of 12 http://session.masteringphysics.com/myct Assignment Display Mode: View Printable Answers Physys 202 Fall08 HW4 Due at 11:00pm on Wednesday, October 8, 2008 View Grading Details Introduction to Electric Current Description: Mostly conceptual questions about electric current in metals. 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: copper aluminum glass quartz cork plywood table salt gold Part B Metals are good conductors of electric current for which of the following reasons? ANSWER: They possess high concentrations of protons. They possess low concentrations of protons. They possess high concentrations of free electrons. They possess low concentrations of free electrons. 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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## This note was uploaded on 11/02/2009 for the course MASTERING PHYS taught by Professor All during the Spring '09 term at Kettering.

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hw4sols - MasteringPhysics 10/19/08 6:50 PM Assignment...

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