Thermal Equilibrium

• The laws of thermodynamics were first developed during the Industrial Revolution when the quest was on to develop a better steam engine. Thermal equilibrium is a basic concept in thermodynamics, and is known as Zeroth's law. If two objects are the same temperature, no heat exchange takes place. However, if the objects are at different temperatures, heat flows from the object at the higher temperature to the object at the lower temperature to achieve thermal equilibrium.
  
  

Conduction

• Conduction is the flow of heat generated by the vibrations and collisions of atoms and molecules moving at varying speeds because of temperature differences. The molecules at a higher temperature vibrate faster than the molecules at the same or lower temperature. When the molecules collide, energy is transferred from the more energetic molecules to the less energetic molecules. Heat always flows to the coldest temperature.
  
  

Convection

• Convection is the transfer of heat in fluids. Hot liquid is not as dense as cool liquid. Convection currents in the fluid exerts an upward, buoyant force on the hot liquid. The hotter liquid is displaced and risesto the surface, while the cooler, denser liquid sinks to the bottom. The molecules in the hot liquid continuously transfer heat to the cooler liquid.
  
  

Thermal Radiation

• Thermal radiation is the emission of energy from surfaces of different temperatures in the form of electromagnetic waves. When the surface temperatures are the same, the amount of energy received and the amount of energy given off is in equilibrium. No temperature change occurs. However, when surface temperatures vary, the hotter surfaces give off more energy in the form of radiation than they absorb and the colder surfaces absorb more energy in the form of radiation than they give off. There is a net heat transfer between surfaces of different temperatures.