Hot answers tagged

4

When lightning is fatal, it is usually due to an electrical discharge-induced heart attack. Since lightning is essentially a electrostatic discharge event, we can roughly view it as analogous to a capacitive discharge. IEC 60470-1 provides threshold values of various physiological effects of capacitive discharge current, including threshold for ventricular ...


3

For monatomic gases the only form of internal energy is translational i.e. the kinetic energy of the atoms. However for molecules we can also have rotational and vibrational energy as well as the translational energy. Each active rotational mode gets $\tfrac12 kT$ of energy just like each translational mode. However each active vibrational mode gets $kT$ of ...


3

There is the generally well-known identity $$\frac{\mathrm d}{\mathrm dx}\ln f(x)=\frac{f'(x)}{f(x)},$$ which can be quickly deduced using the chain rule. Because of this identity, $\frac{f'}{f}$ is also called the logarithmic derivative of $f$. Now they just apply this identity to $$\frac{\sum_{n=0}^\infty n\alpha e^{-n\alpha}}{\sum_{n=0}^\infty e^{-n\alpha}...


2

At constant temperature and pressure, Gibb's free energy is the criteria which determines equilibrium for a system (*). More precisely put, the system will keep changing it's state till it falls into a state where $ \Delta G = 0$ for the constraint conditions mentioned mentioned above. Similar to the principles of potential, the reaction is driven so as to ...


2

I found Ryder on amazon "look inside" and the equations are there. It is indeed off shell. If we define the off-shell energy as $E=m+T$, with $|{\bf p}|$, $T$ small I think he means that the denominator is approximately $$ p^2-m^2= E^2-{\bf p}^2-m^2 =(E-\sqrt{{\bf p}^2+m^2})(E+\sqrt{{\bf p}^2+m^2)}\\ \approx (E-m -{\bf p}^2/2m)(E+m) \approx (T-{\...


2

People use the word "tension" in different ways, to mean different things. I think there is a disconnect between what you think it means and what the author thinks it means. I would not use the word the way the author is using it. Your understanding of the PE in a string is correct. The potential energy in a transverse wave of a string is stored ...


1

In 4-space, the "distance" a photon travels is zero, even if it is emitted on Alpha Centauri and detected here on Earth. The Newton's Cradle analogy isn't too outlandish if all the balls between the front and the back are eliminated, the front ball is the emitter and the back ball is the detector. The positions and times of the emission and ...


1

The Michelson Morley experiment showed definitively that there is no luminiferous aether, that the energy of light is propelled through vacuum, no medium needed. Photons are found experimentally to be the underlying structure of classical electromagnetism.This single photon experiment shows the effect of photons scattering on a double slit and the build up ...


1

Just some observations here: By symmetry $\langle x\rangle=0$ for your problem, and so does $\langle p\rangle=0$. Thus, for instance, \begin{align} (\Delta p)^2=\langle p^2\rangle -\langle p\rangle^2=\langle p^2\rangle \propto \langle T\rangle \end{align} in this case.


1

This is very close to a philosophical question, since certainly Potential Energy is neither observable nor directly measurable. I have on occasion had to listen to non-Physicists insist that PE is entirely a "fudge factor" to maintain the law of conservation of total energy. However, since every experiment to date shows that the PE models lead to ...


1

Work is not "force times displacement". Work is an integral $$W=\int\mathbf F\cdot \text d\mathbf x$$ which becomes $W=Fx$ under certain conditions. The work done by a conservative force is always equal to the negative change in potential energy associated with that force: $$W_\text{cons}=\int\mathbf F\cdot\text d\mathbf x=\int-\nabla U\cdot\text d\...


1

In "The Principles of Chemical Equilibrium," by K.G. Denbigh, in section 2.4 discusses, in a very precise way, how this applies to (a) a closed system at constant temperature and pressure and (b) an open system operating at steady state in contact solely with a constant temperature reservoir. In the former case he discusses a specific example of ...


1

Energy transferred by current is defined as : $$ E = I Q R $$ Where $I$ is current strength in amperes, $Q$ - transmitted charge and $R$ conductor resistance in $\text[ohms]$. Typical lightning bolt current is about $30~000 ~\text{[A]}$, and transmits $15 ~\text{[C]}$ charge. If lightning passes through internal body structures, then one needs to account ...


1

I am providing a new answer in view of the additional details you have provided and our related discussions. I have also retracted my vote to close the post. First of all, I believe we both agree that process $2$ to $2_{rev}$ in the third diagram and process $1$ to $2$ in the fourth diagram are impossible if adiabatic, based on the first law. The issue has ...


1

Your calculation is correct, and if you do it by climbing you accelerate an decelerate at every step, it is really very ineffective . So better think of pulling up the 90kg over a suspended wheel, or a pulley.


Only top voted, non community-wiki answers of a minimum length are eligible