Hot answers tagged

42

Get together a collection of charges. As many different ways to generate a charge as you can think of. Go ahead and invite your friends so they can think of some more. (As a practical matter you make static charges just before you use them, but still...) Now, test them pair wise to see if they attract or repel one-another. Keep careful records. Find the ...


25

I agree with DanielSank that the question is asking (wholly, not partly) about the historical development of the concept of electrical charge, not our modern description of it - "how did they know?" not "how can we know?" The latter (answered by dmckee) is the end result of more than two centuries of observation, experiment, theorising and debate, and ...


16

The Hall Effect shows that negative charge is moving. In the Hall effect, one passes a current through a wide strip of metal exposed to a perpendicular magnetic field. If positive charges moved, we'd expect the positive charges to be travelling in the same direction as $\vec{I}$, and the magnetic force $q\vec{v}\times\vec{B}$ would be to the right. Thus, ...


11

In an atom the electrons do not have a position. They are delocalised over the whole atom and in the absence of any external field this results in a symmetric charge distribution. So an isolated neutral atom is spherical and has no external field. However if you bring two atoms near each other then their charge densities will develop fluctuations that are ...


11

Physics's don't know that only negatively-charged particles move. We can create ion currents on demand in many environments. We do know that the current flowing in a metal wire is negatively charged particles in motion. As for how to determine that, you do a Hall effect measurement. The measurement works by subjecting a current in a relatively wide bar to ...


8

A human can produce about 100 W power continuously. That is equivalent to a couple of lightbulbs. So, while pedalling continuously you can keep one room reasonably lit. In one hour, that means you can generate a total energy of 0.1 kWh. A top athlete will be able to do better, so let's say it is possible to generate 0.2 kWh in 1 hour. That is still nowhere ...


6

Initially, when first glass rods were systematically being rubbed, the "charging" phenomena was observed. The electric charges were hypothesized to be positive and negative, and the pioneer (Franklin? forgot the name...) pretty much arbitrarily decided to call one positive and the other negative. Further experiments helped him deduce that two like charges ...


6

Atoms are in the quantum mechanical regime. The electrons around the nucleus occupy orbitals ( not orbits) i.e. probability loci where a measurement will find an electron. These orbitals have quantum numbers that give them shape. When the angular momentum quantum number l is equal to zero, the orbital is isotropic and neutrality is maintained. But for ...


5

Strictly speaking, yes, I believe neutral atoms have an electric field (though very weak). For example, if you have a hydrogen atom in the ground state, the absolute magnitude of the wave function of the electron decreases exponentially with radius (for a sufficiently large radius), so there is a (very small) positive total charge within a sphere with an ...


5

There are some good answers here, but I think I want to try to abstract Franklin's work a little bit. Because Franklin found just two options - "repel" and "attract", he was forced to consider only two kinds of charges. Consider the experiment, where glass-glass repels, plastic-plastic repels, and glass-plastic attracts. If all glass is the same, the glass ...


4

Technically "potential difference" is the difference in electrical potential, i.e. $\Delta V$, not the difference in electrical potential energy, $\Delta U$. Potential difference ($\Delta V$) is also called voltage, in certain contexts. However, many people and sources are sloppy about their terminology, and they will say just "potential" when they really ...


4

None of the above. Electrons are negatively charged, always. They do not become positively charged under any circumstances. In DC circuits they flow (or rather 'drift' at about 0.1 mm/s) only in one direction, from the -ve terminal to the +ve. In AC circuits they flow forwards and backwards in the wire, changing direction 50 times per second. They don't ...


3

While the paper linked to in sammy's answer does provide an equivalent circuit (page 5988), it seems of little practical use, since even in (unstable) equilibrium the current in L2 rises linearly with time. Within seconds (at most) it will reach a limit: saturation of the transformer core or maximum current (either I=U/R where R is the resistance of the ...


3

Let's assume we don't know how many types of charges exist. But we know that there are bodies which either attract or repel each other. Now we perform an experiment We find all such bodies that repel each other and put them in separate categories. After extensive experimentation we observe that they only belong to two piles. Furthermore we also observe ...


3

Please note that this is not so much an answer as an extended comment. At this website (which is the Google Book website for Contemporary Newtonian Research by Z. Bechler) it says It is well known that Newton became convinced towards the end of his life that electricity played a vital role in the operations of nature. In the famous final paragraph of ...


2

If in one frame you have equal steady positive charges densities in the two wires and equal steady negative charge densities in the two wires but the negative charges are moving at a steady speed in the same direction in the two wires. Then, in the frame of the negative charges, the two wires are positively charged. So yes, the negative charges feel a force ...


2

So long as your electric generator is spinning then you will get electricity out of it The BIG problem with this construction is conservation of energy. You will have to supply energy to the windmill in order to get electric energy out of it. So to get an "infinite" amount of energy out of your windmill youd have to supply more than that to keep it ...


2

Among competing hypotheses, the one with the fewest assumptions should be selected. Some electrified objects repel, some attract. This can be explained by two kinds of charge. Nothing that cannot be explained by two charges can be explained by adding a third kind of charge. So we continue to describe electricity as occurring in two kinds.


2

1) It is necessary for a plane EM wave. If one assumes solutions to the Maxwell's equation to be plane waves, it is not hard to show that $\vec B \cdot \vec E = 0$. Namely, take the third Maxwell's equation and dot both sides with $\vec E$. $$\nabla \times \vec E = - {{\partial \vec B} \over {\partial t}}$$ $$i\vec k \times \vec E = i\omega \vec B{\rm{...


2

The following paper is devoted to electrical analogues to mechanical oscillators. Fig. 6 illustrates a simple coupled LC circuit which imitates the inverted pendulum on a cart given in Fig. 5. It does not include a circuit to balance the 'pendulum' as in the video. https://core.ac.uk/download/files/508/12941830.pdf I also found the following which may be ...


2

Can Light Really be Matter? I suppose it depends on what you mean by "be". Matter will be created from light within a year, claim scientists It just is counter-intuitive to me. This is the fallacy of reasoning from personal incredulity. It isn't a good guide. I find most of QM and relativity counter-intuitive - it doesn't seem to be a simple ...


2

If one understand "carry particle" in the meaning that the particles move with the same velocity as the photons, than clearly no: Photons by itself could not carry particles. Photons move with the velocity of light $c$ and massive particles could not be accelerated to this velocity. This is both an observation and the basis of the Special and General ...


2

Good question :-) In the "ancient" times there was this old problem about the particle-wave duality. At the time, there was a Pilot wave theory, on which the particles and the waves are also different entities in some interaction. But it didn't live too long. Probably it failed some sophisticated experiments. On the current theory, there are fields, and ...


2

1) Electric flux lines model helps us to understand the behavior of an electric field much simply and it's pretty easy to visualize it. The definition of electric flux is the number of filed lines passing a given area normal to it. The field lines show the direction and magnitude of electric force at some point. The density of field lines at some region of ...


2

Typical domestic electricity consumption is 3,300kWh per year, or about 64kWh per week. Gas consumption is 16,500kWh per year; if you do not have a gas supply, your total power consumption would be about 380kWh per week. The maximum current which can be drawn per house from the mains is 100A. This is set by a fuse installed by your supplier close to ...


1

Potential difference (or voltage) is equivalent to a pressure difference in a pipe system of water. If there is equal pressure at two points, then no water has any reason to move. If there is a difference, water flows (is pushed) towards lower pressure. Similarly, charges will want to move towards the point of lowest possible potential. The larger the ...


1

By analogy, you are comparing the properities of a solid material, in your first paragraph, to a gas mixture, the atmosphere, in the second. So the path taken would flow through the "weakest" or most conductive sections of air. No matter how small a volume of air you take, the conductivity/ionisation path is very unlikely to be consistent enough to ...


1

Courtesy of AnnaV : By heating cord your book probably means the wire or flex which conducts electricity from the power supply to the heater. This is made of copper because it has very little resistance to current, compared with the resistance of the heating element, which is very high. The electrical power dissipated in a resistance $r$ when current $I$ ...


1

If you had one sweaty hand on the isolated circuit, and one bare foot on the wet Earth, there would be no path by which current could flow through your body, and return to the isolated circuit. A (probably) trivial amount of current could flow because of the AC voltage, and (probably) very weak capacitive coupling between the Earth and the circuit. But, I'...


1

The picture is correct. By the passive sign convention, the reference direction for current is into the positive labeled terminal of the circuit element and thus the circuit element is absorbs (not necessarily dissipates) power when the product of the voltage across and current through is positive. However, the reference direction for $I_S$ is out of the ...



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