# UF PHY 2054 - Power lines (16 pages)

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## Power lines

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## Power lines

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Pages:
16
School:
University of Florida-Gainesville
Course:
Phy 2054 - Physics 2
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Unformatted text preview:

Power lines At large distances the resistance of power lines becomes significant To transmit maximum power is it better to transmit high V low I or high I low V a high V low I b low V high I c makes no difference Why Power loss is I2R so want to minimize current do birds sitting on a high voltage power line survive They are not touching high and low potential simultaneously to form a circuit that can conduct current PHY2054 Chapter 18 18 Resistors Current flows through a light bulb If a wire is now connected across the bulb as shown what happens a Bulb remains at same brightness b Bulb dims to 1 4 its former brightness 1 2 current c Bulb goes out d None of the above The wire shunt has almost no resistance and it is in parallel with a bulb having resistance Therefore voltage across shunt and bulb is 0 Thus almost all the current follows the zero or extremely low resistance path PHY2054 Chapter 18 19 Circuits Two light bulbs A and B are connected in series to a constant voltage source When a wire is connected across B what will happen to bulb A a burns more brightly than before b burns as brightly as before c burns more dimly than before d goes out The wire shunt effectively eliminates the second resistance hence increasing the current in the circuit by 2x The first bulb burns 4x brighter I2 R PHY2054 Chapter 18 20 Voltage Drops in Series Circuit Series circuit Battery Current V resistors R1 and R2 i V R1 R2 Voltage across R1 IR1 V R1 R1 R2 Voltage across R2 IR2 V R2 R1 R2 So voltage drops in proportion to resistance Let V 24 R1 8 R2 4 V1 24 8 12 16 V1 24 4 12 8 Useful when considering how resistors divide up voltage PHY2054 Chapter 18 21 Circuits Consider the network of resistors shown below When the switch S is closed then What happens to the voltage across R1 R2 R3 R4 What happens to the current through R1 R2 R3 R4 What happens to the total power output of the battery Let R1 R2 R3 R4 90 and V 45 V Find the current through each resistor before and after closing the

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