Author Topic: Episode #609  (Read 3946 times)

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Offline amysrevenge

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Re: Episode #609
« Reply #60 on: March 16, 2017, 10:57:19 AM »
Not quite... Power is E^2/R, so it would get 4x the power in Europe (twice the voltage at twice the current). But that assumes three things:

1. The kettle in America has the same load resistance as the one in Europe.
2. The effect of temperature on your load resistance is negligible. If the heating element is copper, it has a thermal coefficient of 0.393% per degree C. If the higher power delivered to it in Europe makes it 100 degrees C hotter, you will get significantly less than 4x increase in power -- though still more than 2x.
3. Plugging a kettle designed for US voltages (and therefore having perhaps a lower load resistance) into a European power grid doesn't burn out the heating element or blow a fuse.

However, given no other information, I'd assume that someone designing a kettle for US use would give it a lower load resistance so that it gets just as hot as one designed for European use, which invalidates assumption #1.

Right and wrong at the same time.  The maths all line up, but the premise doesn't (it's assumption 1 that isn't going to be true).

We're not looking at kettles with the same R and just doubling the V (or E).  We're looking at keeping the I constant, and designing a new kettle from scratch to match the V we are provided with.

So the formula you're looking for isn't P = V^2/R, where R is constant and V is ~doubled.  The formula you're looking for is P = V * I, where I is constant and V is ~doubled.
Big Mike
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Re: Episode #609
« Reply #61 on: March 16, 2017, 12:18:48 PM »

Ohm's law.  Holding voltage constant, lower resistance = higher current = more power.

But I'm thinking the resistance IS the heat.  Less resistance means more current, but nothing's getting hot.   

I must be missing something basic... maybe I'm thinking wrong about the way the resistors are wired up.



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Offline Pusher Robot

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Re: Episode #609
« Reply #62 on: March 16, 2017, 03:35:30 PM »
15 amp circuits require 14-guage wiring.  20 amp circuits require 20-guage wiring. 

I think you have a typo and mean 12-gauge wire for 20 amps. (Gauge is bigger as the number goes down and you got it right for 40 amp.)

Also, everyone needs to keep in mind the NEC 80% rule, which is to load circuits to no more than 80% of the maximum capacity. On a 20 amp circuit, you should have no more than a 16 amp continuous load.

Just waiting for someone to bring up VA vs. Watts, and power factor too.

Indeed, I was thinking "12" and somehow typed "20".  What can I say, they both have a "2".
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Offline Pusher Robot

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Re: Episode #609
« Reply #63 on: March 16, 2017, 03:42:19 PM »

Ohm's law.  Holding voltage constant, lower resistance = higher current = more power.

But I'm thinking the resistance IS the heat.  Less resistance means more current, but nothing's getting hot.   

I must be missing something basic... maybe I'm thinking wrong about the way the resistors are wired up.

Resistance causes heat, but the power determines the amount of heat, and the lower the resistance, the more power will flow through your circuit.  That's why short-circuits, where the resistance is tiny, will weld themselves from the intense heat.  Consider a water turbine by analogy: the turbine is a resistor that converts current flow into heat, but that doesn't mean you get the most power out of a turbine that does the most resistance to the flow of water - after all, if the water is barely flowing because the turbine is so hard to move, you're not getting much power output.  Instead (holding efficiency constant), you want a turbine that restricts the flow as little as possible for maximum power output.
A novice was trying to fix a broken Lisp machine by turning the power off and on.
Knight, seeing what the student was doing, spoke sternly: “You cannot fix a machine by just power-cycling it with no understanding of what is going wrong.”
Knight turned the machine off and on.
The machine worked.

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Re: Episode #609
« Reply #64 on: March 16, 2017, 03:54:09 PM »
Resistance causes heat, but the power determines the amount of heat, and the lower the resistance, the more power will flow through your circuit.

Well, Google comes through, and I am schooled.

Quote
Does a heating element need a high or a low resistance?

You might think a heating element would need to have a really high resistance—after all, it's the resistance that allows the material to generate heat. But that's not actually the case. What generates heat is the current flowing through the element, not the amount of resistance it feels. Getting the maximum current flowing through a heating element is much more important than forcing that current through a large resistance. This might seem confusing and counter-intuitive, but it's quite easy to see why it is (and must be) true, both intuitively and mathematically.

Intuitively...

Suppose you made the resistance of your heating element as big as you possibly could—infinitely big, in fact. Then Ohm's law (voltage = current × resistance or V = IR) tells us the current flowing through your element would have to be infinitely small (if I = V/R, I approaches zero as R approaches infinity). You'd have a whopping great resistance, no current, and therefore no heat produced. Right, so what if we went to the opposite extreme and made the resistance infinitely tiny. Then we'd have a different problem. Although the current I might be huge, R would be virtually zero, so the current would zip through the element like an express train without even stopping, producing no heat at all.

What we need in a heating element is therefore a balance between the two extremes: enough resistance to produce heat, but not so it reduces the current too much. Nichrome is a great choice. The resistance of a nichrome wire is (roughly) 100 times higher than that of a wire the same size made from copper (an excellent conductor), but only a quarter as much as a similar-sized graphite rod (a fairly good conductor) and maybe only a million trillionth that of a really good insulator such as glass. The numbers speak for themselves: nichrome is an average conductor with only moderate resistance, and not remotely an insulator!

Mathematically...

We can reach exactly the same conclusion with math. The power produced or consumed by a flow of electricity is equal to the voltage times the current (watts = volts × amps or P = VI). We also know from Ohm's law that V = IR. Eliminate V from these equations and we find the power dissipated in our element is I2R. In other words, the heat is proportional to the resistance, but also proportional to the square of the current. So the current has much more effect on the heat produced than the resistance. Double the resistance and you double the power (great!), but double the current and you quadruple the power (fantastic!). So the current is what really matters.
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Offline tralfaz

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Re: Episode #609
« Reply #65 on: March 18, 2017, 08:48:55 PM »
I lived in Seattle when Starbucks had 2 stores (both in Seattle).  I graduated from Starbucks (burnt beans) to the expensive boutique stuff.  I cut back on my coffee habit in my 30s.  Now in my late 50s I keep a jar of cheap instant for emergencies.

Offline Fast Eddie B

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Re: Episode #609
« Reply #66 on: March 19, 2017, 08:38:26 AM »
As far as Cara's desire to preserve Mars in its "virginal" state...

Kim Stanley Robinson wrote a trilogy: "Red Mars", "Green Mars" and "Blue Mars".

I'm admittedly bogged down about halfway through the second one, but much of the book centers on the conflicts between those wanting to terraform Mars as quickly as possible and those wishing to maintain it in much its original state.
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Offline Tom Hopper

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Re: Episode #609
« Reply #67 on: March 27, 2017, 11:00:07 AM »
Do Euro homes have the same 15-20 amp circuits as USA homes?  There's no reason USA homes couldn't have bigger capacity 120v circuits - they'd just need bigger wires... but it's probably a code thing not to.

No. European homes typically have 10 Amp circuits. Depending on who you talk to, this choice was either to improve efficiencies (P = I^2 R, meaning you can use smaller wires, which are cheaper), or to make the circuits safer (it's the current kills, and lower-amperage circuits combined with smaller wires make for safer household electrical, despite the higher voltage).

Offline Morvis13

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Re: Episode #609
« Reply #68 on: March 27, 2017, 01:32:38 PM »
so how do we get science articles to Steve for Science or Fiction?
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Re: Episode #609
« Reply #69 on: March 27, 2017, 01:46:39 PM »
Do Euro homes have the same 15-20 amp circuits as USA homes?  There's no reason USA homes couldn't have bigger capacity 120v circuits - they'd just need bigger wires... but it's probably a code thing not to.

No. European homes typically have 10 Amp circuits. Depending on who you talk to, this choice was either to improve efficiencies (P = I^2 R, meaning you can use smaller wires, which are cheaper), or to make the circuits safer (it's the current kills, and lower-amperage circuits combined with smaller wires make for safer household electrical, despite the higher voltage).

Thanks.  I was thinking about this thread this morning while I was ironing my clothes for work.  When the iron heater kicks on the lights in the room dim, come back part way, and then back to normal when the heater cycles off.  I wondered if 220v houses have that same effect.
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Offline Tom Hopper

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Re: Episode #609
« Reply #70 on: March 27, 2017, 02:03:42 PM »
Do Euro homes have the same 15-20 amp circuits as USA homes?  There's no reason USA homes couldn't have bigger capacity 120v circuits - they'd just need bigger wires... but it's probably a code thing not to.

No. European homes typically have 10 Amp circuits. Depending on who you talk to, this choice was either to improve efficiencies (P = I^2 R, meaning you can use smaller wires, which are cheaper), or to make the circuits safer (it's the current kills, and lower-amperage circuits combined with smaller wires make for safer household electrical, despite the higher voltage).

Thanks.  I was thinking about this thread this morning while I was ironing my clothes for work.  When the iron heater kicks on the lights in the room dim, come back part way, and then back to normal when the heater cycles off.  I wondered if 220v houses have that same effect.

That's a wiring problem; maybe you have too much on that line connected in series (which could lead to overheating of the wires), or you don't have a good ground. In any case, you're looking at damage to other electronics. I suggest finding a good electrician.

Offline Ah.hell

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Re: Episode #609
« Reply #71 on: March 27, 2017, 02:09:38 PM »
As far as Cara's desire to preserve Mars in its "virginal" state...

Kim Stanley Robinson wrote a trilogy: "Red Mars", "Green Mars" and "Blue Mars".

I'm admittedly bogged down about halfway through the second one, but much of the book centers on the conflicts between those wanting to terraform Mars as quickly as possible and those wishing to maintain it in much its original state.
I enjoyed the series but as you noted, the soap opera aspects of the story got old quickly IMHO.  The exploration of mars and its terraforming was pretty good though.

Offline amysrevenge

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Re: Episode #609
« Reply #72 on: March 27, 2017, 03:25:15 PM »
I switched off the audiobook about 5 hours in.  Just didn't click with me.
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Re: Episode #609
« Reply #73 on: March 27, 2017, 05:50:47 PM »
I was out of town and off-line when this episode came out, so I'm just going to throw in my two cents on several topics:

Coffee: When I lived in Mexico I was able to buy Columbian coffee, roasted in Italy, and imported via the U.S. This was superb coffee. Being recently roasted is not nearly as important as being freshly ground. This coffee took weeks from the roaster in Italy to the coffee shop, and was excellent if freshly ground. In contrast, coffee that was ground a week ago is mediocre even if it was roasted the day before it was ground. People give too much importance to coffee being recently roasted.

Mars: They never explained how the scientists proposed to give Mars a magnetic field. Since the proposal seems far-fetched to me, I want to know how they intended to accomplish it. The Earth gets its magnetic fiend from its molten iron core. I think it unlikely they can create a molten iron core for Mars.

Heavy rockets: One of the rogues (I cannot tell Evan, Jay, and Bob apart) asserted that the Saturn 5 was "better" than modern rockets, because it can lift more. I'd argue that a "better" definition of "better" would be the cost per kilogram of payload. The Saturn 5 could lift more weight than the Falcon Heavy, but the Falcon Heavy does it at a much lower cost per kilogram. Any deep space program will require many launches. It's no big deal if the rockets are smaller. All that matters is the total cost of the program. Falcon is far better than Saturn 5 because for the same money we can put more stuff up there.

And a linguistic nitpick: Bob (I know it was him only because one of the others addressed him by name during the segment) kept saying that the researchers had "proved" that they had produced a supersolid by this or that means. A better choice of words would be that they had "demonstrated" or "found evidence" that they had a supersolid. "Proved" sounds too much like the language of pseudoscience advocates.
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Offline Swagomatic

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Re: Episode #609
« Reply #74 on: March 27, 2017, 07:06:23 PM »
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