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Assignment 04

# Assignment 04 - Class PEP112S2007 Assignment 4 Assignment...

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Class PEP112S2007 Assignment 4 Assignment is due at 11:00pm on Sunday, February 18, 2007 Credit for problems submitted late will decrease to 0% over the course of 10 hour(s) after the deadline has passed. The wrong answer penalty is 1% per part. Multiple choice questions are penalized as described in the online help. The unopened hint bonus is 1% per part. You are allowed 10 attempts per answer. 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

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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? A. copper B. aluminum C. glass D. quartz E. cork F. plywood G. table salt H. gold Type the letters corresponding to the right answers in alphabetical order. Do not use commas. For instance, if you think that only materials C and D are good conductors, type CD . ANSWER: ABH Part B Metals are good conductors of electric current for which of the following reasons? ANSWER: They possess high concentrations of protons.
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