Hot answers tagged electricity
22
Ohm's law is generally NOT correct, it's called a law for historical reasons only!! It's a law in the same sense in which Hooke's law is a law... it holds only for certain systems under certain conditions, but it's widely known because it's simple and linear!
It's not just superconductors, diodes are a neat everyday example of Ohm's law failing to hold. But ...
13
Ohm's law works for ordinary conductors for a reason: the particles carrying the current (usually, but not always electrons) scatter incoherently and inelastically from features of the conductor (in the case of an electron current these are generally the individual atoms of the conductor). As long as those conditions pertain, you can expect Ohm's law to be a ...
10
AC or DC, you only get electrocuted if current passes through your body. (Current passing through any part of your body can be dangerous, and possibly cause an electrical burn, but current passing across your heart is the one that's really dangerous.) Touching just one wire at a time gives the current nowhere much to go. You are right to think that some ...
5
Not really. A magnetic field alone doesn't create electricity. A changing magnetic field does. The Earth's magnetic field does change a tiny bit but not enough to really generate much.
The other option is to move the inductor in the magnetic field. The Earth's magnetic field is quite homogeneous over short distances though so the coil would need to move ...
5
Looking at your question from the perspective of ideal circuit theory, an ideal resistor has the following I-V relationship:
$V_R = I_R R$
The voltage across the resistor is proportional to the current through the resistor with constant of proportionality equal to $R$.
In ideal circuit theory, an ideal conductor can be thought of as a "zero ohm resistor". ...
4
This is because the neutrality of polarity can be changed by electric field in this case. When you create - charge in the comb and you expose the pieces of paper to the electric field created by the charge, you will polarise them so that the part closer to the comb will be + and the other will be -.
Here, see the electric field. The same polarities do not ...
4
An ampere passing through your heart can give you a heart attack. An ampere passing through a wire will not.
The human body has a fairly large resistance ($10000\ \mathrm{\Omega}$ perhaps?), so the same voltage that can make a large current pass through a copper wire will not necessarily make any significant current flow through a person.
4
Thermistor with this particular temperature behavior are commonly semiconductors. In a semi-conductor, there is an energy gap between the (filled) valence and the (empty) conduction band. At zero temperature, no charges are in the conduction band and the resistance should be infinite as the system behaves basically like an insulator.
If you turn on the ...
3
First off, what you describe only happens for highly amplified speakers. The current change when you touch the wires is pretty tiny and you'll only hear a sound when that signal is being amplified significantly before being sent to the speakers.
There are many reasons why a tiny bit of electric current flows from your body to the speaker wire so I'll only ...
3
Our current understanding suggests that black holes can have electric charge, and that in addition to mass and angular momentum, these are the only ways that black holes can have distinguishable physical properties. This is a result of the famous no-hair theorem, although keep in mind that no definitive proof of this theorem yet exists.
As this wikipedia ...
2
Take a simple analogy: if you take a bar of metal and put it in a potential gradient (e.g. bring a charge near, but not touching, one end) then you will attract electrons to the positive end. So one end of the bar acquires a negative charge and the other acquires a positive charge. If you now cut the bar in half you have two pieces of metal, one with a ...
2
Yes. Let's assume that the charge density is fixed relative to the surface of a sphere of radius $R$, then spinning the sphere with angular velocity $\vec\omega = \omega \hat{z}$ would create a surface charge density $\vec K$ given by
$$
\vec K(\theta, \phi) = \omega R\sin\theta\sigma(\theta, \phi)\hat\phi(\theta, \phi)
$$
where $\theta$ and $\phi$ are ...
2
When you touch a hydro wire, and ONLY a hydro wire -- nothing else -- there's no potential difference across your body. Your entire body is at the same potential, which is that of the hydro line (often thousands of volts). This is why birds don't get electrocuted if their feet touch only one wire. Plenty of birds, particularly eagles, have been fried because ...
2
What we mean here by the word stationary is that the (macroscopic) charge density is constant , even if charges are moving actually.(An explanation for the word macroscopic comes in the next paragraph). A famous example is a rotating sphere with charge density $\rho$ , which is (electrically) equivalent to a non-rotating sphere.(But has a magnetic field due ...
2
In ideal circuit theory, the parallel connection of two voltage sources results in an inconsistent equation, e.g., a 3V and 2V source connected in parallel, by KVL, gives the equation: 3 = 2.
In the real world, batteries are not ideal voltage sources; batteries can supply a limited current and the voltage across the battery does, in fact, depend on the ...
2
These are two separate questions. It's better if you don't try to combine two questions into one.
In answer to the first question, yes, a black hole can have a measurable charge. You measure it the same way you'd measure any other charge. This is all purely theoretical, however. Many real-life black holes have been observed and characterized by their ...
2
why does the voltmeter not form a closed loop with the circuit and hence cause energy to be dissipated by internal resistance
It does, but the voltmeter probably has an impedance of 10,000,000 ohms - as most multimeters do - so the current flowing is negligibly small (0.6 microAmps) and therefore the voltage drop due to battery internal resistance is ...
1
You are probably asking the question because you haven't gotten to alternating currents yet. One typical and simple example I can think of is the AC-DC War, AC won because it uses very high voltage and very low amperage to transmit electricity, this leads to very low power loss, compared to DC transmission. Although I have heard extremely high DC ...
1
The voltmeter does indeed draw current, but very small. Its internal resistance should be high, unless it's a very cheap voltmeter.
Putting the voltmeter's internal resistance, let us say 1MOhm just to make up a value, in series with the battery's internal resistance of a fraction of one Ohm, makes a voltage divider providing practically the entire ...
1
There are two phenomena in your question.
(1) Let us first understand how magnetic field can be considered to "arise" because of relativity. Imagine a frame of reference in which a charge $Q$ is at rest. If another charge $q$ is brought in its vicinity, it will experience only an electrostatic force. Now get on to another inertial frame of reference moving ...
1
You should not connect different batteries in parallel.
If you do, the battery with the highest voltage will discharge into the other one, until they end up with equal voltages. If the second battery (the lower voltage one) is a rechargeable, then it will be charged by the first one, again until the two have the same voltage. In this case the end voltage ...
1
First, one additional point. It's not just $R_{copper}$ limiting current, but also the battery's internal resistance. This is modeled as one more resistor in series. For most small batteries you might get your hands on, it's less than one Ohm.
People playing with wire and low-voltage batteries don't get zapped because the resistance of Human, when ...
1
Ohm's Law does not have a problem here any more than any other formula in the sciences which involves dividing by a denominator which can go to zero.
Ohm's Law exhibits a singularity when there is no resistance, but a nonzero voltage. An ideal voltage source cannot be connected in parallel with a zero resistance, because that implies that infinite current ...
1
What is the point of batteries? Obviously higher W will drain them faster.
If question is wattage -> luminocity function then I think that there will be two components. One is that every watt is converted into a lumen (683 lm/W, to be exact according to Wikipedia). It is like $E=mc^2$: more mass = more energy. They are equivalent. So more power <=> more ...
1
There are many materials that can be charged by triboelectric effect. Tipicaly you can observe this effect rubbing a material like wool and amber.
The phenomenon is quite complex but it's in great part because the different electron affinity of the materials (one loses easily an electron and the other captures an electron).
1
The net charge of any of those internally connected pairs of plates is always zero. That is, when you charge the capacitors, charge doesn't leave the wire between C and D, it only moves along it, and is held in place by the electric field of the adjacent plates. If a circuit is completed that allows charge to flow from D's negative plate to A's positive ...
1
For flow of charge, the circuit should be closed. In open circuit, no charge flows. If we connect both the capacitor plates it makes closed circuit, charge flows in the circuit, as a result charges on the plates neutralizes to zero.
If only +ve plate of the capacitor is only connected to ground there is no closed circuit. no charges flows from the ground.
...
1
I think every fundamental definition is kind of going in circle.
I would say an electric charge is something that obeys Maxwell's laws.
But to write those laws, you have to know $\vec{E}$ and $\vec{B}$ which need a definition of an electric charge.
At the end you just group things that look/react alike and named them.
The problem arise when you have to ...
1
Practically, for a macroscopic body such as a chunk of metal, the charge on that body is the difference between the number of electrons and protons in the body. It is hard to knock a proton out (can be done, though), but for conductors we can push and pull electrons out by supplying a bit of energy (back to that in a moment). However, instead of saying the ...
1
When you run two different materials together you will usually transfer electrons from one material to the other. For example if you rub glass with silk the electrons transfer from the glass to the silk and you end up with positively charged glass and negatively charged silk. That's why the two glass rods repel each other: they both carry a positive charge ...
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