Conceptual Physics

3. Divide the dissipated power by the generated power to find the fraction of the power dissipated by the lines.


4. Repeat this calculation for the last question asked above.

Physics principles and equations

Power as a function of potential difference and current

P = IǻV

Power dissipated by a resistor

P = I^2 R

Power in an electric circuit

P = (ǻV)^2 /R

Step-by-step solution

First we calculate the current in the transmission lines when the potential difference is 500 kV. We do this using the power for the power plant

and the potential difference across the wires, which are both stated above.

First, we find the power dissipated as heat by the wire. Then we divide that by the amount of power generated by the plant to determine what

fraction is wasted.

For the second case we repeat the same calculations with a lesser potential difference.

The differing rates of power consumption explain why power companies transmit power at high potential differences. The potential difference is

lowered at transformers near the consumer. (Transformers are discussed further in the chapter on electromagnetic induction.) The potential

difference is lowered to 120 V, the standard potential difference of a power outlet in a house, just before it enters the house or building using

the power.

Step Reason

1. power equation

2. solve for I

3. evaluate

Step Reason

4. power dissipated by a resistor

5. evaluate

6. divide by generated power

Step Reason

7. evaluate equation in step 2

8. power dissipated by resistor

9. evaluate

(^468) Copyright 2000-2007 Kinetic Books Co. Chapter 25