Whenever materials of unequal temperature are in contact, heat will flow from one material to the other. Drop an ice cube in a glass of water and the ice warms up and melts while the water cools down. Pour a kettle of boiling water into a tepid bath and the boiling water cools while the bath warms up. This is true of any two materials. Touch a car that has been sitting in the hot sun and the heat will flow from the metal to your finger. Or think of what happens when an auditorium fills with people. Body temperature is warmer than room temperature, so humans radiate heat. So heat will flow from the people to the air in the room, and the room gets hotter.
Ice melting provides an example of entropy?increasing and heat flow from the water to the ice. Click here for original source URL
Our everyday experience shows that heat tends to flow from a hotter material to a colder material. The study of how heat moves is called thermodynamics. Some of the first experiments in this subject were done by Isaac Newton, who found that the rate of heat transfer between two adjacent objects or regions is proportional to the difference in their temperatures. If there is no temperature difference between an object and its surroundings, there will be no heat flow. But the larger the temperature difference, the faster heat transfers from the warm region to the cool region. Using the mathematical form of this law, Newton calculated how much time is needed for various objects to cool from a given temperature.
These everyday examples illustrate the concept of equilibrium. When different regions of a system have different temperatures, heat energy will flow and the system is said to be out of equilibrium. When all regions have the same temperature, no heat energy flows and the system is in equilibrium. So while the ice cube is melting, heat energy is flowing from the water in the glass to the ice. After the ice cube has melted, all the water has the same temperature, cooler than it was before. As you pour boiling water into a bath, heat energy is flowing into the bath water. After the boiling water is mixed in, all the water has the same temperature, warmer than it was before. The end result of both actions is a situation of equilibrium.
This principle of equilibrium operates at the microscopic level as well as at the macroscopic level. Temperature is just a measure of the violence of the motion of individual atoms or molecules. The tiny particles in a piece of very hot metal are moving faster than the particles in the skin of your hand. When you touch the hot metal, the fast moving metal particles agitate the particles in the skin of your hand and make them move faster. Heat is transferred and your finger gets hot. The molecules in an ice cube are vibrating more slowly than the molecules in water. As the ice melts, the molecules in the surrounding water slow down and the water cools down.
Heat engine diagram and an example of heat flow. Click here for original source URL.
We can even make the concept of heat flow visible by imagining a set of balls on a pool table. Let’s imagine that they are analogous to atoms. Suppose that they are not moving, which would correspond to zero temperature or no heat. Now roll a cue ball fast across the table so that it bounces hard off all the cushions. This incoming "atom" has a high velocity and so a high temperature. As the cue ball collides with the stationary balls, they start to move and the cue ball slows down. The cue ball has transferred energy to the other balls. As a result, the cue ball gets "colder" and the other balls get "hotter." This is just like the example where a drop of boiling water falls into a cooler liquid.
Hot atoms or molecules move faster than cold ones. If we have a hot body of material next to a colder body of material, then energy will flow from the hot one to the cold one. The average kinetic energy of molecules in the hot one will decrease and the average kinetic energy of molecules in the cold one will increase. When the average kinetic energy per molecule is the same in both objects — that is when they reach the same temperature — they are in equilibrium and no more net heat flow will occur between them. Nature displays a universal tendency to move towards equilibrium.
Types of heat transfer for different stars. Click here for original source URL.
Heat transfer and the idea of equilibrium come up repeatedly in the study of planets and stars. Equilibrium is a quite general physical principle, so when dealing with a situation of heat flow we will talk about thermal equilibrium. Put in simple words, this means that unequal temperatures will tend to even out and become equal.