Kara Sears
The law of conservation of energy states that energy can neither be created nor destroyed, only transformed from one form to another. For example, chemical energy from food is converted to thermal energy when it is broken down in the body. The total energy of an isolated system remains constant over time. However, there is debate over whether the law can be broken. Some argue that the law is far from being empirically true, and that potential energy is ill-defined. Others claim that general relativity on a cosmological scale may violate the law. Despite these arguments, there is no known example of a violation of the principle of conservation of energy.
| Characteristics | Values |
|---|---|
| Law of Conservation of Energy | Energy can neither be created nor destroyed |
| Energy can be transformed from one form to another | |
| In an isolated system, the total energy of the system remains constant | |
| In a closed system, the total energy within the system can only change through energy entering or leaving the system | |
| In a closed system, the total energy of the system is conserved | |
| The law of conservation of energy can be violated by general relativity on a cosmological scale | |
| The law of conservation of energy can be violated by 'expansion' redshifts | |
| The law of conservation of energy cannot be proven |
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What You'll Learn
Perpetual motion machines
The law of conservation of energy states that energy can neither be created nor destroyed, only transformed or transferred from one form to another. In a closed system, the total amount of energy within the system can only change if energy enters or leaves.
Perpetual motion is the motion of bodies that continue forever in an unperturbed system. A perpetual motion machine (PMM) is a hypothetical machine that can do work indefinitely without an external energy source. This kind of machine is impossible, as its existence would violate the first and/or second laws of thermodynamics. These laws apply regardless of the size of the system.
The history of perpetual motion machines dates back to the Middle Ages. For millennia, it was unclear whether perpetual motion devices were possible or not, until modern theories of thermodynamics showed they were impossible. Despite this, many have tried to create such machines, continuing into modern times.
There are three kinds of perpetual motion machines:
- The first kind includes devices that deliver more energy from a falling or turning body than is required to restore the device to its original state. An example is the overbalanced wheel, with flexible arms attached to the outer rim of a vertically mounted wheel. This violates the first law of thermodynamics, which states that the total energy of a system is always constant.
- The second kind attempts to violate the second law of thermodynamics, which states that some energy is always lost in converting heat into work. An example is the ammonia-filled "zeromotor" developed in the 1880s by John Gamgee.
- The third kind is defined as one that completely eliminates friction and other dissipative forces to maintain motion forever due to its mass inertia. However, this is impossible, as dissipation can never be completely eliminated in a mechanical system.
Some machines are sometimes referred to as perpetual motion machines because they comply with both laws of thermodynamics by accessing energy from unconventional sources. For example, clocks and other low-power machines have been designed to run on differences in barometric pressure or temperature between night and day. However, these machines are not truly perpetual motion machines because they are consuming energy from an external source and are not isolated systems.
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General relativity
The law of conservation of energy states that energy cannot be created or destroyed, only transformed from one form to another. In the context of general relativity, this law is seemingly broken.
In flat spacetime, energy conservation can be phrased as a differential equation or an equation involving integrals. However, when generalised to curved spacetimes, this equivalence breaks down. The differential form extends without issues, but the integral form does not.
There are multiple definitions of energy as a conserved global quantity in general relativity. The three main ones are Komar energy, which requires a stationary spacetime, and Bondi and ADM energy, which require asymptotic flatness. Local energy conservation is built into the theory, but it is challenging to extend this to a global law.
Some theorists argue that general relativity does not have time-translation invariance because the gravitational field is not invariant. However, this can be addressed by including the gravitational field as a dynamical field with its own time-translation invariance.
Despite these arguments, it is essential to note that energy has been conserved in all circumstances where it can be experimentally tested. It is also conserved according to theory in any system with time-translation invariance, including general relativity.
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Quantum mechanics
The law of conservation of energy is a fundamental principle in physics, stating that the total amount of energy in a closed system remains constant. While this law generally holds true, there are certain scenarios and theories that challenge its absolute validity, particularly in the realm of quantum mechanics.
The Many-Worlds interpretation of quantum mechanics offers a different perspective. In this formulation, the wavefunction of the universe is perfectly conserved, and the creation of new universes does not require additional energy. However, from the perspective of observers within a particular universe, energy conservation appears to be violated during quantum measurements. This paradoxical situation has led to ongoing debates and investigations within the field of quantum mechanics.
Additionally, the uncertainty principle, as proposed by Heisenberg, suggests that a particle's energy level can fluctuate for short periods as long as the product of the change in energy and the duration is less than a specific value. While this may not directly violate the law of conservation of energy, it highlights the intricate and probabilistic nature of quantum mechanics, where precise predictions are challenging.
Furthermore, the expanding universe presents another context in which the law of conservation of energy becomes intricate. As the volume of space increases, the energy density of matter decreases, while the energy density of starlight and radiation diminishes at a faster rate, seemingly without conversion into other forms of energy. This asymmetry in time poses challenges to the traditional understanding of energy conservation.
While these quantum mechanical phenomena and interpretations may seem to suggest violations of the law of conservation of energy, it is important to recognize that the field of quantum mechanics is still evolving. Physicists are actively working to reconcile these apparent contradictions and develop a more comprehensive understanding of energy conservation within the quantum realm.
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Inconsistent conservation
The law of conservation of energy states that energy can neither be created nor destroyed in an isolated system. However, it can be transformed from one form to another. For example, chemical energy is converted to kinetic energy when a stick of dynamite explodes. The total energy within a closed system can only be changed through energy entering or leaving the system.
While the law of conservation of energy has never been proven to be violated, there are some inconsistencies and limitations to consider. One example is the concept of “potential energy”, which has been described as a “fudge factor” to fix inconsistencies in the theory. Potential energy is difficult to empirically measure and verify, and some argue that it is simply a bookkeeping method to ensure that energy appears to be conserved. This is because, in some cases, the law of conservation of energy does not seem to hold true when only considering measurable natural agents that form work. In these cases, “potential energies” are postulated, where actual energy may disappear or originate, but there is no empirical evidence or measurement of these energies.
Another example of inconsistent conservation is the phenomenon of “expansion” redshifts, where two objects are stationary relative to each other, but the space between them is expanding. This scenario creates a violation of the conservation of energy.
Additionally, in the field of quantum mechanics, while Noether's theorem applies to the expected value, it is debated whether individual conservation-violating events could exist or be observed.
Furthermore, early understandings of the law of conservation of energy focused on the conservation of momentum, which was later found to be inadequate for practical calculation. This led to the development of more comprehensive theories, such as the law of conservation of vis viva championed by the Bernoullis.
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Energy creation
The law of conservation of energy states that energy cannot be created or destroyed, only transformed or transferred from one form to another. This is also known as the principle of energy conservation. While energy cannot be created from nothing, it can be converted from one form to another. For example, chemical energy from food is converted to thermal energy when broken down by the body to keep it warm. Similarly, chemical energy is converted to kinetic energy when a stick of dynamite explodes.
The principle of energy conservation can be applied to closed systems, where the total amount of energy within the system can only change if energy enters or leaves the system. For instance, in a steam turbine, fuel is burned to produce hot water and steam, and the steam powers a turbine that drives a generator, converting mechanical (kinetic) energy into electrical energy.
There are various methods to convert different forms of energy into electrical energy, including rotating electric generators, photovoltaic systems, and batteries. Electric generators transform kinetic energy into electricity, and this is the most common method for generating electricity based on Faraday's law. Electromagnetic induction is used in almost all commercial electrical generation, where mechanical energy forces a generator to rotate.
While the law of conservation of energy holds true, there are speculative methods to recover energy, such as proposed fusion reactor designs that aim to extract energy from intense magnetic fields. Additionally, in the field of quantum mechanics, while Noether's theorem applies to the expected value, making any consistent conservation violation impossible, it is debated whether individual conservation-violating events could exist or be observed.
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Frequently asked questions
The law of conservation of energy cannot be broken. It states that energy cannot be created or destroyed, only transformed from one form to another.
Chemical energy is converted to kinetic energy when a stick of dynamite explodes. The total amount of energy remains the same.
Einstein's theory of mass-energy equivalence shows that mass is a form of energy. The amount of mass is directly related to the amount of energy.
Potential energy is a controversial aspect of the law of conservation of energy. Some argue that it is a "fudge factor" to explain the conservation of energy, while others argue that it is a reliable bookkeeping method.
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