New answers tagged

0

The concept of electric charge is introduced to explain experiments (originally from static electricity). It is found that only two types of charges are necessary and to distinguish them and to distinguish between they are given labels. The most convenient label is positive and negative (that has some mathematical advantages). It is pure convention that ...


-1

All particles seems to be grouped under two distinct polarities based on the manner of attraction or repulsion. Those particles repelling one another are said to have like charges. Those that attract one another have different charges. Being a positive or negative charge, is a matter of convention already accepted by world scientific community. A Charge is ...


1

Force is a vector. Potential energy is a scaler. Forces which have associated potential energy functions as called conservative forces. Conservative forces act in such a direction that, if released from rest, the potential energy function associated with that force will decrease (and the kinetic energy will thus increase) with the velocity increasing, until ...


0

If you take down as positive then displacement $s = +30$ m, initial velocity $v_i = +8$ ms$^{-1}$ and acceleration $a = +10$ ms$^{-2}$. Using the constant acceleration kinematic equation $s = v_i t + \frac 1 2 a t^2$ where $t$ is the time gives $$(+30) = (+8)t+\frac 1 2 (+10)t^2$$ If you take up as positive then displacement $s = -30$ m, initial velocity ...


1

The question is rather incomplete and confusing. By the way, it is used to consider surfaces as vectors when needed for computing surface integrals, like flux integrals, where the scalar product between a vector field $\vec A$ and a infinitesimal surface $\mathrm d\vec S$ is considered: $\vec A\cdot\mathrm d\vec S$. To this aim, the differential surface is ...


2

From the paper, which states fiber Bragg gratings (FBG) have been demonstrated to exhibit temperature dependent shifts in resonant wavelength of 10 pm/K it is fairly clear that the unit is picometer per kelvin. That is, you have some device with a resonance wavelength $\lambda_\mathrm{R}$ which depends on temperature, ...


1

An empirical answer: Metals (often copper) can be used as insulating support structures in superconducting magnets. Compared to ~0 resistivity of the coil, the resistivity of metals makes for very good insulating properties! Or, going in the other direction, a 50-watt VandeGraaff-type power supply may output 500KV at 100uA. Such a supply has an internal ...


0

As is often the case, the answer to this is actually slightly context-dependent. For many everyday purposes, the answer of Cort Ammon that it is all a matter of degree is correct. However, one other context worth mentioning is when condensed matter physicists speak of whether a particular state of matter is a "conducting" (or "metallic") state or an ...


8

Another way of distinguishing conductors and non-conductors or insulators is with band gap - for good conductors the fermi level of electrons is inside a band - semiconductors have a small band gap and good insulators have large band gaps... Electrons in solids lie in energy bands, whereas in atoms and molecules they have generally sharp levels. If you ...


20

As you have expected, there is no sharp divide between the groups. The divide is man made. Since all conductors have some resistance, (except superconductors - follow this link to find out more) and all insulators will conduct some current if they are forced to, this means there is no absolute dividing line between conductors and insulators. Since ...


0

Without the factor $1/2$ for a complex field all observables constructed out of the Lagrangian in the standard way vie Noether theorem, like the energy $H:= \int T_{00} dx$ or the momentum $P_i = \int T_{i0} dx$, turn out automatically to be the ones of a system of identical particles of two types, {\em proper particles} and {\em anti particles}. E.g., $$H ...


1

One tends to forget these 'rules', and so the way to go about is to check that the relative error of the final answer is not less than the relative errors of the quantities involved when multiplying or dividing. When adding or subtracting, the absolute error of the final answer should not be less than the absolute errors of the contributing quantities.


0

I believe that the potential at point X should be 12−60I and the potential at Y should be 12−30I. However, in the several problems that I've tried, this is not the case. Please explain, why? A proper way to write this is (in terms of the node voltages and branch currents) is $$V_X = 12V - I_X \cdot 60 \Omega$$ $$V_Y = 12V - I_Y \cdot 30 \Omega$$ ...


3

Imagine actually doing some measurements. Say you are measuring the length of a train. You measure all of the cars together and get 1234.5 meters (plus or minus 5 cm). Then you measure the locomotive and get 12.34 meters (plus or minus 5 mm). How precisely do you know the length of the train? Does it matter how short one of the parts you measure is ...


2

It is sometimes easier to visualise what is happening by using the idea of potential. To do this make one point in the circuit 0 V. This is a totally arbitrary choice. It is the bottom right hand corner of your circuit. Note to make the sums easier I have change the emf of the battery to 90 V so 2 A flows through the battery and 1 A through each of the ...


1

The equation $\Delta U = Q - W$ is complete in itself. The confusion arises in the definition of enthalpy. We tend to think that the "pressure energy" $PV$ and the work done by the system $W$ are somehow different. But the fact is that there isn't a well defined physical meaning to the $PV$ term. It is confusing because when we add $U$ and $PV$, we think ...


0

Since work done by a force $\vec F$ undergoing a displacement $d\vec r$ is defined as $\vec F \cdot d\vec r$ when this dot product is positive the force and displacement are in the same direction and is negative when they are in opposite directions. The work done by a frictional force does not always have to be negative. Imagine a block $A$ on top of block ...


0

Force is a vector, meaning magnitude and direction. Work done by a force is relative to the direction of a force is the scalar value obtained by performing the vector dot product of the force and the displacement (which is also a vector). If something isn't coming out to what you expect when you compute work, make sure you have the right magnitude and ...


0

I want to find out the potential energy of a electric dipole in a uniform electric field by another process which gives the result to $U= -P.E$. Purpose A, B are the position of the point charges $q$ and $-q$ and seperated by small distance D and O be the position of the origin. The distance from O to $-q$ is R. So potential energy, $U=-q.v(R) q.v(R D)$ ...


2

The reason is that all experiments known can be explained by having two types of electric charge. To distinguish between the two types of charge them it is necessary to introduce labels, conventionally the labels were taken be "positive" and "negative". Because of history, electrons are given the label "negative".


0

The straight answer to your question is no, the value of the components of the stress energy tensor do not change according to the metric signature. Qmechanich showed you how the formulas change, depending on the metric signature, just in order to keep the component values the same.


0

To place a charge in the vicinity of an electric field, you should do work against the electrostatic force on the charge. This work done to bring a charge q to an electric field of some other charge configuration from infinity to a distance r, in the field is what we call the potential at the point r. To do a work to move a charge q from a potential V to a ...


1

Relation between Electric field and potential The relationship between electric field $\bf E$ and scalar potential $\varphi$ is given as $$\mathbf E= -\mathbf \nabla\,\varphi$$ where $\mathbf \nabla \equiv \textrm{gradient operator}\;.$ I am unable to understand from this - sign comes. It is worthy to quote from Purcell: The minus sign came in ...


0

Work done is $\vec F \cdot \Delta \vec x$. If $\vec F$ and $\Delta \vec x$ are in the same direction then the work done is positive. If $\vec F$ and $\Delta \vec x$ are in opposite directions then the work done is negative. Consider a spring fixed at one end as a system and an external force $\vec F$ stretching the spring. If the external force $\vec ...


0

Potential energy of a body is its capacity to do work by virtue of its position in a conservative force field.So if the body is free to move it will do so in such a way as to reduce its potential energy and the reduction in the potential energy will be equal to the positive work done by the body which could result in the raising of a weight or displacement ...


0

It is just a convention. If we would use the opposite convention, we would get for conservative forces $$\vec F=+\vec\nabla U.$$ You can easily see (think in the one dimensional case) that the particle would move to points of maximal potential energy. This only sounds strange because we are used to the opposite. The mechanical energy principle would retain ...


0

The decrease or increase in potential energy is converted as mechanical work either to increase or decrease it's kinetic energy (we are considering here conservative systems). Suppose you have a system at rest at some height above the ground level, say, a ball placed on the top of a mountain. The work done in order to bring the ball of mass m to a height h ...


0

It's a convention. The real reason is so that we can have: $$\begin{align} \Delta {\rm KE} &= W_{\rm ext}\\ \Delta {\rm KE} &= W_{\rm con} + W_{\rm noncon}\\ \Delta{\rm KE} &= -\Delta {\rm PE} + W_{\rm noncon}\\ \Delta{\rm KE} + \Delta {\rm PE} &= W_{\rm noncon}\\ \Delta {\rm E} &= W_{\rm noncon} \end{align}$$ Which doesn't work ...


0

It would be better to say that potential energy is the amount of work that a system can do. Say you have a system consisting of two masses - a brick and the Earth. As the brick moves down U decreases, the force pulling the brick and the Earth together acts downwards on the brick and it can do some positive work. On the other hand, if brick moves up, U ...


0

For non-hermitian products of Dirac field operators the parity is not well defined and depends on the phase $\eta=\pm1,\,\pm i$ of the parity transformation $\eta \gamma^0$. For example, $I_P = -\eta^2 I$, where $I = \overline{\Psi_C}\Phi$. In the $S$-matrix elements, however, all phases go away eventually, because creation and annihilation operators come ...


-3

The constant is only $8\pi$ when you're working in units where $G=1$. The number $G$ was originally defined as the constant of proportionality in Newton's $$F=G\frac{m_1m_2}{r}.$$ So in some sense the only thing we can do is to pass to the Newtonian limit and calculate the force between two point masses.


0

In general in Feynman diagrams an incoming particle can be read as an outgoing antiparticle and W+ is the antiparticle of W- and vice verso. Quantum number conservation holds at the vertices. (charge , lepton number..) The reaction studied in 11 is a change of a proton to a neutron through the weak interaction. The charge of the proton has to go to the ...


0

What do you mean by positive electrons? There are no positive electrons. Once an electron is lost from an atom, it's an ion, not a positive electron. The ions cannot move because they are tightly packed. It's the electrons that are free to move. Now the flow of electron from positive terminal to negative terminal is related to conservation of energy. The ...


0

Just think in terms of their properties. A concave lens diverges the rays incident on it. So the image will be formed behind the lens. So the image distance becomes negative. (The image distance is measured from the principal point to the point at which the image is formed.) Now the convex lens converges the incident rays. Hence the rays converge to a ...



Top 50 recent answers are included