# Tag Info

### What makes energy "the" conserved quantity associated with temporal translation symmetry?

The OP's question is basically stating that in a system with time-translation invariant dynamics, we can define a conserved quantity by arbitrarily assigning a real number to each orbit; when the ...
• 4,585

### What makes energy "the" conserved quantity associated with temporal translation symmetry?

What is special about the conserved quantity $Q(x, p) := \frac{1}{2} (x^2 + p^2)$, when also the quantity $Q_2(x, p) = \sin(x^2 + p^2)$ is conserved too, by the same temporal translational symmetry? ...
• 6,831

### Where does all the energy in black holes go?

Energy inside black holes doesn't "go" anywhere. Energy and mass are the same thing ($E=mc^2$). There's a "no hair" theorem that says that black holes can be completely described ...
• 5,619
Accepted

### Where is the energy going in this simple harmonic motion

In case 1, the collision between the two masses is inelastic (because they stick together), and so some of the energy is "lost", or rather, some of the energy is converted to thermal energy ...
• 3,767

• 50.6k
Accepted

### Why don't perpetual motion machines with superconducting magnets work?

A perpetual motion machine that merely 100% conserves energy and thus operates forever like a perfect flywheel is of little interest and is not the same as as a perpetual motion where you can ...
• 6,892

### What exactly is the limiting factor of the efficiency of a heat engine?

What exactly is the limiting factor of the efficiency of a heat engine? The short answer to the title of your post, the limiting factors are the maximum and minimum temperatures between which the ...
• 55.5k

### A Tough Mechanics Problem

I’ll just outline a method of obtaining the equations of motion. I don’t know if you’re familiar with Lagrangian mechanics. If you are, use polar coordinates for position of $B$ which specifies the ...
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• 9,154
1 vote

### Where does an electromagnetic wave's energy come from?

since EM waves carry energy; where exactly does it come from according to the laws of thermodynamics? From other available energy around the radiating body. Energy conservation is local, if some ...
• 27.8k
1 vote

### Where does an electromagnetic wave's energy come from?

One way to think about it is to realize that any work done on charges, (imagine touching it with something) means some type of electromagnetic interaction. And when charges interact and move, the ...
• 11.7k
1 vote

### Where does an electromagnetic wave's energy come from?

This is a famously messy problem. The most developed classical model is that of Abraham and Lorentz. Whether this is satisfactory depends a lot on your taste. In practice, it's not used much. For ...
• 3,159
1 vote

### Angular Momentum Conservation and Conservation of Energy

The sphere is initially slipping. Only when its speed drops to $5u/7$ does it start rolling. The sphere's speed drops because it is moving on a rough surface. The friction between the slipping sphere ...
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1 vote

### What exactly is the limiting factor of the efficiency of a heat engine?

Let us consider gas, cylinder, and a piston. The only energy that the gas has is thermal energy. The only work the gas does is pushing the piston From 1 and 2 it follows that: lost thermal energy =...
• 1,495
1 vote

1 vote

### How does stimulated emission work, exactly?

I just don't understand one thing, if photons carry energy, shouldn't the electron be forced to higher excited states? This typically does not happen because the photon usually considered when ...
• 1,600

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