Consider dropping a ball from rest. This ball moves from astate of high gravitational potential energy to one of lowgravitational potential energy as it falls to the ground. Similarly, charges move from a state of high electric potentialenergy to one of low electric potential energy.

Mustang Sally just finished restoring her1965 Ford Mustang car. To save money, she did not get a newbattery. When she tries to start the car, she discovers that thebattery is dead and she needs a jump start. While unhooking thejumper cables, the positive and negative cables almost touch and aspark jumps between the ends of the cables. This spark is caused bythe movement of electrons through the air between the batteryterminals. In what direction are the electrons traveling?

Aheat engine running backward is called a refrigerator if its purpose is to extract heat from a cold reservoir. the same engine running backward is called a heat pump if its purpose is to exhaust warm air into the hot reservoir. heat pumps are widely used for home heating. you can think of a heat pump as a refrigerator that is cooling the already cold outdoors and, with its exhaust heat qh, warming the indoors. perhaps this seems a little silly, but consider the following. electricity can be directly used to heat a home by passing an electric current through a heating coil. this is a direct, 100% conversion of work to heat. that is, 19.0 \rm kw of electric power (generated by doing work at the rate 19.0 kj/s at the power plant) produces heat energy inside the home at a rate of 19.0 kj/s. suppose that the neighbor's home has a heat pump with a coefficient of performance of 4.00, a realistic value. note: with a refrigerator, "what you get" is heat removed. but with a heat pump, "what you get" is heat delivered. so the coefficient of performance of a heat pump is k=qh/win. an average price for electricity is about 40 mj per dollar. a furnace or heat pump will run typically 200 hours per month during the winter. what does one month's heating cost in the home with a 16.0 kw electric heater? what does one month's heating cost in the home of a neighbor who uses a heat pump to provide the same amount of heating?