POWER INPUT TO A SOURCE
Suppose that the lower rectangle in Fig. 26-19a is itself a source, with an emf larger that of the upper source and with its emf opposite to that of the upper source. Figure 26-20 shows a practical example, an automobile battery (the upper circuit element) ing charged by the car’s alternator (the lower element). The current I in the circuit is n opposite to that shown in Fig. 26-19; the lower source is pushing current backward through the upper source.
Because of this reversal of current, instead ofEq. (26-15) we for the upper source and instead of Eq. (26-19), we have P = V,bl =cl + 12r. (26-20) Work is being done on, rather than by, the agent that causes the non-electrostatic force in the upper source. There is a conversion of electrical energy into non-electrical energy in the upper source at a rate el. The term Pr in Eq. (26-20) is again the rate of dissipation of energy in the internal resistance of the upper source, and the sum cI + 12r is the total electrical power input to the upper source. This is what happens when a rechargeable battery (a storage battery) is connected to a charger.
The charger supplies electrical energy to the battery; part of it is converted to chemical energy, to be reconverted later, and the remainder is dissipated (wasted) in the battery’s internal resistance, either warming the battery or causing a heat flow out of it. If you have a power tool or laptop computer with a rechargeable battery, you may have noticed. that it gets warm while it is charging.