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Energy, heat, and relativity

  1. Oct 5, 2007 #1
    I read in a book that as an object gains more energy that it gains more mass. That if an object heats up that it actually starts to get heavier. It said this has not been proven because it is several orders or magnitude away. Say you have a specific amount of energy in a battery in the form of electrons (what battery is used for). Then you have another battery that is dead with no electrons but is heated up with the same amount of energy. Shouldn't the two then be equal in weight according to this? thanks
     
  2. jcsd
  3. Oct 5, 2007 #2
    Electric energy is being confused for electrons. E=mc2 calls for a charged battery to weigh more than the same uncharged. Slightly irrelevant here is that the electrolyte can be seen to increase from the discharged to the charged state. Don't add water to an uncharged battery as it may overflow when charged. The energy is stored chemically and the mass change detection does not equate to the volume difference in the electrolyte. Discharging changes acid to water. Charging puts back the acidity, increasing the volume.

    The instructor for the mine repair class on battery powered scoop cars told of the old grizzled Kentucky mine repairman. The instructor was showing how to detect a bad cell by difference in density with a hygrometer. The old boy said he done it different and showed him by putting his finger in the cell and tasting it he pronounced the cell "a mite weak." My class laughed and said that I would probably try that. I said I might but I would use my wheezer. The instructor thought I was being smart and asked how I would taste it. I looked at my boss and said I'd have him do the tastin'.
     
    Last edited: Oct 5, 2007
  4. Oct 5, 2007 #3
    Yes, charging the battery or heating the battery are two ways to increase its internal energy and, therefore, it mass, according to [itex]E = mc^2 [/itex].

    Eugene.
     
  5. Oct 5, 2007 #4
    Normally the heat will equalize to optimum levels in 8 hours for a 1200 amp hour lead acid battery after it has charged for 8 hours. The electro/chemical energy stored in the electrolyte will also dissipate, but at the rate of 3% to 20% a month. If the tops are dirty and sulfation present there the rate will be at the high end. But even a new battery when charged is internally discharging. Heat is not the desired energy that we want stored in the battery.
    I've had to change the receptacle when loose cable connections cause heating when drawing the 1000 amps over a period of time. No way to disconnect the + and - cables from the cells. The receptacle connections were an inch apart. These batteries were 20,000 pound monsters. Small compared to submarines batteries, but cables there could be disconnected from the cells before working on them. No room for error. Boy I miss my old job. Like a sore wheezer.
     
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