A microscopic approach to the concept of Heat as energy transferred between two ideal gases held in contact.
Differences between Heat and Internal Energy.

Heat refers to an energy in transit between two systems held in contact that are at different temperatures.

The word heat is used in ordinary language with a meaning different from the previous definition. This often causes the student to approach this concept starting from a wrong idea.

WARNING: before answering the questions make sure that the Test Control Window is open, if not click here.
EXPERIENCE: Energy transfer between two bodies at different temperature.

Consider two systems A and B (for instance, two ideal gases) made up of 200 and 100 particles, respectively. System A (white particles) possess a certain internal energy and temperature, as shown in the corresponding windows. System B (red particles) has zero internal energy, and then its temperature is zero. 

Two systems will be said to be in thermal contact when their particles are allowed to collide between them, and therefore can exchange energy. The proposed situation is certainly an extreme case, as the particles of B are at rest, which corresponds to the absolute zero temperature. Moreover, the systems are two-dimensional in order to get a most efficient interaction between the particles. 

The distance between the two systems is controlled through the slider "CONTACT", initially at 100. Slide it until the value 0 is reached, when thermal contact will be achieved. The collisions between particles of both systems result in system A giving some energy to system B. The energy transferred in this way is the Heat Q exchanged between the two systems. Record the initial value of the energy of system A, and compare it with the final values of the energies in both systems. 

Looking at the results obtained in the previous experience choose one of the following options: 
The final energy of system A is less than that of B.
The sum of the final energies of the two systems equals the initial energy of A.
The energy lost by A is less than the energy gained by B.

Check your conclusions by clicking here


These three concepts are clearly different, though there exist relations between them. An experience is proposed to help in the understanding of the following ideas:

Run the simulation, carry out the following experiences controlling the number of particles and the energy in each system, and then analyze the results obtained when the two systems are put in contact in each situation. 

Two bodies A and B 

  • The temperature and internal energy of A are higher than those of B. 
  • The temperature of A is lower than that of B, but its internal energy is higher. 
  • The temperature of B is lower than that of A, but its internal energy is higher. 
  • The temperature and internal energy of B are higher than those of A. 
From the results obtained, choose one of the following options: 
There is always a transfer of energy from the body with higher internal energy to the body with less internal energy.
There is always a transfer of energy from the body at higher temperature to the body at lower temperature, independently of the values of the internal energy.
There is no energy transfer if the internal energies of the two bodies are the same.
Index 1.Introduction 2.Pressure 3.Temperature 4.internal Energy
6.Work 7.First Law 8.Entropy 9.Velocity Distribution 10.Specific Heat