Let's consider two situations:

a) A solid metal box (not hollow) where some heat is put on its left side. After some time, the heat will have moved to the right and also have spread out throughout the box.

b) A fully charged battery is connected to a motor and to the motor is a long massive rod attached in the direction of the axis of rotation of the rotor. All these objects are in a container (hollow this time, but not important for the story). The motor and battery lay together at the left side of the container. The rod goes from the rotor all the way to the right side of the container. The rod and rotor are not rotating at first. Then, the motor is turned on and energy starts "flowing" from the battery into the rod and thereby converting electrical enrgy into rotational energy. This goes on until all energy from the battery is put into the rod's rotation.

In both situations no energy is able to escape to the environment, thus both are isolated systems. In my eyes, energy flows from left to right in both cases. I know that when a mass moves from one place to another by pushing itself off of the container, it obbeys Newton's laws of motion and thus causing displacement of the mass and the container.

My question is, is there in any situation displacement of the systems/containers due to energy "flowing" (to the right or left)?

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    $\begingroup$ For me, the question is unclear. $\endgroup$ – toliveira Oct 5 '15 at 18:57
  • $\begingroup$ In situation b), the energy was in the battery at first (on the left of the container), then after some time it was put into the rod's rotation. Battery is empty now, and the rod is rotating, thus the energy is now on the right side of the container. It must've moved or "flowed" somehow to the right, which I am questioning wether that causes displacement of the objects themselves or the container. $\endgroup$ – Arundel Oct 5 '15 at 19:03
  • $\begingroup$ Situation (a) is irrelevant to the question, as it seems, since in that case, energy does not move to right side, it spreads throughout. In situation (b), situation (a) has no relevance for if you keep the rod perpendicular to bottom of box, i.e. the rod projects towards the top of the box, then also energy moves from bottom to top , not left to right. So what??? Question is unclear. $\endgroup$ – SchrodingersCat Oct 5 '15 at 19:18
  • $\begingroup$ I see. I think the answer is simply "no, energy flowing from one place to another in an object doesn't cause displacement of that object." As you can see in both examples, the objects remain in their position. Even the centre of mass of the rod, which is turning, is not moving. $\endgroup$ – toliveira Oct 5 '15 at 19:50
  • $\begingroup$ @Aniket, the energy does move to the right side in situation a) AND I assumed that due to the nature of heat that is unevenly distributed (more energy on the left to start with) thermal equilibrium will eventually be reached throughout the whole box, and I therefore mentioned the energy spreading throughout aswell as moving to the right. I think it's still relevant to compare to situation b). And wether it's up or down: it don't matter; left and right were just an example.And the rod is actually parallel to the bottom of the box and parallel to the rotor(also just as an example from my story) $\endgroup$ – Arundel Oct 5 '15 at 20:30

For case 1), heat exists in forms of oscillations of molecules. That is to say, the microparticles move back and forth in random directions but assumed never synchronized in one direction and never come back. The heat flow direction is the same as the kinetic energy gradient direction, but nothing about the displacement of the system. In the end, I would say the displacement of the system maintains zero.

Case 2), moved are the electrons in a closed circle, but assumed no escapes. Therefore, no displacement will be observed for the system as well.

  • $\begingroup$ Electrons moving in a circle I understand, but what after the battery is empty, when all stored energy is used up and put into the rotation of the rod? The energy had to "travel" along the rod in some way in my eyes. The battery was on the left, the rod on the right, therefore energy has flowed from left to right. Does that flow not move the system or parts of it? Because when for example a mass pushes itself to go to the right, it provides an opposite reaction to the container, thus making the container displaced to the left. $\endgroup$ – Arundel Oct 6 '15 at 17:24
  • $\begingroup$ For a current movement, you can think of it as electrons moving from negative pole to positive pole to neutralize the battery. If you would like to treat electrons' mass as non-negligible, sure, electrons move. But normally I would say the mass of electrons can be ignored and hence the system is not moving to anywhere under usual observations. $\endgroup$ – Xiaodong Qi Oct 6 '15 at 19:11
  • $\begingroup$ Yes, I understand what you say but I will keep on repeating: situation 2 wasn't about electrons travelling in a circle, it was about electrical energy being converted to rotational kinetic energy in the rod, thus in my eyes energy must have moved from left to right (because the battery was on the left and the rod on the right), and since energy and mass are related, my question is if a colliding/moving mass can cause displacent to another mass, can moving energy cause displacement too to a mass? $\endgroup$ – Arundel Oct 11 '15 at 17:22
  • $\begingroup$ Unless you consider the rotation is so fast that you have to consider relativity effects, energy has nothing to do with mass for situation b. $\endgroup$ – Xiaodong Qi Oct 11 '15 at 23:35

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