Heat and work are two different forms of energy transfer with the change of system state, and they are the measures of different forms of energy transfer. They are all related to the intermediate process of state change, so they are not functions of system state. Nutrients that can produce calories in food are protein, fat, sugar and carbohydrate. They are oxidized to generate heat for the body to maintain life, grow and develop and exercise.
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The relationship between heat energy and internal energy is just like the relationship between work and mechanical energy. Heat is a measure of the change of energy inside an object. If the thermal balance between the two areas has not been reached, the heat will be transferred to the other side with low temperature at the place with high temperature between them. Any substance has a certain internal energy, which is related to the disorderly movement of atoms and molecules that make up the substance.
When two substances with different temperatures are in thermal contact, they exchange internal energy until the temperature is the same, that is, they reach thermal equilibrium. Here, the energy transferred is equal to the heat exchanged. Many people confuse heat energy with internal energy. Heat actually refers to the change of internal energy and the work done by the system. Heat describes the change of internal energy, which is a state quantity and state function of the system, corresponding to a state point of the system.
Above content reference: Baidu Encyclopedia-Heat