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169

You get burned because energy is transferred from the hot object to your hand until they are both at the same temperature. The more energy transferred, the more damage done to you. Aluminium, like most metals, has a lower heat capacity than water (ie you) so transferring a small amount of energy lowers the temperature of aluminium more than it heats you (...

125

While it's true that the difference in specific heat capacity is to your advantage, its effect is really dwarfed by the mass difference. Typical aluminium foil is 0.016 mm thick and weighs 0.043 kg/m², while human skin is about 1.3 mm thick (and even thicker on your palms/fingers) and weighs about 1.3 kg/m², assuming 1000 kg/m³ density. So there's about 30 ...

95

"Pure" water is a very poor conductor (resistivity is actually used as a measure of purity). "Real" water is not pure - it contains electrolytes and is quite conductive. Also - when your skin is wet, its resistivity is significantly lower. For example - "pure" water has a resistivity of (about) 18.2 M$\Omega\cdot\rm{cm}$. With ...

83

It depends on the frequency. DC electricity travels through the bulk cross section of the wire. A changing electrical current (AC) experiences the skin-effect where the electricity flows more easily in the surface layers. The higher the frequency the thinner the surface layer that is usable in a wire. At normal household AC (50/60hz) the skin depth is ...

51

The speed of electricity is conceptually the speed of the electromagnetic signal in the wire, which is somewhat similar to the concept of the speed of light in a transparent medium. So it is normally lower, but not too much lower than the speed of light in the vacuum. The speed also depends on the cable construction. The cable geometry and the insulation ...

42

Without getting into the quantum mechanical details, here’s a cartoon depiction of what’s going on. The vertical axis represents energy. Like other answers have already pointed out, metals don’t have actual free electrons. In the cartoon this is given by the grey region. If electrons have enough energy to be in the grey region, they’re free. In individual ...

38

The amount of heat generated by current flowing through a resistor (whether from lightning or more ordinary sources) is directly related to the power dissipated by the resistor, which is $$P = I^2 R.$$ $R$ is small for objects made from good conductors, which many metals are, and large for objects that are made from bad conductors like plastic or wood. ...

36

If you suddenly removed all the electrons from a piece of material, or even just the valence electrons, you would be left with a huge concentration of positive ions in a small volume, which would exert a huge electrostatic repulsion on each other. Since you no longer have the bonding influence of the electrons to counteract this repulsion, the material would ...

32

Stranded wire is used because it bends more easily, but it has essentially the same conductive properties. Current flows throughout the entire wire. This is easily tested by measuring the resistance of round wires - the resistance will fall quadratically with the radius, indicating that it's the cross-sectional area that matters. Amendment: this answer is ...

31

The problem is in the definition what a "good" or "bad" conductor is. In school children are taught that everything is quite easy: there are "good" conductors like metals and "bad" conductors like plastic. Short question before: What do you think is the difference in electrical conductivity between silver (the best non-...

26

A band is essentially a (near) continuous collection of momentum eigenstates. Within the band the electrons can be treated as free to a reasonable approximation, so their eigenstates are just plane waves. The symmetry means that for every eigenstate there is another with an equal and opposite momentum. So if we populate every momentum eigenstate the net ...

23

Electrons are bound to the metal by the attraction of the nuclei. After screening of the nuclei by other electrons in the metal, there is a net electric field creating a potential barrier for the electrons to escape. This potential barrier is called the work function and is defined with respect to the Fermi energy of the electrons. The work function is ...

21

Like everything else in physics it makes sense to talk about some quantity like that in the context of a model. And we bother because the model is (at least occasionally) useful. Consider the question "Why doesn't the current in a household circuit with a bend in the wire radiate at high power?" Clearly charge is accelerated by going around the bend, ...

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 ...

20

Electrons are not being created nor they are destroyed. The electrons already exist in the metal. The metal consists of layers of positive ions (depicted by orange circles in the picture) in which the electrons (depicted by cyan colored circles in the picture) are in continuous motion. Hence, the charge on the metal is overall neutral. Where did the ...

19

The naive reasoning which leads to the conclusion that charges $Q_1$ and $Q_2$ of two touching conducting spheres with radii $R_1$ and $R_2$ are related by the relation $Q_1 = Q_2\frac{R_1}{R_2}$ is wrong. This formula holds only when the distance between the spheres $L$ is large compared to $R1$ and $R_2$, $L\gg R_1,R_2$, and the spheres are connected by a ...

18

Loosely speaking, the gradient of a scalar field (such as the electrostatic potential) points in the direction of that field's greatest change. Since no change occurs in the field when you go along the surface, the gradient shouldn't have a component in that direction. Here is another intuitive explanation: Imagine for a moment that the electric field was ...

17

Your confusion stems from a fundamental misunderstanding about drift velocity. Drift velocity is not the average speed of electron motion, but instead is the average velocity vector. The average speed of free electron motion in a metal can be approximated to be the Fermi speed $$v_F = \sqrt{\frac{2E_F}{m_e}}$$ where $E_F$ is the Fermi energy. This is ...

16

No band is special. A partially full valence band does conduct, just like a partially full conduction band. On the other hand, a perfectly full band conducts just as well as a perfectly empty band: No conduction at all. Now, nobody is surprised when you say an empty band can't conduct, but at first it seems surprising that a full band is the same way. ...

16

The electrons in the conductor which are not free are also travelling at high speed but they are bound to particular atoms. It requires energy to remove them. The 'free' electrons in the conductor are not really free. They are not bound to individual atoms but they are shared by and bound to a large number of atoms which form a microscopic crystal called a '...

15

Because of electromagnetic forces, all of the electrons in the wire are displaced towards A with a certain velocity causing a positive current towards B. The electrons have a small drift velocity, not moving much. Although your light turns on very quickly when you flip the switch, and you find it impossible to flip off the light and get in bed before ...

15

In high school my chemistry teacher, Mr Stratton, set up an experiment. He had a light bulb fixture on a wooden paddle, with a power cord attached and two metal prongs projecting at right angles, such that the paddle could be set over a beaker of water with the prongs dangling into the water. He first filled the beaker with distilled water and screwed a ...

15

Also, does the speed of the electricity depend on the voltage applied or the resistance of the conductor? Not just the resistance of the conductors, but the inductance. And also the capacitance to ground and/or to the other conductor. Remember that an electric circuit requires a complete loop, unlike a laser. Wiring to carry electricity normally includes ...

15

Even when an isolated atom has a filled shell, the electron bands in a solid crystal may be partially filled. The reason is that bands that originate from different atomic orbitals may actually overlap in energy. This is shown on the left in the figure below. When the bands overlap, the lowest-energy state has some electrons displaced from the top of the ...

14

Your analogy is faulty. Think instead of the falling ball as having already reached a constant terminal velocity because of air drag when its gravitational potential energy is 10 J. Then since it’s velocity is constant on the way down there is no change in its kinetic energy. Its loss of gravitational potential energy equals the heat dissipated in the air ...

14

Here's a self-contained proof, with no hand-waving. Let the surface $S$ be the boundary between the empty cavity and the conducting medium that surrounds it. Things may be discontinuous at the surface $S$, but that doesn't matter. The only thing that matters is that the conductor imposes a boundary condition on the electric field $\mathbf{E}$ inside the ...

14

It has been pointed out to me that the Feynman lectures address this very problem. This is the answer given by Yasir, though he is a little economical with his text so for completeness I will go through the argument in detail here. It is also equivalent to the answer given by S. McGrew. Suppose we have a field in the cavity then there must be field lines in ...

13

You can totally transfer charge using protons. Or using Na+ or any kind of charged particle. It happens all the time - if you look at how a wet cell battery works, you'll find that while charge across the wire is carried by electrons, current flows along the salt bridge via charged ions (theoretically protons could be among these). I bet if you could somehow ...

13

I’m going to second dmckee’s notion that concepts like velocity need context. Every variable in every theory needs interpretation to connect it to our everyday intuition. For example, here’s a theory: $F=ma$. This theory has no meaning beyond the math of a differential equation if we don’t interpret $F$ to be something that makes sense in light of our ...

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