Laws of thermodynamics - RationalWiki
The intimate connection between matter and energy has been a source of of the fundamental concepts of energy and heat and the relation between them. which is incorporated into the modern concept of "conservation of energy". When God created the world, He "caused some of its parts to push. The change in internal energy that accompanies the transfer of heat, q, or work, w , into or out of a system can be calculated using the following equation. This principle, also known as the “conservation of energy principle” (Cengel . of power input and the rate at which heat is released in the First Law equation.
In other words, Walter, and only his motional energy, his kinetic energy, was the only energy we were keeping track of, that's why we said that, initially, there was just Walter's kinetic energy.
And this sheet of ice was external to our system, not part of our system, that's why it exerted a negative external work, removed the energy from the system, and Walter ended up with no kinetic energy. But there's an alternate way to go about this calculation. You could say, "Alright, instead of just considering "Walter and Walter alone to be part of our system, "let's go ahead and include the ice as part of our system.
So an alternate way to solve these problems, is to use this same formula, but now, Walter and the ice are both part of our system. Our system would still start with the kinetic energy that Walter had at the beginning, that doesn't change. But now there would be no external work, not because force of friction isn't acting, there's still a force of friction, but that's an internal force between objects in our system. So there's no external work done now.
That might be a new or confusing idea to some people, so let me just say, if there's forces between objects within your system, then those forces cannot exert external work and they cannot change the total energy of your system. Only forces exerted on objects within your system from outside of your system, can change the total energy of your system. So when this ice was not part of our system, it was exerting an outside external force on Walter, and the energy of our system changed.
We started with kinetic energy, we ended with no energy. But now that the ice and Walter are part of our system, this force of friction is no longer external. It's internal, exerted between objects within our system, and so it does not exert any external work.
It's just going to transform energies between different objects within our system. So that's why we write this zero here, there'd be no external work done if we choose the ice and Walter as part of our system. And this would have to equal the final energy, and we know where this energy ends up. It started with kinetic energy, and it ends as this extra thermal energy in the snow, and Walter's feathery coat. So I could write that as e thermal. But I know how much thermal energy was generated, this just has to equal the amount of work done by friction.
So even though this work done is not external, it still transfers energy between objects within our system, so when we write that the work was negative f k d down here, we mean that the force of friction took f k d from something and turned it into something else, and that's all we need up here.
We need an expression for thermal energy. But if friction took f k d and turned it into something else, the thing it turned it into was the thermal energy so that value of f k d, that magnitude of the work done, was how much energy ended up as thermal energy. People might find that confusing, they might be like, "Wait a minute, why do we have this "with a positive here and not a negative?
If you take energy from something, you're doing negative work on it. If I gave energy to something, I'd be doing positive work. So this negative sign in the work done, just means that the force of friction took this much energy from something, and turned it into thermal energy. So when we want to write down how much thermal energy did we end with, well, we ended with the amount that we took.
So we took f k d, the thermal energy ended with f k d. And I can still set this equal to the kinetic energy that Walter started with, and I get the same formula I ended up with over here, because I had to.
Because we're describing the same universe and the same situation, so no matter what story you tell, you should get the same physics in the end. And we do, but some people prefer one to the other.
Thermal energy from friction
Some people like thinking of friction as a negative external work, and not including the energy within the surfaces as part of their system. And some people like including those surfaces as part of their energy system, and just including that thermal energy on the e final side.
Which is fine, you can do either, you just can't do both. Either the surface is part of your system, and you include it in your final energy, or the surface is not part of your system, and you include it as external work. But you can't say it does external work and it gains some final energy over here because it's got to be either part of your system, or not part of your system. Our article on conservation of energy further explores this concept. Thermal energy from friction Consider the example of a man pushing a box across a rough floor at a constant velocity as shown in Figure 1.
Since the friction force is non-conservativethe work done is not stored as potential energy.
What is thermal energy? (article) | Khan Academy
All the work done by the friction force results in a transfer of energy into thermal energy of the box-floor system. This thermal energy flows as heat within the box and floor, ultimately raising the temperature of both of these objects.
- Laws of thermodynamics
- What is the second law of thermodynamics?
Man pushing a box opposed by friction. Recall that the box is moving at constant velocity; this means that the force of friction and the applied force are equal in magnitude. The work done by both these forces is therefore also equal.
James Joule himself predicted that it would be. It has been calculated that at Niagra falls, that complete conversion of the potential energy of 1 kg of water at the top into kinetic energy when it hits the plunge pool 58 meters below will result in a temperature increase of about 0.
More on internal energy
But there are lots of complications. For example, some of the water breaks up into tiny droplets as it falls, and water evaporates from droplets quite rapidly, producing a cooling effect. Chemical energy can also be converted, at least partially, into electrical energy: But as we mentioned above, when an object is dropped onto the floor, or when an exothermic chemical reaction heats surrounding matter, the kinetic energy gets dispersed into the molecular units in the environment.
We refer to this as "thermalized" kinetic energy, or more commonly simply as thermal energy.
We observe the effects of this as a rise in the temperature of the surroundings. The temperature of a body is direct measure of the quantity of thermal energy is contains. Convsersion of thermal energy to heat is never completely recoverable Once kinetic energy is thermalized, only a portion of it can be converted back into potential energy. The remainder simply gets dispersed and diluted into the environment, and is effectively lost. Potential energy can be converted entirely into kinetic energy.
Potential energy can also be converted, with varying degrees of efficiency,into electrical energy.