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29

Trees are not as good an insulator as you might think. This source suggests a typical conductivity of living tree sap is 0.01 S/m with a relative permittivity of 80. So not an insulator, though a poor conductor. Typical advice when using electric fencing is that you do not use wooden posts! Presumably because wet wood is also conductive to some extent. In ...


15

"Wood is an insulator" is a very broad statement. In fact it's wrong. "Wood is not a good conductor" would be a more accurate statement, but moisture in a living tree would easily carry enough current to give you a shock and have enough resistance not to trip out the breaker/discharge the battery powering the fence.


3

It's given that volumetric heat capacity = c is 3.45 joules per kelvin per cc that is equal that the heat necessary to heat up 1 cc of cooper for 1 K is c, so $\Delta Q = CV \ \Delta T$ where $V = lA$ is a volume.


3

The light from a fluorescent lamp is produced by an electric current discharging through a gas, typically mercury vapour, which releases photons in the UV-range. These are then absorbed by a thin phosphor coating on the inside of the glass tube, which re-emit a photon in the visible spectrum. In order to generate the required voltage and maintain an arc ...


3

It is because of the Lorentz force, a basic law of electromagnetism. Its expression is $\vec{F}=q(\vec{E}+\vec{v} \times \vec{B})$ What this means is that an electrostatic charge is only influenced by a magnetic field if it is moving (the first term is just the electrostatic force). The other is that the force is perpendicular to both the magnetic field ...


3

Why are electrons attracted to a magnetic field? Perhaps nobody knows. You can know the name of a bird in all the languages of the world, but when you're finished, you'll know absolutely nothing whatever about the bird... So let's look at the bird and see what it's doing — that's what counts. I learned very early the difference between knowing the ...


3

The torque on electricity generators is continuously adjusted to keep them running at a constant speed (e.g. 60Hz in the US and 50Hz in the UK). When you turn on some electrical item the current it consumes places a greater load on some electricity generator somewhere and this reduces the speed. To counter this, at the generating station more torque is ...


2

It would be the current that would cause damage and assuming your laptop had a 3 amp fuse in the plug top then yes across you chest 10 - 20 milli amp can stop your heart. Electrical engieer


2

Kirchhoff's current law says that the current entering any junction is equal to the current leaving that junction. The current through $R_1$ must therefore add up to the current through the three legs of the circuit. $$ I_{\mathrm{R}_1} = I_{\mathrm{cap}} + I_{\mathrm{nonlinear\ R}} + I_{\mathrm{R}_2} $$ Using the terms in the diagram, two of these ...


2

I already got the answer. because transformers simply doesn't work in high voltage environments. High voltage environments causes dielectric breakdown which reduce the transformers function which is bad. Tesla coils fix this problem.


2

Yes, a force is still acting on the particle. Moving perpendicular to the lines of force may result in no work being done ($W = \mathbf{F}\cdot \mathbf{r}$) , but since the particle is charged, it will be experiencing a force when moving through any electric field. If the particle is (initially) moving in the $y$ direction and the electric field is in the ...


2

Physicist137 took a crack at showing why this follows from the structure of the electric field in the absence of time-dependent magnetic fields. That doesn't hold once you have induction in the system, however. So let's look at a simpler approach. Anything that you want to treat as a potential (whether it is a real potential or not) has to have a simple ...


2

Every time two dissimilar insulators rub together, there will be a relative buildup of charge due to the triboelectric effect. In essence the two materials stick together briefly, and when they unstick the electrons may prefer to stay with one surface and not the other. This happens summer or winter. The reason you get "shocked" more in winter has to do ...


2

Power generation and supply management is not easy and it is to their credit that most of the time power companies supply people with AC at the same voltage no matter what the demand for power is. So when we turn on appliances we do not see the voltage drop as a result. Or more realistically when everyone gets home from work and starts cooking/ boiling ...


1

Looking at the expression you wrote in the comment to @nvvm's answer, you are getting your units messed up. If you just convert everything to SI units, and write the units during your evaluation, you should see where your problem is. You wrote I would rewrite as $$\begin{align}\\ \frac{dT}{dt} &= \frac{I^2R}{\ell\cdot A\cdot c_v}\\ ...


1

In 1999, the president of the IEEE Power Engineering Society, Robert Dent, noted that: "The degree or intensity of the corona discharge and the resulting audible noise are affected by the condition of the air--that is, by humidity, air density, wind and water in the form of rain, drizzle and fog. Water increases the conductivity of the air and so ...


1

The rotational of the electric field: $$ \nabla\times\mathbf E = -\frac{\partial\mathbf B}{\partial t} $$ Using Stokes' Theorem on this equation, we get the integral form of this equation: $$ \varepsilon = \oint_{\gamma}\mathbf E\cdot\mathbf{dl} = -\frac{d}{dt}\iint_S\mathbf B\cdot\mathbf{dS} = -\frac{d\Phi}{dt} $$ Which means, the electric field in a ...


1

The first equation is when you want to solve for either the voltage, current, or power already knowing the other two, similar for $P=I^2R$, when you want to solve for an unknown already having knowledge of the other two. The last equation you get by noting that, $P=IV=I^2R$, hence $V=IR$ or $I=\frac{V}{R}$ plugging this into, the first equation you get ...


1

It depends which side of the transformer the cable is broken. The cable from the mains outlet to the transformer carries 250V AC. If you are NOT well earthered, for example, you are on nylon carpet, or wearing rubber-soled shoes, then the electricity will not travel through you very well, and will get a severe shock. But if are well earthed, ie., there is a ...


1

It won't kill you if you touched the side after the transformer, that is the side that connects your laptop. The transformer is the big black box. However if you touched the side that is connected to the the Power Socket and were properly earthed you would have died assuming the current flowing from the the power board was not limited by placing some kind ...


1

You seem to be asking about mode transformers; these are extensively used in antennas as they connect to waveguide feeds. The waveguides usually employ TE10 (rectangular) or TE11 (circular) modes, but if you want to feed a horn, say, then you have to shape the field properly to avoid reflection, reduce sidelobes, and reduce cross-polarization coupling, etc. ...


1

The live found a route to earth, discharging it's energy on whatever load was connected between live and earth, and then luckily the RCD cut off the supply before you cooked yourself.



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