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# First law of thermodynamics / internal energy

First law of thermodynamic and internal energy. Created by Sal Khan.

## Want to join the conversation?

• Does internal energy include nuclear energy?
• Yes nuclear energy is potential energy therefore it is internal energy. Similarly, if you had a hydrocarbon like propane, it has potential energy and therefore internal energy and if you add heat, Q, then it will do work by releasing some of its internal energy.
• at , Sal describes the first law of thermodynamics. however, isn't it the law of conservation of energy?
• Indeed the first law of thermodynamics is a version of the law of conservation of energy.
• What is difference between heat and temperature
• Temperature (in kelvin) is a measure of the average kinetic energy of molecules in an object.
Heat is like mechanical work, in that an object cannot possess heat, but rather is acted upon by heat, changing the internal energy of the object.
The relationship between the temperature and the heat transferred to the object is given by the heat capacity of the object.
If you heat up a pot of water and a pot of copper (of the same mass, using the same heating method) the temperature of the copper will increase faster than the water, even though the heat transferred to them both are equal, due to copper having a lower heat capacity.
• "It is never possible to find internal change however we can find the change in internal energy.how?"
• Internal energy, as Sal explained, are those found in the micro-states (kinetic energy of each atom, energy between bonds in molecules, maybe potential energy of electrons, rotational energy too to some extent and vibrational energy of the atoms/molecules). That too is going to differ every time those particles bump into each other or the walls of the container. Don't you think trying to chart all that for each particle of your system is going to be next to impossible? Hence the statement "It is never possible to find internal change"
But, if we are able to do some work ON the system, or if the system USES the internal energy it possesses to do work (work done BY the system) then we can calculate the change in it's internal energy.
Hope I was explicit enough
• If energy cannot be created,how did it come to existence on earth?
• Are you asking how the universe was created? No one knows that
(1 vote)
• At about when he says it's intuitive for Heat to be represented as Q, because heat does not start with Q, was he being sarcastic, or am I missing some reason as to why Q is heat?
• Sal is being sarcastic about Q being an intuitive representation of heat, since Q is completely unrelated to the word heat. However, he is serious about the use of Q to represent heat, as an accepted method.
• If i take the example of a ball being thrown at a wall, what happens to the energy after the ball hits the wall? Doesnt it get destroyed. if not how are we able to extract or reuse the same energy from the wall.
• The energy doesn't get destroyed, it is conserved. There are a number of things that the energy can go into. There is the sound that the ball makes hitting the wall. There is also the deformation of the ball that causes the ball to increase in temperature and vibrate, there is deformation and vibration of the wall.

Most of the "lost" energy ends up as heat, which is basically random kinetic motion. Unless there is a cooler area that we can get the heat energy to flow to we can't use the energy to do any useful work. This idea of energy that is useful for work and energy that isn't is closely tied to the concept of entropy.
• If temperature is the measure of the average kinetic energy of the molecules in an object/substance, why are certain things - such as a fridge, for example - able to cool hot objects? Surely molecules with lesser average kinetic energy colliding with molecules with greater average kinetic energy would gain momentum and energy and therefore become hotter.
• Refrigerators use coolants like Freon to absorb heat from inside the unit and transfer it to outside the unit.
• Take a walled mirror room and put it on a scale (for measuring the rooms mass with you and your components and what all you have with you) accurate to the mass of anything. Go inside the room with a torch. Now switch the torch on. Does the mass of the room change?
• Consider you started the experiment after entering the room, and that the room is closed.
Chemical reactions do not change the total mass of the substances involved, which include burning stuff. So, if you don't allow gas to escape by closing the room, the mass contained within the room won't change.
Since the total mass inside the room must be constant, and the room's volume is also constant, then the room's density doesn't change. Therefore, you also have no changes in the buoyant forces acting on the room.