Conservation of mass and energy

In summary, mass is not always conserved due to particle creation and annihilation, but invariant mass is always conserved. Rest-mass is often taken to mean the same thing as invariant mass, but in this context, it is not conserved due to particle creation and annihilation. However, energy is always conserved, but it is important to consider all forms of energy and transformations such as E=mc^2. The more general conservation law is the conservation of mass-energy.
  • #1
noobphysicist
26
0
Hmm, I
s matter always conserved?
Is mass always conserved?
Is rest-mass always conserved?
Is energy always conserved?

are there any exceptions?
 
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  • #2
noobphysicist said:
Is matter always conserved?

No - matter/anti-matter annihilation and creation.

noobphysicist said:
Is mass always conserved?

No ... see above.

noobphysicist said:
Is rest-mass always conserved?

No ... you can change the rest mass of a pile of potatoes by heating them.

noobphysicist said:
Is energy always conserved?

Yes - but you need to take into account all forms of energy, and transformations such as E=mc^2.
With the possible exception of General Relativity ... but if GR does not apply then energy is conserved.
 
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  • #4
UltrafastPED said:
No - matter/anti-matter annihilation and creation.



No ... see above.


I just found that mass is conserved because mass equals energy divided by c^2, and energy is always conserved, no?
 
  • #5
noobphysicist said:
I just found that mass is conserved because mass equals energy divided by c^2, and energy is always conserved, no?

The more general conservation law here is the conservation of mass-energy, not just mass, not just energy.

Zz.
 
  • #6
noobphysicist said:
I just found that mass is conserved because mass equals energy divided by c^2, and energy is always conserved, no?

No - mass has energy, but energy does not necessarily have mass.

For example, light has no mass, but does have energy.

Nor does kinetic energy contribute to the mass:
http://en.wikipedia.org/wiki/Energy–momentum_relation
 
  • #7
The OP asked if mass is conserved and if rest mass is conserved. Mass is often taken to mean the same thing as rest mass, but in this context, you could also interpret mass to mean invariant mass. Rest-mass is not conserved because of particle creation/annihilation, but invariant mass is conserved (at least, not counting any general relativity weirdness). This is because a pair of photons traveling in opposite directions has a combined invariant mass.

In fact, invariant mass is independent of observer frame, so I think it should be conserved even in GR, since the problems of parallel transport do not apply.
 
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Related to Conservation of mass and energy

1. What is the law of conservation of mass and energy?

The law of conservation of mass and energy states that the total amount of mass and energy in a closed system remains constant over time. This means that mass cannot be created or destroyed, only transformed into different forms, and energy cannot be created or destroyed, only converted into different types.

2. Why is the conservation of mass and energy important?

The conservation of mass and energy is important because it is a fundamental law of nature that allows us to understand and predict the behavior of physical systems. It also has practical applications in fields such as chemistry, physics, and engineering.

3. How does the conservation of mass and energy apply to everyday life?

The conservation of mass and energy applies to everyday life in many ways. For example, when we burn fuel in a car, the energy from the fuel is converted into motion, but the total amount of mass and energy remains the same. It also applies to the food we eat, as the energy from the food is converted into the energy our bodies need to function.

4. Is the law of conservation of mass and energy always true?

Yes, the law of conservation of mass and energy is always true in a closed system. This means that no mass or energy can enter or leave the system, and all transformations must occur within the system. However, in certain extreme situations, such as black holes or nuclear reactions, the law may appear to be violated, but this is due to our limited understanding of these phenomena.

5. How is the conservation of mass and energy related to the concept of entropy?

The conservation of mass and energy is related to entropy, which is a measure of the disorder or randomness in a system. The law of conservation of mass and energy states that the total amount of mass and energy in a closed system remains constant, but the entropy of the system tends to increase over time, resulting in a decrease in the available energy. This is why systems tend to move towards a state of greater disorder and less usable energy, in accordance with the second law of thermodynamics.

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