101–110 of 117 posts
Now I'm wondering if anyone has built a very large grid of resistors to try to approximate / curve fit this. Surely there's a youtube video in this ...
Earlier quoted context omitted.
You're describing thin film resistors, and they exist. They also just convert current to heat though. Some amount of current moving through a material with some amount of resistance always produces a fixed amount of current in accordance with Ohm's law. You can't really get away from that.
Thanks. So Is there a physical reason resistors have to make heat? Is it theoretically possible to find a material that limits current but produces very little heat? I guess the explanations always confuse me. Let’s say a short circuit with no resistors has a certain amount of power. Then we add a resistor and the power in the circuit goes down. The resistor isn’t turning the difference in power into heat, right.
Earlier quoted context omitted.
You're describing thin film resistors, and they exist. They also just convert current to heat though. Some amount of current moving through a material with some amount of resistance always produces a fixed amount of current in accordance with Ohm's law. You can't really get away from that.
Thanks. So Is there a physical reason resistors have to make heat? Is it theoretically possible to find a material that limits current but produces very little heat? I guess the explanations always confuse me. Let’s say a short circuit with no resistors has a certain amount of power. Then we add a resistor and the power in the circuit goes down. The resistor isn’t turning the difference in power into heat, right.
But let's talk about short-circuiting lithium batteries for example. They have a roughly 50 milliohm (aka 0.05 ohm) of "equivalent series resistance".
That means, if you short circuit a lithium battery with a superconductive wire (aka with 0 resistance), the circuit has a resistance of 0.05 ohms. We can compute the current with Ohm's law: I=V/R. V is typically 3.6 volts for li-ion batteries, R is 0.05 ohm, so I (aka current) is 3.6/0.05 = 72 amperes. 72 amperes * 3.6 volts is 259 watts. Now in the real world, the battery's chemistry would step in here and limit current in complicated ways, but this means that under the assumption that our battery would work as an ideal voltage source + a 0.05 ohm resistor, and if there was no extra heat coming from the chemical reactions, a shorted battery would produce 259 watts of heat.
We can add a 1 ohm resistor to the circuit, which means our circuit's combined resistance would be 1.05 ohm. Using Ohm's law again, we find that the current would be 3.6/1.05 = approx 3.43 amperes. 3.43 amperes * 3.6 volts is 12.35 watts of heat.
So thanks to our resistor, we're now producing 12.35 watts instead of 259 watts of heat, because the resistor limits the current going through the circuit. With a higher resistance resistor we'd produce even less heat.
A core idea here is that power consumptions equals heat. I don't understand the physics reasons why, but "this thing consumes 10 watts of power" means the same as "this thing produces 10 watts of heat". Higher resistance means less current which means less watts, which means both less heat and less power consumption because those are the same.
People think this is not relevant to real world problems but it actually is, albeit all the calculations aren't that relevant. Silicon substrate's resistance is basically an infinitely large grid of unut resistances at the distances relevant for a local point of an IC. Note that silicon substrate is often heavily doped (p-type) and all info you get from the fab is it's resistivity (often somewhere between 1 to 100 oh…
I'd argue the case you're describing is mathematically simpler precisely because it is continuous.
Earlier quoted context omitted.
Thanks. So Is there a physical reason resistors have to make heat? Is it theoretically possible to find a material that limits current but produces very little heat? I guess the explanations always confuse me. Let’s say a short circuit with no resistors has a certain amount of power. Then we add a resistor and the power in the circuit goes down. The resistor isn’t turning the difference in power into heat, right.
There is no such thing as a short circuit with no resistance, because everything (other than superconductors) has a resistance. If you had a circuit with a magical ideal voltage source and no resistance, you'd have infinite current. But let's talk about short-circuiting lithium batteries for example. They have a roughly 50 milliohm (aka 0.05 ohm) of "equivalent series resistance". That means, if you short circuit a l…
Earlier quoted context omitted.
You're describing thin film resistors, and they exist. They also just convert current to heat though. Some amount of current moving through a material with some amount of resistance always produces a fixed amount of current in accordance with Ohm's law. You can't really get away from that.
Thanks. So Is there a physical reason resistors have to make heat? Is it theoretically possible to find a material that limits current but produces very little heat? I guess the explanations always confuse me. Let’s say a short circuit with no resistors has a certain amount of power. Then we add a resistor and the power in the circuit goes down. The resistor isn’t turning the difference in power into heat, right.
Look at it like this: resistors produce heat by definition.
What you are describing isn't a resistor. It's actually a switched power supply, but that material is probably more heterogeneous than you wanted.
Earlier quoted context omitted.
You're describing thin film resistors, and they exist. They also just convert current to heat though. Some amount of current moving through a material with some amount of resistance always produces a fixed amount of current in accordance with Ohm's law. You can't really get away from that.
Thanks. So Is there a physical reason resistors have to make heat? Is it theoretically possible to find a material that limits current but produces very little heat? I guess the explanations always confuse me. Let’s say a short circuit with no resistors has a certain amount of power. Then we add a resistor and the power in the circuit goes down. The resistor isn’t turning the difference in power into heat, right.
There are ways to limit the current through some components regardless of the applied voltage (to a sane level) without producing much heat. These are active switching mode DC-DC converters widely used eg. for driving LEDs in (higher quality) light bulbs and charging batteries.
I don't get why EE education emphasizes problems of this sort. The infinite grid is an extreme example, but solving weirdly complicated problems involving Kirchoff's laws and Thevenin's theorem was a common way to torture students back in my day... Here, I don't think it's even useful to look at this problem in electronic terms. It's a pure math puzzle centered around an "infinite grid of linear A=B/C equations". Not…
Aside: Veritasium had a great video similar to this on the paths that light takes. I'll link to the part where they do the best physics demo I have ever seen : https://www.youtube.com/watch?v=qJZ1Ez28C-A&t=1500
Now, the same underlying theory also explains why there will always be diffraction from any finite boundary resulting in a reality which is indistinguishable from one where light actually takes all possible paths. But to argue that it actually does is arguably more meta-physics than physics. The demonstration is further compromised by the fact that the laser's diffraction performance is also presumably far from the physical limit.
So a cynic seeing that demo would say "Isn't that just due to some of the light from the laser being off-axis" -- and it absolutely is. The physics means that some of the light will always be off-axis, but the demonstration does nothing to establish that.
People think this is not relevant to real world problems but it actually is, albeit all the calculations aren't that relevant. Silicon substrate's resistance is basically an infinitely large grid of unut resistances at the distances relevant for a local point of an IC. Note that silicon substrate is often heavily doped (p-type) and all info you get from the fab is it's resistivity (often somewhere between 1 to 100 oh…