# UT Arlington PHYS 1444 - Lecture Notes (16 pages)

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## Lecture Notes

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## Lecture Notes

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Lecture Notes

Pages:
16
School:
University of Texas at Arlington
Course:
Phys 1444 - General Technical Physics Ii
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PHYS 1444 Section 004 Lecture 12 Wednesday Mar 26 2007 Dr Andrew Brandt EMF and Terminal Voltage Resistors in Series and Parallel Energy loss in Resistors HW6 due Fri 3 30 at 8 pm Wednesday Mar 21 2007 PHYS 1444 004 Spring 2007 Dr Andrew Brandt 1 EMF and Terminal Voltage What do we need to have current in an electric circuit A device that provides a potential difference such as battery or generator typically it converts some type of energy into electric energy These devices are called sources of electromotive force emf This does NOT refer to a real force The potential difference between terminals of the source when no current flows to an external circuit is called the emf of the source A battery itself has some internal resistance r due to the flow of charges in the electrolyte Why do headlights dim when you start the car The starter needs a large amount of current but the battery cannot provide charge fast enough to supply current to both the starter and the headlights Wednesday Mar 21 2007 PHYS 1444 004 Spring 2007 Dr Andrew Brandt 2 EMF and Terminal Voltage Since the internal resistance is inside the battery we cannot separate the two So the terminal voltage difference is Vab Va Vb When no current is drawn from the battery the terminal voltage equals the emf which is determined by the chemical reaction Vab However when the current I flows from the battery there is an internal drop in voltage which is equal to Ir Thus the actual delivered terminal voltage is Vab Ir Wednesday Mar 21 2007 PHYS 1444 004 Spring 2007 Dr Andrew Brandt 3 Resistors in Series Resistors are in series when two or more of them are connected end to end These resistors represent simple electrical devices in a circuit such as light bulbs heaters dryers etc What is common in a circuit connected in series the current is the same through all the elements in series Potential difference across each element in the circuit is V1 IR1 V2 IR2 and V3 IR3 Since the total potential difference is V we obtain

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