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Unformatted text preview: Lesson 3.3 Electric Current and Direct Current Circuits 1 . Flow of electric charges . You have seen earlier that a positive charge placed in an electric filed will experience a force and move in the direction of decreasing potential. In other words, a positive charge moves from a high potential point to a low potential point. Similarly a negative charge will move from a low potential point to a high potential point. + + + + + + + +  If a negatively charged plate is connected to a positively charged plate by a conducting wire as shown in the figure above, an electric field is propagated through the wire and the free charges in it will move because of the force they experience in the electric field. Since the free charges are electrons, the electrons in the conducting wire will drift from the negative end to the positive end. While the direction of the electric field is from the positive plate to the negative plate, the drift of electrons takes place from the negative to the positive plate as the direction of motion of electrons is opposite to the direction of the electric field. The drift of electrons in this way caused by a potential difference between the two plates is called an electric current . The electric field propagates through the wire almost instantaneously at the speed of light as soon as the connection is established and the electrons through the length of the wire begins to drift from the negative plate to the positive plate. This drift of electrons through the conductor constitutes an electric current. Electric current in a conductor is defined as the rate of flow of charges. The current is represented by I. If a charge dQ crosses a section of the conductor in a time dt, then dQ I dt = I is the instantaneous current in the conductor. The unit of current is the coulomb per second ( C.s1 ) which is called the ampere (A). One ampere is one coulomb per second. Since electric current is caused by the transport of electric charges through a conductor, a knowledge of current I in a conductor will help us to obtain a measure of the amount of Charge Q that passes through a conductor during a given period of time. dQ I dt dQ Idt = = In order to find the charge Q that passes through the conductor from time t = t 1 to t = t 2 can be obtained by integrating dQ 2 1 t t Q Idt = ∫ Example 1: The quantity of charge q passing through a surface of area 2.0 cm 2 varies with time according to the equation q = 4.0t 3 + 3.0t + 10 where q is in coulombs and t is in seconds. What is the instantaneous current through the surface at t = 1.0 s? Solution: 2 1 12 3 15 3 12 t dQ I t dt I A = = = + = + = Example 2 : An electric current is given by the expression I (t) = 75 sin(60 π t) where I is in amperes and t is in seconds. What is the total charge carried by the current from t = 0 to t = 1/120 s?...
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This note was uploaded on 05/01/2011 for the course PHY 2049 taught by Professor George during the Spring '11 term at Edison State College.
 Spring '11
 George
 Physics, Charge, Current, Force

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