Thermal energy is used to cook food. Thermal energy is heat. It's converted from either electrical potential energy or chemical potential energy, depending on the cooking appliance. An electric stove converts electric potential energy to thermal energy. It makes use of electrical resistance which creates heat in the heating element and in coils beneath the burners. Electrical energy is converted to heat when electrons pass through substances with high resistance to flow of electricity. The electrons slow down and heat up the substance. 

A natural gas cooking appliance converts chemical potential energy to thermal energy. The breaking and reforming of chemical bonds during combustion releases energy in the form of heat and light.

Thermal energy is transferred to food either through conduction, convection or radiation. When a metal cooking vessel is in contact with a burner, heat is conducted through the metal and into the food. In an oven, heat is transferred by convection, or moving air currents, to food that is cooking. Microwaves are an example of radiation energy being transferred to food.