Re: Lost Mass in Chemical Reactions

From: N:dlzc D:aol T:com \(dlzc\) (net_at_nospam.com)
Date: 02/01/05 

Date: Tue, 1 Feb 2005 12:05:43 -0700

Dear Marshall Dudley:

"Marshall Dudley" <mdudley@king-cart.com> wrote in message
news:41FFCEC7.EB7060E4@king-cart.com...
> "N:dlzc D:aol T:com (dlzc)" wrote:
>
>> Dear Marshall Dudley:
>>
>> "Marshall Dudley" <mdudley@king-cart.com> wrote in message
>> news:41FFB758.5C7F02B7@king-cart.com...
>> > Lloyd Parker wrote:
>> >
>> >> In article <ctm8of$1ucp$1@pc-news.cogsci.ed.ac.uk>,
>> >> richard@cogsci.ed.ac.uk (Richard Tobin) wrote:
>> >> >In article <ctluhm$bc0$1@puck.cc.emory.edu>,
>> >> >Lloyd Parker <lparker@emory.edu> wrote:
>> >> >
>> >> >>>Mass does not change in a chemical reaction. In an exothermic
>> >> >>>reaction, total energy of the system changes but mass does not.
>> >> >
>> >> >>No, it does. The energy given off comes from a loss of mass
>> >> >>(calculated
>> >> by
>> >> >>E = mc2). In chemical reactions, the energy change is so tiny, the
>> >> >>mass
>> >> >>change is negligible; only in nuclear reactions is the energy large
>> >> enough
>> >> >>to make the mass change significant. However, any energy given off
>> >> >>comes
>> >> >>from a conversion of some mass to energy.
>> >> >
>> >> >Surely the total mass - the mass of the matter and its energy -
>> >> >remains the same. It's just that some of the mass which was
>> >> >"condensed" electrical potential energy has changed to some other
>> >> >form
>> >> >of energy such as heat (kinetic energy). Only if the heat is lost
>> >> >from the reactants will their mass change.
>> >> >
>> >> >-- Richard
>> >> >
>> >> You're saying energy has mass. That's not the normal meaning of the
>> >> term
>> >> "mass" -- the quantity of matter something contains. Scientists
>> >> sometimes
>> >> use the term mass-energy to refer to both together, but I've never
>> >> seen
>> >> "mass" used to refer to both.
>> >
>> > Of course energy has mass. Look up the mass of a photon, it is
>> > non-zero.
>>
>> It is less-than-or-equal-to the resolution available for the
>> measurement.
>> It is "inclusive of zero". In theory, photons cannot have mass, and it
>> provides for various observational difficulties (predicts things that
>> are
>> not seen).
>
> A photon has no rest mass. But since it is traveling at the speed of
> light, it
> has inertia. Thus they appear to have mass.

The term you are looking for may be "invariant mass".

> If you had a box with perfect
> mirrors on each side full of photons, then each side would have a slight
> pressure from the photons inside, but they would balance. If you shift
> the
> box, then the side that is shifted toward the center of the box will be
> struck
> by photons that have a shorter wavelength (doppler effect) then those of
> the
> opposite side, and will thus have a higher "pressure" while accelerating.

This may very well correlate to what we call inertia. Never can tell. The
mass of the box is such a "box"... and photons are contained "within".

> This
> from the outside would would look just like an additional mass
> contributed by
> the photons.

Additionally, pairs of photons in their center of momentum frame have rest
mass. Your "mirrored" box likely has a number of (essentially)
box-stationary pairs.

> That is how solar sails work.
>
> Speaking of solar sails. This article in New Scientist (
> http://www.newscientist.com/article.ns?id=dn3895 ) claims they violate
> the laws
> of physics and cannot work. Their reasoning is:
>
> ---------------------------
> Solar sails are designed to be perfect mirrors, meaning that they reflect
> all
> the photons that strike them. Gold argues that when photons are reflected
> by a
> perfect mirror, they do not suffer a drop in temperature.
> --------------------------
>
> Now, if the spacecraft is travelling away from the sun, then the photons
> that
> are reflected will be a longer wavelength then those that struck the
> mirror.
> The color temperature thus drops, as well as the energy. Can anyone
> figure out
> why they are saying that it doesn't? Are they saying that there is no
> doppler
> drop, or something else?

It is a frame jump on your part. In the frame of the sail, momentum before
= -momentum after. So the incident photon's momentum would simply be
reversed (neglecting the impulse delivered to the sail by the single
photon), with no Doppler effect. In the frame of some object the sail was
moving wrt, the incident light would be a different
color/energy/temperature, and this would be a function of the sail motion
wrt to the observing object.

David A. Smith

Relevant Pages

  • Re: Can inverse gravity waves cancel out Earths gravity in selected areas?
    ... Are you both telling me that radiation pressure can be caused by massless ... photons have zero _REST_ mass. ... that's what a solar sail is. ...
    (sci.physics.relativity)
  • Re: Light in a box
    ... the box is the mechanism for this increase in weight. ... But if the walls are mirrors then it will still add mass to the box ... "The mass of a gas of massless photons," H. Kolbenstvedt, Am. J. Phys. ...
    (sci.physics)
  • Re: Can inverse gravity waves cancel out Earths gravity in selected areas?
    ... Are you both telling me that radiation pressure can be caused by ... photons have zero _REST_ mass. ... that's what a solar sail is. ...
    (sci.physics.relativity)
  • Re: Experimental Evidence for Special Relativity
    ... Pound-Rebka and subsequent experiments of that nature ... for sound reasons states that the energy W of a photon ... There are two methods of measuring mass you either measure the ... The fact is photons have mass. ...
    (sci.physics.relativity)
  • Re: Is light a wave or a particle?
    ... You wanted to have a word to distinguish a certain type of particles ... >>Depends on if you talk about rest mass or relativistic mass. ... The "energy of motion" factor for photons is p*c. ... > because it is created from a moving light wave and a virtual particle. ...
    (sci.physics)