# Questions tagged [conservation-laws]

The statement that a property of a system does not change if the system is isolated.

2,738 questions
Filter by
Sorted by
Tagged with
11 views

### According to the standard model, if the universe were not expanding and treated as a single, closed system, would energy be conserved?

It is said that energy conservation requires the reversibility of time, but space expanding breaks that condition. If the universe was not expanding and treated as a single, closed system, is energy ...
60 views

### Incompressible fluid flow in tilted pipe setup - Continuity Equation, Flow Rate, Mass Conservation

Say that we have water flowing horizontally in a pipe of constant diameter. The continuity equation for incompressible fluids, which is a statement of mass conservation, guarantees that the flow rate ...
• 209
25 views

### How do we visualise multiplication and division (reciprocal multiplication) in physical equations?

All formulas have terms generally multiplied or divided to represent another physical quantity. Like $F=ma$, $I=Q/t$, $W=F.s$ etc. Technically this is so because of ratios and proportionalities, the ...
1 vote
38 views

### Continuity equation for charge density

Let $\rho$ be the charge density and $M_i$ the momentum density. The article I am reading states that the continuity equations for this system are given by, \frac{\partial \rho}{\...
53 views

### Local conservation of charge does not imply global conservation of charge?

In order for a set of $\rho, \vec{J}$ functions to conserve charge locally. $$\nabla \cdot \vec{J} = - \frac{\partial \rho}{\partial t}$$ Consider $$\vec{J} = x \hat i$$ Does this have a valid ...
• 5,235
79 views

### Proof that conservation of momentum is Lorentz invariant

In classical mechanics, if $$\frac{\mathrm d}{\mathrm d t}\sum_i m_i\vec{v_i}=0$$is true for one frame of reference, then it is easy to prove that this is true for all frames (since different frames ...
• 1,286
29 views

### How to determine the flow of a conserved vector quantity?

I know that if $F$ is a conserved quantity, then the Hamiltonian is invariant under the flow $\psi^{\lambda}(q^{\alpha},p_{\alpha})=((q^{\lambda})^{\alpha}(p,q), (p^{\lambda})_{\alpha}(p,q))$, that ...
• 235
12 views

### Conservation of momentum flux

When is momentum flux conserved in a fluid? How do these conditions compare to the conservation of momentum? What is an example when momentum flux is conserved, and momentum not?
90 views

### Why Electron Quantum Field Wants Little Energy But Photon Field Doesn't

In this Quora post: https://qr.ae/pv5tac, it states that the electron quantum field "wants" to reduce the energy it has, so when a particle and an anti-particle interact and the charges ...
28 views

### What is conserved under spacetime translation?

Is it the energy momentum tensor: $$T_{\mu \nu}$$ Or the four momentum: $$P_{\mu}$$ I know they are essentially the same thing, but it seems that one is the outcome of spacetime translation invariance ...
• 1,138
1 vote
82 views

### Transfer of momentum at quantum level

The clack of two billiard balls signifies the transference of their momentum and the mathematics of resulting vectors is fairly straight forward. At the quantum level, I understand that the transfer ...
39 views

### Time dependent Quantum Generators

We know a space-translation generator can be written as: T(\textbf{r}_{0})|\alpha\rangle=e^{-i\frac{\textbf{p}\cdot\textbf{r} _{0}}{\hbar}}|\alpha\rangle=|\alpha'\rangle. \end{...
• 11
36 views

### Euler-Lagrange Equation, $\dot{P_{\alpha}}\neq\partial_{q_{\alpha}}L$

We know $$\frac{d(\partial_{\dot{q}_{\alpha}}T) }{dt}=\partial_{q_{\alpha}}(T-U) \\ \partial_{\dot{q}_{\alpha}}U=0 \rightarrow\dot{P_{\alpha}}=\partial_{q_{\alpha}}L$$ ...
• 11
1 vote
46 views

### Physical interpretation regarding heat equation [closed]

I have included image of a problem from Oxford ( from a Math course). I was able to do the question. However, i am stuck at the last part which asks about the physical interpretation. I don't have a ...
• 13
41 views

• 15