Chapter26_Notes - Physics 184 Physics for Scientists &...

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10/6/2011 Physics for Scientists & Engineers 2 1 Physics 184 Physics for Scientists & Engineers 2 Fall Semester 2011 Chapter 26: Direct Current Circuits
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10/6/2011 Physics for Scientists & Engineers 2 2 Complex Circuits In each circuit, which resistors are in series and parallel? Some circuits can be constructed that cannot be resolved into series or parallel systems of capacitors or resistors
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10/6/2011 Physics for Scientists & Engineers 2 3 Kirchhoff’s Rules To handle these types of circuits, we must apply Kirchhoff’s Rules. Kirchhoff’s Rules can be stated as Kirchhoff’s Junction Rule The sum of the currents entering a junction must equal the sum of the currents leaving a junction Kirchhoff’s Loop Rule The sum of voltage drops around a complete circuit loop must sum to zero
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10/6/2011 Physics for Scientists & Engineers 2 4 Kirchhoff’s Junction Rule A junction is a point in a circuit where three or more wires (or other elements) are connected to each other. Each connection between two junctions is a branch . A branch can contain any number of circuit elements. Kirchhoff’s Junction Rule is a direct consequence of the conservation of charge. In a conductor, charge cannot be created or destroyed. At a junction: all charges streaming into the junction must also leave the junction. i 1 i 2 i 3
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A loop in a circuit is any set of elements forming a closed path. If you follow a loop, you will eventually return to where you started. You can follow a loop in either direction as long as you follow it all the way around. If we sum the potential difference across each element as we cross it in our path around the loop, we will then obtain the total potential difference of the complete path around the loop. Kirchhoff’s Loop Rule states: The potential difference around a complete circuit loop is zero. 10/6/2011 Physics for Scientists & Engineers 2 5 Kirchhoff’s Loop Rule
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10/6/2011 Physics for Scientists & Engineers 2 6 Kirchhoff’s Loop Rule Kirchhoff’s Loop Rule is a direct consequence of the conservation of electric potential energy In terms of our water analogy, water can flow out of a reservoir, do work as it falls (e.g., by turning a turbine), and then be pumped back into the reservoir. However, when it returns to the top of the reservoir, its potential energy is the same as it was when it began at the top of the reservoir. We will need to adopt conventions regarding the potential drop across each element of the circuit.
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10/6/2011 Physics for Scientists & Engineers 2 7 EMF Devices - Directions An emf device (e.g., a battery) keeps the positive terminal (labeled +) at a higher electrical potential than the negative terminal (labeled -) When a battery is connected in a circuit, its internal chemistry causes a net current inside the battery : positive charge carriers move from the negative to the positive terminal, in direction of the emf arrow Or: Positive charge carriers move from a region of low electric potential (negative
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Chapter26_Notes - Physics 184 Physics for Scientists &...

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