# Tag Info

61

It definitely does consume electrical energy. Why? Because there's some opposing force faced by it while it rotates, and this force is often known as air drag/air resistance. You can see the effect of air drag once you switch off the fan. The fan decelerates from its original angular velocity until it stops completely. This deceleration is due to the motion ...

38

Suppose an inductor is connected to a source and then the source is disconnected. The inductor will have energy stored in the form of magnetic field. But there is no way/path to discharge this energy? Short answer: It will find a way/path to discharge this energy. Longer answer: Let's have this simple electric circuit consisting of a battery (...

28

It depends. You cannot disconnect an ideal inductor from an ideal voltage source with an ideal switch. These ideal things will break your calculations and you will get an infinite voltage on disconnect. A real inductor has its coil resistance, a capacitance between coils and an insulation between coils that has some great, but pretty much nonlinear ...

20

when rotating at constant speed, the fan disc is continuously performing work on the air drawn through it by imparting momentum to it. To perform that work requires a constant input of energy from the motor, and therefore the motor is continually absorbing electrical power while the fan is running. Some of that work is wasted in overcoming drag, but most ...

12

As stated in the other answers, it is true that a fan rotating with a uniform angular velocity consumes electric energy due to the presence of energy dissipation. But it's not only due to the energy transferred to the air molecules (as others state as "air drag"), but also due to other factors like - friction in the bearing, Joule heating and electromagnetic ...

11

But there is no way/path to discharge this energy? What will happen to the stored energy, current and voltage of the inductor in this case? In that case it makes its own circuit with its own path to ground. Often, that is through dielectric breakdown at the switch itself, but the details are highly unpredictable and depend very sharply on environmental ...

11

Mass is not a "concentrated" form of energy. I'll come to this later but first let's understand the relation between mass and energy. In relativity, energy and momentum are parts of a unified object called the four momentum vector $(E, \mathbf{p}$). Now, just like usual vectors in Euclidean geometry, there is a magnitude attached to this vector which is ...

9

Usually this extra energy creates a spark due to the high back emf produced. But it is not always possible for a coil to create sparks. It is clear If we try out the experiment. So what happens to the magnetic energy if no sparks are generated? firstly, The sudden switching off would create a potential. difference between the ends of the coil. This means ...

8

Indeed a rotating fan does not consume any energy to maintain the same angular velocity... in a vacuum. But if a medium is preset (eg. air, water...), its kinetic energy is increasing (that is the scope of a fan!)

7

Does a car need fuel to drive on a motorway? You already know what happens when you let go of the gas: friction, mostly from the air. This is exactly the same situation. It makes no difference whether the momentum is linear or angular.

7

An important point which is somewhat addressed by others but perhaps not clearly enough is (quoting Scotty) "Y' canna break the laws of Physics". You can make everything ideal - semiconductor wire, perfect instantaneously acting switch, infinite insulation - and the basic "rules" governing an inductor still apply. The fact that current flow cannot ...

5

The fact that mass and energy are two realisations of the same thing is a direct consequence of the theory of special relativity. Nothing more is required to find this other than Einstein postulates. The fact that mass and energy are the same thing, although was considered a very strange result at the time, has been countlessly proven experimentally. In ...

4

No, the energy wouldn't be the same if you move an object the same distance with a weaker force. Let's see this explicitly. The energy that is given to the object during this process will be reflected in what property of the object? It will be reflected in its speed. So, if the speed of the object at the end of the process is smaller for the weaker force ...

4

I understand that in an adiabatic compression, we will do it fast enough to not allow any heat to escape the cylinder. That's one way for the process to be adiabatic, if the piston/cylinder is not thermally insulated. But suppose I can't do it fast enough. If you don't do it fast enough, and the piston/cylinder is not thermally insulated, the process ...

4

You are correct that you have to explain the "mechanism" with which the general expectation of conservation laws would be satisfied. Or, in other words, the expectations of the conservation laws should play well the specific laws of dynamics, otherwise, we really wouldn't have a reason to think the conservation laws are true in the first place. ...

4

Here we will for fun try to reproduce the first few terms in a perturbative series for the ground state energy $E_0$ of the 1D TISE \begin{align} H\psi_0~=~&E_0\psi_0, \qquad H~=~\frac{p^2}{2}+\frac{\omega^2}{2}q^2+V(q), \cr V(q)~=~&gq^3, \qquad g~=~\frac{\lambda}{6},\end{align} \tag{A} using an Euclidean path integral in 0+1D \begin{align} e^{... 3 The total energy of the particles will depend on the reference frame. Any particle in its rest frame has only its rest mass energy E = mc^2. In any other frame, it will also have energy of motion in addition to rest mass energy. In the center of mass frame, then, if neither particle is moving the total energy would be E_\text{cm} = (m_1+m_2)c^2. You ... 3 The inductor becomes an active inductor. The energy is still stored in it, and the total flux it produces remains the same. If you connect it to another circuit, (Say, with just a resistor), it will momentarily act as a source of current i.e. the first current that flows through the circuit will be the same as the one that last flew through it (to maintain ... 3 It depends on the process that causes the transitions. If you have only the quantum well and nothing else, the electron will forever stay in the state where one puts it. The transitions occur when the electron is also coupled, e.g., to a photon or phonon field, so it can go to lower stats by emitting photons/phonons with energy\hbar\omega_{n\rightarrow n-...

3

Explanation Your force method give you the point where the net force on the body is zero, or in other words, the equilibrium point. Whereas, the energy method gives you the point where the velocity of the block is zero. Note that $v=0\nRightarrow a=0$. In other words, zero velocity doesn't imply zero acceleration, and thus zero velocity doesn't imply zero ...

3

In this passage, he's talking about a situation in statics, where the kinetic energy of every component is zero. This system is also non-dissipative (meaning that there are no mechanisms like friction in which energy can leak out of the system), so mechanical energy is also conserved. Mechanical energy is the sum of kinetic and potential energy: $$E=K+V$$ ...

3

You're wrong that the factor $1/2$ in the capacitor energy implies any energy has been "dissipated away". This factor is simply a result of the fact that the voltage across a capacitor is not constant, but rather a function of the charge stored on the capacitor; so moving charges across a capacitor becomes harder over time. Since the defining relation of a ...

3

In your example, you can do this analytically as the unperturbed potential in the harmonic oscillator, for which there are analytical solutions for the eigenenergies and eigenfunctions. General formulae Use perturbation theory and a sensible choice of your unperturbed basis. The energy $E_n$ will be written as: E_n = E_n^{(0)} + E_n^{(1)} + E_n^{(2)} + \...

3

A neutral particle can decay to a charged particle/antiparticle pair, which can then annihilate to two photons. For example, the Higgs boson can decay to two photons in this way. However, in the Standard Model of particle physics, photons directly couple only to charged particles. In some extensions of the Standard Model, photons couple directly to the ...

3

That's a good question. Note first of all that it is inconsistent to use proper time $\tau$ as the world-line (WL) parameter $\lambda$ for the principle of stationary action (PSA). The point is that the WL parameter $\lambda$ is never varied in the PSA, but the action $S$ happens to be proportional to $\tau$, which we are trying to maximize. In particular, ...

2

I assume you mean an indoor cooling fan (either stand-alone or a ceiling fan). You're right that total energy is always conserved but: Kinetic energy is just one type of energy. Heat is relevant here too. Note that heat is basically the kinetic energy of randomly moving particles. The fan is not a closed system. It's connected to the power supply (as you ...

2

But that constant angular velocity is not cost free. It has to be maintained. As others have already mentioned. Air drag of the spinning fan, mechanical friction all tend to slow down the fan. You need power to maintain the velocity. That is why most fans are connected directly or indirectly to electrical power source.

2

For an ideal battery it is indeed true that the potential difference across its terminals is constant, which means that a fixed amount of energy is transferred per coulomb of charge that flows through any device connected across its terminals. This doesn't imply that any charge will flow. For example if nothing is connected to the battery, there is air ...

2

The extinction in the atmosphere is a function of atmospheric conditions, dust and aerosol concentrations, the angle to the vertical the laser is fired at, and of course the wavelength. A typical value at 589 mm (sodium laser) in a clean atmosphere and a vertical beam would be about a 15% attenuation. It could easily be a factor of a few worse in more ...

2

An inductor contains a magnetic circuit. Changing the magnetic flux through it induces voltage in the coils that, if it is allowed to develop into a current, builds up until the produced current compensates the flux change. Separating the circuit blocks this process. Without the regular release of the magnetic energy through the coils, the magnetic ...

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