Electricity heats these elements which, in turn, fills the insulated oven cavity with hot air or thermal radiation. In a conventional oven you bake by dry heat that envelops the food. You must preheat the oven so that the temperature is consistent. Once the desired temperature is reached, the bottom element only turns on enough to maintain the temperature, and the pan bottom absorbs most of the radiant energy. The heat then migrates toward the coolest part of the food, its center, by conduction and slowly bakes from the outside in.
Cooking can be slow because thermal ovens cook from the outside in.
If the temperature is too high or too low, the surface can dry out and overcook while the interior is undercooked. If you're in a hurry or come home late there may not be enough time to prepare a meal that must be cooked in the oven. The microwave has been used commercially since the 1940s but was introduced to home consumers in the late 1970s and caught on in the 1980s. It is almost as common as the thermal oven today, and many ranges come with both thermal and microwave ovens. Microwaves are fast and energy efficient because they only heat the food, not the space around it.