A demanding game can make a phone feel warm within minutes. The processor, graphics chip, screen and radio circuits are all transferring electrical energy at a high rate. Power is the rate at which energy is transferred. When a game makes the processor perform billions of calculations, the phone draws more electrical power from its battery. Not all of that energy becomes useful light, sound or computation. Some is dissipated as thermal energy because components have electrical resistance and because real devices are not perfectly efficient. Phones spread this thermal energy through their case and may reduce processor speed if the temperature rises too far. That slowdown is called thermal throttling and helps protect the electronics.

The useful part of this physics is that it helps people predict and design real things. Engineers often begin with the same simplified model used in school, test where it works, then add more detail when measurements show that the real situation is more complicated.

Power is the rate of energy transfer: a high-power device transfers a lot of energy each second. Electrical power can be found from P = IV, while energy transferred is E = Pt. Efficiency compares the useful energy output with the total input, and the remainder is not destroyed—it is transferred to less useful stores, often by heating or sound.