Heat is energy transferred between the system and the environment. Consequently, the SI unit of heat is the joule. Historically, before the connection between heat and work had been recognized, a unit for measuring heat, the calorie, had been defined as
Once Joule established that heat is energy, it was apparent that the calorie is really a unit of energy. In today’s SI units, the conversion is
The calorie you know in relation to food is not the same as the heat calorie. The food calorie, abbreviated Cal with a capital C, is
We will not use calories in this textbook, but there are some fields of science and engineering where calories are still widely used. All the calculations you learn to do with joules can equally well be done with calories.