6.1 BASIC PRINCIPLES AND FLUIDS 119 Some materials are better conductors of heat than others: metals are considered good conductors. Why? Thermal energy, also called heat energy, is produced when a rise in temperature causes atoms and molecules to move faster and collide with each other. ONLINE RESOURCES Atoms and matter Kinetic and potential energy branch: branca, ramo conversely: al contrario to cool down: raffreddarsi due to: a causa di eventually: alla fine heat sink: dissipatore warming up: riscaldamento HEAT Heat pervades our lives: heating and cooling systems are present in our homes, schools and workplaces to control the temperature and to keep us as comfortable as possible. Energy transfer Heat is present only when there is energy transfer: for example, if we place a very hot mug of coffee with a temperature of 80°C on the kitchen table, after a while, both the coffee and the mug will cool down and will eventually reach room temperature. The coffee and the mug have transferred heat to the surrounding air. The fact that the mug and the coffee lower their temperature is a sign that the average kinetic energy of their particles is decreasing, and they are both losing energy which is transferred to the colder surroundings. Heat We refer to this transfer of energy from the coffee and the mug to the surrounding air as heat, that is to say the transfer of energy from a hot object to a colder object. Conversely, an object increases its temperature by gaining energy in the form of heat from its surroundings. Both the warming up and the cooling down of objects work in the same way. Thermodynamics Thermodynamics is the branch of physics that deals with the relationships between heat and other forms of energy. It developed in the 19th century when scientists were first discovering how to build and operate steam engines. In particular, it describes how thermal energy is converted into and from other forms of energy and how it affects matter. Thermal energy is the energy a substance or system has due to its temperature, i.e. the energy of moving or vibrating molecules. The first law of thermodynamics The first law of thermodynamics states that heat is a form of energy, and that thermodynamic processes are therefore subject to the principle of conservation of energy. This means that heat energy cannot be created or destroyed; it can, however, be transferred from one location to another and converted into and from other forms of energy. 262 EnErgy MechPower.indb 262 30/01/24 17:09