Dr. Lewin Responds, Admits error "EMF is not larger due to ohmic resistance" youtube.com/watch?v=ototT…
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πŸ‘€︎ u/sandalcandal
πŸ“…︎ Nov 18 2018
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How can you have ohmic resistors if resistance causes heat increase and heat increase causes higher resistance?

So filament lamps are non-ohmic because if you increase the voltage across them then the charge carriers disipate more energy in the resistance of the lamp, causing it to heat up, increasing the resistance, and therefore reducing the increase in current with each increase in voltage. Therefore they are not proportional and therefore it is non-ohmic.

With an ohmic resistor, surely as current passes through the resistor, the resistor heats up and increases in resistance just like with the filament lamp and therefore not ohmic because the resistance is changing?

Or am I missing something?

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πŸ‘€︎ u/Spacecow60
πŸ“…︎ May 22 2016
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How do you calculate antenna efficiency?

Recently ive been diving into antenna theory and learned about gain. However theres some value that looks like an epsilon with a subscript of A i think and i dont know how to calculate it.

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πŸ‘€︎ u/Abxsmal_Void
πŸ“…︎ Jan 15 2022
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Efficiency of Water Electrolysis to make hydrogen

I have been struggling with a question that I cannot seem to make a good answer for. We know that the electrolysis of water to make hydrogen gas is not 100% efficient, but I am not sure what exactly is driving these loses in efficiency if it's not just the energy barrier that the reaction must past in order to be executed (because it is non-spontaneous) What makes electrolysis of water inefficient?

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πŸ‘€︎ u/allsapiens
πŸ“…︎ Nov 23 2021
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Tafel-Plot and temperature dependent electrode performance

Hello everyone, for a project at my university i have to evaluate the temperature dependence of the electrodes of a DMFC. I have gathered the iv-plots for cathode and anode via a measurement with a standard hydrogen reference electode. Now i want to derive the exchange current density by Tafel-plotting my Data in Excel (i have got no other data processing program). Best case scenario is a rising exchange current density with rising temperture (at least thats what i think will happen out of "arrhenius-reasons"). So my question is, is the exchange current density a good/fitting indicator to explain the temperature dependence of both electrodes independently. As far as theory is concerned the current exchange density shows a strong temperature dependence. But does tafel-plotting the real life iv-plots for each electrode and temperature and deriving the current exchange density work. I tried Tafel-plotting my Data without any success till now, bit first things first.

For further insight: i am mainly focusing on deriving the current exchange density for the anode because it is the main reason for temperature dependence of the fuel cell according to my Dataset (i think this is beacuse of the methanol oxidation on Platinum-Ruthenium Catalyst being far more complicated and showing a bigger activation-overptential than the electrocatalytic ORR on Platinum and thus the anode is more volatile to a change in temperature than the cathode) So i want to compare i0 of the cathode and anode with rising temperature.

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πŸ‘€︎ u/dauerschreiber
πŸ“…︎ Oct 11 2021
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Something interesting about V=IR

This is an academic post but it is something I've always found interesting because nearly every person in the world believes V=IR is Ohm's Law. But that is actually incorrect, read on to find out why!!

When asked what is Ohm's Law, you would probably say back V=IR, or an equivalent form of that equation such as I=V/R or R=V/I. And that is correct, Ohms Law is exactly "voltage through a conductor is directly proportional to the current through that conductor". So Ohm's Law is V=IR. But what is interesting is that this is actually a very strict definition. When two values are directly proportional, they are always directly proportional by the same value. This value is called the constant of proportionality. The constant of proportionality in this instance is the resistance of the conductor. So in other words what this means is as long as the resistance of the conductor doesn't change, then the conductor obeys Ohm's Law. But what happens when the resistance does change?

As we vapers know, when you pass electricity through a conductor, the conductor will heat up because of its resistance to allow electricity to pass through it. When some materials heat up, its resistance changes. Take a coil made of stainless steel for instance and say it has a resistance of 0.4 ohms at room temperature. When that coil heats up, its resistance will change (inceease). After firing for a few seconds, the coil may now be 0.45 ohms. (How much the resistance changes depends on how much the temperature increases). But what is interesting here is that this material does not follow Ohm's Law because the resistance is not constant (remember, the constant of proportionality cannot change). Stainless steel is called a non-ohmic material because Ohm's Law can't be used to model its behavior. If Ohm's Law doesn't work, what equation can be used to model electricity passing through it? We can still use V=IR. V=IR still works even if the resistance changes. So what does this mean? It means that Ohm's Law is V=IR but V=IR is not Ohm's Law. This is not a paradox, it's just a one way definition (doesn't work backwards). V=IR applies to every situation while Ohm's Law only applies when the resistance of the material doesn't change.

So what does this mean for vaping? Not much really, you still use I=V/R when calculating amp draw, but if you are using a non-ohmic material such as stainless steel or nichrome the coil resistance will slightly increase as it heats up

... keep reading on reddit ➑

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πŸ‘€︎ u/Wags43
πŸ“…︎ Nov 11 2021
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why are CT burdens rated in VA? doesn’t it make much more sense to be rated in ohms since it’s just the resistance connected to the secondary of the CT? is there a significance in the VA rating?
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πŸ‘€︎ u/bmtkwaku
πŸ“…︎ Sep 12 2021
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Why is it so hard to find a decent explanation?
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πŸ‘€︎ u/Ikebear2
πŸ“…︎ Apr 26 2021
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SERIOUS: This subreddit needs to understand what a "dad joke" really means.

I don't want to step on anybody's toes here, but the amount of non-dad jokes here in this subreddit really annoys me. First of all, dad jokes CAN be NSFW, it clearly says so in the sub rules. Secondly, it doesn't automatically make it a dad joke if it's from a conversation between you and your child. Most importantly, the jokes that your CHILDREN tell YOU are not dad jokes. The point of a dad joke is that it's so cheesy only a dad who's trying to be funny would make such a joke. That's it. They are stupid plays on words, lame puns and so on. There has to be a clever pun or wordplay for it to be considered a dad joke.

Again, to all the fellow dads, I apologise if I'm sounding too harsh. But I just needed to get it off my chest.

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πŸ‘€︎ u/anywhereiroa
πŸ“…︎ Jan 15 2022
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Blind Girl Here. Give Me Your Best Blind Jokes!

Do your worst!

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πŸ‘€︎ u/Leckzsluthor
πŸ“…︎ Jan 02 2022
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French fries weren’t cooked in France.

They were cooked in Greece.

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πŸ“…︎ Jan 20 2022
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This subreddit is 10 years old now.

I'm surprised it hasn't decade.

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πŸ‘€︎ u/frexyincdude
πŸ“…︎ Jan 14 2022
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You've been hit by
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πŸ‘€︎ u/mordrathe
πŸ“…︎ Jan 20 2022
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I'm sick of you guys posting dumb wordplay in here for awards and upvotes.

Don't you know a good pun is its own reword?

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πŸ‘€︎ u/diggitygiggitycee
πŸ“…︎ Jan 21 2022
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My 4 year oldest favourit joke, which he very proudly memorized and told all his teachers.

Two muffins are in an oven, one muffin looks at the other and says "is it just me, or is it hot in here?"

Then the other muffin says "AHH, TALKING MUFFIN!!!"

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πŸ‘€︎ u/smoffatt34920
πŸ“…︎ Jan 22 2022
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Dropped my best ever dad joke & no one was around to hear it

For context I'm a Refuse Driver (Garbage man) & today I was on food waste. After I'd tipped I was checking the wagon for any defects when I spotted a lone pea balanced on the lifts.

I said "hey look, an escaPEA"

No one near me but it didn't half make me laugh for a good hour or so!

Edit: I can't believe how much this has blown up. Thank you everyone I've had a blast reading through the replies πŸ˜‚

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πŸ‘€︎ u/Vegetable-Acadia
πŸ“…︎ Jan 11 2022
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What starts with a W and ends with a T

It really does, I swear!

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πŸ‘€︎ u/PsychedeIic_Sheep
πŸ“…︎ Jan 13 2022
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Why did Karen press Ctrl+Shift+Delete?

Because she wanted to see the task manager.

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πŸ‘€︎ u/Eoussama
πŸ“…︎ Jan 17 2022
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Steve JOBS would have made a better President than Donald Trump

But that’s comparing apples to oranges

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πŸ‘€︎ u/Ok-Ingenuity4838
πŸ“…︎ Jan 22 2022
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So 2 trees got arrested in the town I live...

Heard they've been doing some shady business.

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πŸ‘€︎ u/K1ll47h3K1n9
πŸ“…︎ Jan 18 2022
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I was almost upset that my coffee tasted like dirt today

but then I remembered it was ground this morning.

Edit: Thank you guys for the awards, they're much nicer than the cardboard sleeve I've been using and reassures me that my jokes aren't stale

Edit 2: I have already been made aware that Men In Black 3 has told a version of this joke before. If the joke is not new to you, please enjoy any of the single origin puns in the comments

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πŸ‘€︎ u/scarf_spheal
πŸ“…︎ Jan 19 2022
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How eggs-traordinary
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πŸ‘€︎ u/Rix27_
πŸ“…︎ Jan 21 2022
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What is the scariest tree?

BamBOO!

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πŸ‘€︎ u/K1ll47h3K1n9
πŸ“…︎ Jan 18 2022
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A queen size statement.
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πŸ‘€︎ u/Flight-less
πŸ“…︎ Jan 22 2022
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What is a a bisexual person doing when they’re not dating anybody?

They’re on standbi

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πŸ‘€︎ u/Toby-the-Cactus
πŸ“…︎ Jan 12 2022
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My ten-year-old daughter came up with this at dinner tonight: What do you get if put a copy of Macbeth on top of a dictionary?

A play on words.

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πŸ‘€︎ u/ah1887
πŸ“…︎ Jan 20 2022
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Difficulty with RTD Temperature Measurements

Morning, Folks.

Summary: Our team is having troubles with temperature measurements of a system at LN2 temperatures. When tested by submerging in LN2, the RTDs report accurately, but when attached to the objects that they are intended to measure, they give non-physical values. These values plateau at temperatures that are both impossible based on the material properties and geometry and constant over very long periods of time implying that this is a steady-state. This behavior began after a long down period during which I rebuilt the object that is cold, but the RTDs themselves were only disconnected and reconnected.

We have, so far, disconnected them from the RTD module and directly measured the resistance to ensure that we are not seeing ohmic heating. The resistance agrees with the non-physical temperatures which should preclude problems with the module. We have removed, tested, and reinstalled the sensors and have gotten slightly different, but still very wrong results. During this time, we also looked carefully for heat leakage paths, but nothing is evident.

Details: The system is a double crystal monochromator (DCM) with a sandwich geometry of directly LN2 cooled copper, silicon, copper, silicon, copper with RTDs wrapped in indium foil and inserted into the holes in the ends of both silicon crystals. Other RTDs measure the temperature of the incoming and outgoing LN2 as well as various points on the assembly. The approximate dimensions of the cold mass are approximately a 6" cube. This cold mass exists inside a large vacuum chamber operating at ~10e-9mbar which serves as both the insulating vacuum and eliminates x-ray-air scatter.

The particular RTDs in which I am interested are those attached to the silicon crystals since they are their protection against over-temperature. For those who don't know silicon is a magical material at LN2 temperatures for two reasons: 1) the thermal conductivity is approximately that of diamond nearly 2000W/m*k and 2) the thermal expansion coefficient goes to (and through) zero. We operate this system with the surface temperature at the zero point to dramatically reduce the thermal strain from a high power and power density x-ray beam striking the crystal. Due to the easily achievable runaway condition of the thermal conductivity increasing with increasing temperature, these RTDs will trip off the beam when needed. Our machine is powerful enough to destroy these crystals easily enough.... Specifically, the RTDs indic

... keep reading on reddit ➑

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πŸ“…︎ Sep 09 2021
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My son, Luke, loves how I named our kids after Star Wars characters...

My daughter, Chewbecca, not so much.

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πŸ‘€︎ u/andersonfmly
πŸ“…︎ Jan 21 2022
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Geddit? No? Only me?
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πŸ‘€︎ u/shampy311
πŸ“…︎ Dec 28 2021
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No gains
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πŸ‘€︎ u/ridi86
πŸ“…︎ Jan 22 2022
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I wanna hear your best airplane puns.

Pilot on me!!

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πŸ‘€︎ u/Paulie_Felice
πŸ“…︎ Jan 07 2022
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I just flew in from Chernobyl

And boy are my arms legs.

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πŸ‘€︎ u/JhopkinsWA
πŸ“…︎ Jan 23 2022
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E or ß?
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πŸ‘€︎ u/Amazekam
πŸ“…︎ Jan 03 2022
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Which actor drives the least?

Christopher Walken

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πŸ‘€︎ u/TR1771N
πŸ“…︎ Jan 18 2022
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What did Spartacus say when the lion ate his wife?

Nothing, he was gladiator.

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πŸ‘€︎ u/rj104
πŸ“…︎ Jan 15 2022
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Pun intended.
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πŸ‘€︎ u/Sharmaji1301
πŸ“…︎ Jan 15 2022
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No spoilers
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πŸ‘€︎ u/Onfour
πŸ“…︎ Jan 06 2022
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Should we create an English word for the 'day after tomorrow'?

Or would that be too forward thinking?

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πŸ‘€︎ u/afunkysquirrel
πŸ“…︎ Jan 19 2022
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Covid problems
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πŸ‘€︎ u/theincrediblebou
πŸ“…︎ Jan 12 2022
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These aren't dad jokes...

Dad jokes are supposed to be jokes you can tell a kid and they will understand it and find it funny.

This sub is mostly just NSFW puns now.

If it needs a NSFW tag it's not a dad joke. There should just be a NSFW puns subreddit for that.

Edit* I'm not replying any longer and turning off notifications but to all those that say "no one cares", there sure are a lot of you arguing about it. Maybe I'm wrong but you people don't need to be rude about it. If you really don't care, don't comment.

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πŸ‘€︎ u/Lance986
πŸ“…︎ Dec 15 2021
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Spi__
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πŸ‘€︎ u/Fast_Echidna_8520
πŸ“…︎ Jan 11 2022
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What did 0 say to 8 ?

What did 0 say to 8 ?

" Nice Belt "

So What did 3 say to 8 ?

" Hey, you two stop making out "

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πŸ‘€︎ u/designjeevan
πŸ“…︎ Jan 03 2022
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I had a vasectomy because I didn’t want any kids.

When I got home, they were still there.

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πŸ‘€︎ u/demotrek
πŸ“…︎ Jan 13 2022
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I dislike karma whores who make posts that imply it's their cake day, simply for upvotes.

I won't be doing that today!

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πŸ‘€︎ u/djcarves
πŸ“…︎ Dec 27 2021
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It is really unfortunate that Islam, Christianity, and Judaism have been fighting each other for centuries.

Hindus, on the other hand, never had any beef.

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πŸ‘€︎ u/Utkarsh_Anand2004
πŸ“…︎ Jan 20 2022
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