When we iron our clothes does the mass of the cloth increase, since energy transformation and transfer happens, Einstein's energy mass relation comes to action and causes increase in mass.

Am I right or wrong?

  • $\begingroup$ Energy transformation and transfer - what's that? Or, what do you mean by that? $\endgroup$ Jul 1 '13 at 14:51
  • $\begingroup$ You know that energy-mass transformation occurs during nuclear reactions. I doubt there is a nuclear reaction going on during ironing. $\endgroup$ Jul 1 '13 at 15:38
  • $\begingroup$ 1 Joule of energy has a mass of around 11 nanograms. $\endgroup$ Jul 3 '13 at 5:38
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    $\begingroup$ Oops! It is actually 110 fantograms/femtograms. So, assuming your cloth is made of 100 grams of uranium, and you heat the cloth by 30 K, then the cloth will gain 33 picograms. $\endgroup$ Jul 3 '13 at 5:44
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    $\begingroup$ And that is less than half the mass of a red blood cell. If the cloth is made of something else, like paraffin wax, then it will be 25 times more, so that is around 12.5 red blood cells. $\endgroup$ Jul 3 '13 at 5:53

Assume that we started with a dry cloth, use a dry iron (no steam) and don't initiate any chemistry (you don't want to burn the collar, after all).

There are two way to look at this.

  • On a macroscopic scale the cloth (taken as a whole) has gained internal energy, so it simply is more massive. No "transformation" is required. The energy exists in the more energetic bounds between atoms and molecules and that is mass. Just as the proton is mostly binding energy rather than the (very small) rest energy of the valence quark content.

  • On the microscopic scale the heat ends up almost entirely in thermal motion of existing molecules without changing or exciting the molecules. So the molecules have the same mass they did before.

If you have excited some of the molecules than those molecules (taken as a whole) are more massive than before, but their unexcited atoms are not. And so on, up and down the energy/distance scales.

Aside: There seems to be a lot of misunderstanding about this. There is no magic which transforms energy from "energy" into "mass". Energy in internal degrees of freedom acts as mass as long as the physics you are interested in can't probe those internal degrees of freedom. When you can probe a particular scale you become able to separate the kinetic energy of the components and their interaction energy and their mass (which may consist partly of binding energy of smaller components, too).

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    $\begingroup$ I think the OP might also be under the impression that the mass increase is permanent. It should be emphasized that the (undetectably small) increase in mass here will vanish when the cloth cools down again. $\endgroup$
    – user10851
    Jul 1 '13 at 17:20
  • $\begingroup$ what should i consider yes or no $\endgroup$
    – gksingh
    Jul 2 '13 at 5:14
  • $\begingroup$ @gkshindia "the cloth (taken as a whole) [...] is more massive". Mind you, it is instructive to calculate the scale of the mass gain involved. $\endgroup$ Jul 2 '13 at 13:55

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