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24

Silicon carbide and similar IR heating elements are semiconductors and have a negative temperature coefficient. Image from here


18

PTC materials are prone to spot-overheating. Part of the wire gets hot, so the resistance increases, so more power is lost there, so it gets hotter, and the rest of the wire gets colder, and ... the wire burns out. For a resistor, you can partly handle this by giving the resistor good thermal conductance, so that it heats up evenly. For a long thin bit of ...


14

... without any loss in it ... First, physics says there must be some loss, because energy is received at the antenna and is available to be consumed by the receiver. ... be received by 100s of receivers ... Because the amount of power received by a typical receiving antenna is minuscule compared to the amount that is transmitted. Let's consider a not-...


8

There are few types of heating elements used in space heaters, all with their peculiarities. The traditional Kantal wire, formed as a spiral/serpentine and supported by mica paper, ceramic body or quartz tube in its place. It has more or less constant resistance between room temperature and working temperture (600 ... 1000K) Modern carbon mesh heaters are ...


7

You're right ... but Let's put some numbers to this. 3 mV (70dBuV) is a good strong signal into an FM receiver, on the 300 ohm (dipole antenna) input. That is a current of 10uA or a power of 30nW, so a perfectly efficient 1W transmitter could power 33 million such radios. The FM transmitter covering London, at Wrotham in Kent transmits 125kW on each main BBC ...


6

If you imagine the radio signal as the sun and each person as a radio. Only those indoors or behind someone's shadow on the ground is attenuated.(yet not completely in the dark) Why not millions of radios? Same results. The difference is that the grid distribution loss is by heat of {Pd= resistance * current²} in a loop back to source. In AM/FM radio ...


5

In both cases (electrical grid and radio), energy is transported by electromagnetic fields. Wires in the electrical grid act as waveguides. Electrical field (voltage) exists mostly between the Live and Neutral wires and current flows in the wires. Think of waveguides as pipes that guide an electromagnetic wave where we want it to go. They're a bit leaky, ...


2

First of all, remember that you're comparing photons (transmitted EM) to electrons (the power grid). This is important. Allow me to use a couple of metaphors When drawing power from an electrical grid, it's like drawing water out of a swimming pool. The individual pipe may draw very little water, and the result may be hard to detect, but the volume of water ...


2

Can we increase the power of the source by decreasing the time of the supply ? Unfortunately, the power of a source is just that but... can we trade off electric power for time? Absolutely we can. If the source has a limited energy then, we can choose to use that energy (within reason) how we like. It's average power that you cannot do much about. For ...


2

Same way you can speak in a room and everyone in the room will hear you regardless of how many people are in the room. Does your professor sound quieter when attendance is higher?


2

When the motor is not turning the current is maximum, because there's no back-EMF to oppose the terminal voltage. All of the energy going into the motor is burnt up in the winding resistance. As seen in your graph, when the torque * RPM is at its maximum, that's exactly when the power output is maximum. At that point, the motor is roughly 50% efficient -- ...


2

Power = Torque * RPM The plot of RPM is not shown but would be a positive linear slope. (X=Y) The plot of Torque due to back EMF is a negative linear slope. The product of the two linear opposing linear slopes starts at zero and ends in zero with a parabolic shape in between.


2

An interesting part about antennas is that if two or more antennas (elements) are closer together than a fraction of a wavelength, they influence each other through mutual coupling effects. The result of this interaction is that having two antennas close together does not result in twice the power collected. Looking at this in a qualitative manner, the ...


1

Your biggest concern when using a small motor as a generator won't be getting too much voltage or current. Your problem will be getting enough voltage and current. My son has a small steam engine. Not that model, but one like it. It produces more power than your home made steam turbine is likely to. We built a generator from a small motor and used it to ...


1

For just one LED, you can use a current limiter circuit built from 2 transistors. Here's an example (simulate it here): More here: Controlling High Current LEDs with an ATmega328


1

I hate these graphs. People look at them and think that they are operational graphs of the motor or something. But they are not. These graphs cover the full range from stalled rotor to overload up to no-load. It is assumed, but not always stated, that the voltage is held fixed. In this case, the input voltage is held at 12 V. Also, understand that the power ...


1

Why does the resistance of a space heater decrease as it gets hot? That's because its heating element has a negative temperature coefficient of resistance (NTC). Space heaters employ NTC heating elements so that the set temperature is attained faster.


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