Lesson_3.3_Printable_PPT

# Lesson_3.3_Printable_PPT - Electric Current and Direct...

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Electric Current and Direct Current Circuits Motion 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. A positive charge moves from a high potential point to a low potential point. negative charge will move from a low + - E 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, 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. + + + + + + + - - - - - - -

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+ + + + - - - - - 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 Electron drift + + + + - - - electrons takes place from the negative to the positive plate. 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.

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Electric current in a conductor is defined as the rate of flow of charges. The current is represented by I If a charge Q crosses a section of the conductor in a time t , then: Q I t = he instantaneous current in a conductor is: The unit of current is the coulomb per second ( C.s -1 ) and is called the ampere ( A ). One ampere is one coulomb per second. One ampere is one coulomb per second. The instantaneous current in a conductor is: dQ I dt =
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 = Q = I dt dQ = I dt In order to find the total charge Q that passes through the conductor from time t = t 1 to t = t 2 we integrate dQ with respect to t from t 1 to t 2 . 1 2 t t Q Idt =

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Conventional current Although electric current constitutes the drift of negative charges from the negative end to the positive end of a circuit, the conventional direction of current is the direction f motion of a positive test + Conventional current of motion of a positive test charge which is from the positive end to the negative end. The conventional direction of current is the direction of the electric field inside the conductor, that is from positive end to the negative end.
An electric current can only flow in a closed circuit. If the closed path is broken, the current stops.

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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.

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Lesson_3.3_Printable_PPT - Electric Current and Direct...

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