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Lecture214Week11

Lecture214Week11 - This week Electrical Circuits Series or...

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This week Electrical Circuits Series or parallel that’s the question . Current, Power and Energy Why does my laptop battery die? Transmission of power to your home Why do we have big transmission towers? Household Appliances Why do fuses blow? 10/13/2011 Physics 214 Fall 2011 1
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10/13/2011 Physics 214 Fall 2011 2 Electrical power and energy In order to separate charge we have to do work and energy is stored. The simplest example is a battery. The quantity we call voltage is related to the energy stored V = PE/q (joules/coulomb = volt) When positive charge moves to the negatively charged plate or vice versa then the stored energy is released. In order for this to happen the two sides of the battery must be joined by a conductor so that the charge can move easily. This is a simple circuit.
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10/13/2011 Physics 214 Fall 2011 3 Electrical circuits All circuits are basically the same. There is an external source of energy which produces a voltage. In a charged battery there is separation of charge. When the circuit is made positive charge flows from high to low voltage or negative charge flows from low to high voltage releasing the stored energy. Normally it is electrons which flow. As they move they “collide” with the atoms of the wire and lose some of their energy in the form of heat. There is resistance to the flow.
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10/13/2011 Physics 214 Fall 2011 4 Current and resistance The rate of flow of charge determines how much energy is released per unit time I = q/t amperes (coulombs/sec) The direction of I is the flow of positive charge or opposite to the flow of negative charge. OHM’s Law R = V/I ohms Every part of a circuit has resistance including an internal resistance in the battery. The higher the resistance the lower is the current for a given voltage difference
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10/13/2011 Physics 214 Fall 2011 5 Electromotive force The electromotive force of a battery , ε , is the voltage difference between the two terminals when no current is being drawn. When it is connected to a simple circuit I = ε /(R circuit + R battery ) A voltage difference is the energy stored or the energy released per unit charge. So if charge +q goes from high to low voltage then the energy released is q ε or q V
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10/13/2011 Physics 214 Fall 2011 6 Series circuit If we add more light bulbs in the circuit in series the total resistance increases and the current will be reduced. The current is the same in all parts of the circuit and the voltage difference across each light is the same ε = I(R battery + R + R + R) The voltage difference across any light = IR
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10/13/2011 Physics 214 Fall 2011 7 Parallel circuit In the circuit shown the voltage difference across each light is the same and the total current is the sum of the three currents I = I 1 + I 2 + I 3 and since a current = V /R V AB /R circuit = V AB /R 1 + V AB /R 2 + V AB /R 3 and 1/R circuit = 1/R 1 + 1/R 2 + 1/R 3 That is the total resistance of the circuit is smaller than any of the single resistances. It is also true that if one bulb fails the other two will stay lit at the same brightness.
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