# Tag Info

47

Varying (due to AC) electromagnetic forces exerted on the components cause them to vibrate thereby causing the hum. Components that typically hum noticeably are transformers (where the coils and cores vibrate/magnetostrict in the varying magnetic field), and under certain circumstances capacitors (typically they have higher resonant frequencies and audible ...

8

Switching power supplies make higher-pitched noise, because they rectify to DC and then drive a small transformer at much higher than mains frequency. It's still the same root cause: magnetic forces vibrating the components, specifically the inductors. I'm not aware of electrostatic attraction/repulsion in capacitors ever being a source of noise. ...

6

Because the diode lit for both ways of connecting your finger and soldering tip to the diode, the soldering tip has an AC voltage on it, and one side of the AC is connected to your building ground (the floor your bare feet were touching). The solder tip is heated by a coil for which it sounds like has 120 VAC on it. There is a short through the insulation ...

6

A battery connected to a capacitor is an RC circuit in the limit $R \to 0$ (i.e., there is no resistor and the resistance of the wire is negligible). One might think that the energy loss is zero in this limit, but this is not the case. For an RC circuit with a battery and an initially (i.e., at $t=0$) uncharged capacitor, we have Q(t) = CV ...

3

If your power supply is sourcing a positive current toward the ground, that corresponds to a flow of positive charge from the supply to ground. This is equivalent to a flow of negative charge from the ground to the power supply. In a real wire, only negative charges can flow, so the second thing will happen: electrons (which have a negative charge) will ...

3

The total energy at the initial point should equal the final energy. i.e. $$V(x)=V(0)+KE$$ ($x,0$ are along the axis of the ring.) The potential at a point on the axis of the ring is given by: $$V(x)=\frac{kQ}{\sqrt{R^2+x^2}}$$ From this, we get: $$-\frac{keQ}{\sqrt{R^2+x^2}}=-\frac{keQ}{R}+\frac{1}{2}mv^2$$ From this you can obtain the velocity. The P.E. ...

2

There are such devices, but they are rather limited in their electrical output and do require you to carry a big chunk of radioactive material around. Ironically their only commercial use was implanted pacemakers! If you are going to use radiation to make energy it's easier to use a safer alpha emmitter and just use the heat to generate electricity by the ...

2

Metal is much more conducting than the human body, but in order for an armor to act as a Faraday cage the shoes and soles also should be metal soldered on the armor. Otherwise the body will be the line to the ground. These insights in instructions for metallic cars is enlightening: Reported incidents and related injuries make it clear that a person ...

2

Wikipedia defines capacitance of parallel plates as $$C=\frac{Q}{V},$$ where $\pm Q$ is the charge on the plates (one sign for each plate) and $V$ is the voltage between them. In other words, yes, if you calculate a capacitance to be negative from, say, $Q<0$, then you can just take the magnitude and call the capacitance positive.

2

First of all, if you are dealing with a network of finite number of resistors, try redrawing it in some form in which you'll be able to recognize the parallel or series connections. Secondly, take a look at Delta-Y Transform which might be really helpful in some cases. If these fail, turn to Kirchoff's laws i.e. put a test generator between the points ...

2

Do electrons always have a probability of being somewhere [in] the same way as when they surround a nucleus? Yes. Of course they don't have a probability a of being somewhere when surrounding a nucleus, they have a frequency of being found somewhere if measured, which is different. You can get a full probability too, but only if you specify even more ...

2

There is already a very useful mechanical equivalent called the hydraulic analogy. You'll find lots of related posts already on this site. All analogies have their limits, but the hydraulic analogy is remarkably good. You can even represent components like capacitors, inductors and even semiconductor junctions.

1

As you may know, You can equate a damper with a capacitor and a spring with an inductor. You'd need to transfer the energy from your wave somehow to those mechanics in order to use the damper and the spring.

1

No, it will rotate at a speed determined by the load. Witness that the current in, and thus the magnetic field produced by the stator coils is either in-phase with or anti-phase with the rotor current, with the $\pi$-phase change triggered by the split ring commutator. So the torque in each half of the rotor's rotation will throb at twice the AC line ...

1

An ideal capacitor never "dissipates" energy, it merely stores it. The amount of energy stored in a capacitor is given by the formula you mentioned: $U = \frac{1}{2}CV^2$. In the case of the LC circuit, the energy stored in the capacitor moves into the inductor in form of magnetic field energy and then goes back and forth from them. In the case of an ideal ...

1

Positively charged means lack of electrons. Remember that protons are stationary only free electrons can move. So if you have excess electrons its negative and if no of protons= no of electrons its neutral. So what happens in above case is that one sphere has lack of electrons and the other is neutral. By joining them by a conductor some electrons from ...

1

The first sentence started with an if. When you start with an if and end with a problem a solution you should consider is that your if never happens. So if the EMF around a zero resistance loop is zero then we don't expect the total magnetic flux through to change. Is that reasonable? Yes. Since it is a zero resistance loop, it can generate any current it ...

1

The power pole boxes you mention, are typically step down voltage transformers, and although it is true that the source of the "hum" could be the coils and the core, I think it is more likely that the transformer enclosure might be loose, causing it to vibrate at the line frequency (50 or 60 Hz).

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