Virial theorem as applied to hydrogen atom

In summary, the virial theorem states that for a potential that goes like 1/r, the average potential energy within a system is exactly twice that of the average kinetic energy. This is a special case of a more general theorem that states for a potential that goes like r^n, the average potential energy is equal to n times the average kinetic energy. The potential energy term in this theorem is not an absolute value, but rather the value it would have at infinite separation. This relationship between potential and kinetic energy can also be seen in the Schrodinger analysis of the hydrogen atom.
  • #1
Dadface
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In the virial theorem the numerical value of the average potential energy within a system is exactly twice that of the average kinetic energy. I know the theorem is proved mathematically but to me it seems a coincidence that one value is exactly twice the other value. I find that interesting.
I find it more interesting and more of a coincidence when I take into account the fact that the potential energy term is not an absolute value but the value it would have if the potential energy at the chosen separation of infinity is given the chosen value of zero.
Can anyone explain, without the maths, why one value is twice the other. I'm trying to get an intuitive feeling of why this should be the case.
Also, does the Schrodinger analysis of the hydrogen atom give a better proof of the virial theorem than the Bohr treatment? I ask this because I think the simple Bohr analysis ignores the kinetic energy of the proton.
 
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  • #2
Dadface said:
In the virial theorem the numerical value of the average potential energy within a system is exactly twice that of the average kinetic energy.

More precisely, in the virial theorem for a potential which goes like 1/r, we have ##2 \langle T \rangle = - \langle V \rangle##, where ##\langle T \rangle## is the time average of the total kinetic energy and ##\langle V \rangle## is the time average of the total potential energy. The minus sign comes from the ##1 / r## potential; it is a special case of a more general theorem which says that, for a potential that goes like ##r^n##, we have ##2 \langle T \rangle = n \langle V \rangle##. The ##1 / r## potential is just the case ##n = -1##.

See here:

https://en.wikipedia.org/wiki/Virial_theorem

Dadface said:
the potential energy term is not an absolute value but the value it would have if the potential energy at the chosen separation of infinity is given the chosen value of zero

Again, this is for a potential that goes like ##1 / r##; it obviously vanishes as ##r \rightarrow \infty##. More generally, a potential that goes like ##r^n## will vanish as ##r \rightarrow \infty## for any ##n < 0##. This makes physical sense since we expect two particles at infinite separation to have zero interaction with each other.
 
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Thank you PeterDonis. The thing that interests is the fact that one value is numerically twice the other value. Without looking into the theory in more detail it seems to be a nice coincidence.
 

1. What is the virial theorem and how does it apply to the hydrogen atom?

The virial theorem is a mathematical relationship that describes the equilibrium state of a system of particles. It states that the average kinetic energy and the average potential energy of the particles in a system are related by a factor of two. In the case of the hydrogen atom, the virial theorem can be used to relate the average kinetic energy of the electron to the average potential energy of the electron due to its interaction with the nucleus.

2. How is the virial theorem derived for the hydrogen atom?

The virial theorem for the hydrogen atom is derived from the Schrödinger equation, which describes the motion of the electron in the atom. By solving the Schrödinger equation and using the properties of the electron and the nucleus, the virial theorem can be mathematically derived for the hydrogen atom.

3. What is the significance of the virial theorem in understanding the hydrogen atom?

The virial theorem provides insight into the balance of forces within the hydrogen atom and helps us understand the stable equilibrium state of the atom. It also helps us calculate the average kinetic and potential energies of the electron, which are important in understanding the behavior and properties of the atom.

4. Can the virial theorem be applied to other atoms besides hydrogen?

Yes, the virial theorem can be applied to any system of particles, including other atoms. However, the specific mathematical relationships may differ depending on the properties of the particles and their interactions. In the case of the hydrogen atom, the virial theorem has a simple and elegant form due to the simplicity of the system.

5. What are some practical applications of the virial theorem in studying the hydrogen atom?

The virial theorem is commonly used in quantum mechanics to calculate the energies of the electron in the hydrogen atom. It also has applications in astrophysics, specifically in understanding the behavior of stars. Additionally, the virial theorem is used in theoretical chemistry to model and predict the behavior of molecules and chemical reactions involving hydrogen atoms.

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