PhysicsLAB CP Workbook
Power Transmission

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Many power companies provide power to cities that are far from the generators.
 
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Consider a city of 100,000 persons who each use continually use 120 W of power (equivalent to the operation of two 60-W light bulbs per person). The power constantly consumed is
 
100,000 persons x 120 W/person = 12 million W (12 MW).
 
What current corresponds to this amount of power at the common 120 V used by consumers?
 

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This is an enormous current, more than can be carried in the thickest of wires without overheating. More power would be dissipated in the form of heat than would reach the faraway city. Fortunately the important quantity is IV and not I alone. Power companies transmit power over long distances at very high voltages so that the current in the wires is low and heating of the power lines is minimized.
If the 12 MW of power is transmitted at 120,000 V, the current in the wires is 

Refer to the following information for the next three questions.

This amount of current can be carried in long-distance power lines with only small power losses due to heating (normally less than 1%). But the corresponding high voltages wired to houses would be very dangerous. So step-down transformers are used in the city.
What ratio of primary turns to secondary turns should be on a transformer to step 120,000 V down to 2400 V? 

What ratio of primary turns to secondary turns should be on a transformer to step 2400 V down to 120 V used in household circuits? 

What is the main benefit of ac compared to dc power? 




 
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