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does voltage affect power in a battrey?

Old 05-05-2010, 09:35 PM
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laextreme
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Default does voltage affect power in a battrey?

well i was looking at lipos trying to figure them out and came across some max amps that are 11.1 volts at 5250. then i came across some villeon 14.8 volt 5000's. my question is what is the more powerful. they basically have the same capacity but different voltage
Old 05-05-2010, 09:39 PM
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yotaman1991
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Default RE: does voltage affect power in a battrey?

volts are like a cars engine size the mah is the gas tank size.
Old 05-05-2010, 09:39 PM
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Default RE: does voltage affect power in a battrey?

higher voltage more tq and RPM's but you'll aslo put more of a strain on ESC and motor as well as transmission

the lug
Old 05-05-2010, 09:52 PM
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laextreme
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Default RE: does voltage affect power in a battrey?



so do you think the vxl esc would be able to handle the 14.8 volts??

Old 05-05-2010, 10:07 PM
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Default RE: does voltage affect power in a battrey?


ORIGINAL: laextreme



so do you think the vxl esc would be able to handle the 14.8 volts??

Nope! It's only rated for 3s lipo which is 11.1 volts. You'll have to watch your temps on 3s with the VXL.
Old 05-05-2010, 10:35 PM
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Default RE: does voltage affect power in a battrey?

k thanks. any clue what type of speed i would be getting with the max amps on a duratrax evader with some grippy road tires and stock gearing??
Old 05-05-2010, 11:11 PM
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Default RE: does voltage affect power in a battrey?

If you have a VXL system then it crazy fast on 3s. You would have to learn throttle control as it would probably wheelie very easily. I know it does in the Traxxas Slash. I'm not familiar with your trucks drivetrain but a VXL system on 3s can be very hard on them. For this reason you might consider going with a 2s lipo instead.

By the way. Lipo cells are 3.7 volts per cell so a 2s lipo is 2 cells which is 7.4 volts. A 3s is 3 cells which is 11.1 volts. A fully charged lipo will charge to 4.2 volts per cell max. Also, a lipo should never be discharged below 3 volts per cell. Your ESC (the VXL) should have a LVC (low voltage cutoff) which keeps your lipo from running too low on voltage. You have to enable the LVC and it shows you how to in the manual.

This video may be close to what you would see but I'm not sure if your gearing is the same. http://www.youtube.com/watch?v=Lml_dwmhTQ8
Old 05-06-2010, 02:53 AM
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Default RE: does voltage affect power in a battrey?

ORIGINAL: laextreme


so do you think the vxl esc would be able to handle the 14.8 volts??

No, above 3s the VXL won't start up. 3s max for that ESC unless they've changed something.
Old 05-06-2010, 03:47 AM
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Default RE: does voltage affect power in a battrey?

Voltage by itself determines the RPM of the motor (ignoring a load being put on it like driving your truck on the ground, etc.). Assuming you're running the VXL 3500 Kv motor, the motor will rev 3500 RPM per volt you feed it. So at 11.1 volts, that's 11.1 x 3500 = 38,850 RPM.

So to answer your basic question as also stated above, battery voltage determines speed (or efficiency by letting the motor rev more but gearing down), and the mah (milli amp hours) rating is the capacity of the battery (and usually it also determines how well the battery can handle higher amp draw).

Going up in voltage however, also will under normal circumstances cause the motor to pull more amps (AMP is what the motor creates torque with, to put it simply) proportionally. Going from 2s lipo to 3s lipo will not only increase your rpm potential 50% faster, but possibly your amp draw and the resulting torque by 50%. This is assuming the motor CAN actually convert the amp it pulls into torque and doesn't max out and instead convert it to heat. And yes so in case you missed it, more voltage can lead to hotter temperatures since the resulting increase in amps (depending on motor efficiency and also on your gearing, and your setup) can cause more system heat.

The more amps your system draws, also the faster you drain the battery, or less runtime in other words. So.... you can either take that hit and go faster, OR you can gear down a few teeth in motor pinion size and go not as fast, but reduce amp draw, resulting in better heat, and run times, and still have a bit more power.

You don't want your motor to reach over 170-180*F (200*F and most motor magnets lose strength permanently!) and same for ESC, and for lipo you DON'T want to let the battery touch 140*F or you risk puffing and ruining the pack. 120*F is plenty warm for them.

These are mostly general rules of thumb type things. Excuse me for rambling if I'm repeating things you already know.

Hope this demystifies some things for you.

Old 05-06-2010, 05:48 AM
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Default RE: does voltage affect power in a battrey?


ORIGINAL: Slo-V Flyer

Voltage by itself determines the RPM of the motor (ignoring a load being put on it like driving your truck on the ground, etc.). Assuming you're running the VXL 3500 Kv motor, the motor will rev 3500 RPM per volt you feed it. So at 11.1 volts, that's 11.1 x 3500 = 38,850 RPM.

So to answer your basic question as also stated above, battery voltage determines speed (or efficiency by letting the motor rev more but gearing down), and the mah (milli amp hours) rating is the capacity of the battery (and usually it also determines how well the battery can handle higher amp draw).

Going up in voltage however, also will under normal circumstances cause the motor to pull more amps (AMP is what the motor creates torque with, to put it simply) proportionally. Going from 2s lipo to 3s lipo will not only increase your rpm potential 50% faster, but possibly your amp draw and the resulting torque by 50%. This is assuming the motor CAN actually convert the amp it pulls into torque and doesn't max out and instead convert it to heat. And yes so in case you missed it, more voltage can lead to hotter temperatures since the resulting increase in amps (depending on motor efficiency and also on your gearing, and your setup) can cause more system heat.

The more amps your system draws, also the faster you drain the battery, or less runtime in other words. So.... you can either take that hit and go faster, OR you can gear down a few teeth in motor pinion size and go not as fast, but reduce amp draw, resulting in better heat, and run times, and still have a bit more power.

You don't want your motor to reach over 170-180*F (200*F and most motor magnets lose strength permanently!) and same for ESC, and for lipo you DON'T want to let the battery touch 140*F or you risk puffing and ruining the pack. 120*F is plenty warm for them.

These are mostly general rules of thumb type things. Excuse me for rambling if I'm repeating things you already know.

Hope this demystifies some things for you.

a higher voltage with the same kv will result in more current draw, yes. But the max power of a 3s is 50%higher, so you can run less kv and get the same results w more battery life
Old 05-06-2010, 08:40 AM
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Default RE: does voltage affect power in a battrey?


ORIGINAL: rpg711


a higher voltage with the same kv will result in more current draw, yes. But the max power of a 3s is 50%higher, so you can run less kv and get the same results w more battery life

Yup I was talking about if you were using the same motor/gearing. But yes, stepping down to a lower kv motor or gearing down will both help as you suggested.

Something my geeky post doesn't mention also is that not only are you getting 50% more raw rpm, but 50% more torque due to the (potential) more amp draw, so in essence you're more than doubling the effective power output. Here's an example:

Say on 2s your motor only draws 50 amps upon normal acceleration (as a rough guestimate of the average amp draw)...

2s lipo - 7.4v x 50 amps = 370 watts

Moving up to 3s or 11.1v and keeping the same motor and gearing (for simplicity's sake) your motor may draw 75 amps:

3s lipo - 11.1v x 75 amps = 832.5 watts

You're potentially able to get over 2.25 x the power just by going up 50% in battery voltage.

Double the original voltage by using 4s lipo 14.8v:

14.8v x 100 amps = 1480 watts. Just going from 2s to 4s lipo (double voltage) the motor is making not double, but 4 times the power!

This is assuming your motor can actually handle the amp draw and is geared to be able to run correctly without overheating.

Cool eh?
Old 05-06-2010, 09:08 AM
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Default RE: does voltage affect power in a battrey?

A lot of these questions come down to a scientific perspective or scientific thinking. You can 'black box' the whole motor/esc and just say it's a system that converts electrical energy to mechanical energy with some efficiency, the lost energy is converted to heat, vibration, etc. All the years of motor development have only changed two things, that efficiency and the physical maximums associated with the motor.

We also know power can be electrical (Volts * Amps) or mechanical (RPM * torque) in this case. The motor/esc system is essentially converting from one to the other.

Once you think of things from this perspective, finding the answer to a lot of these questions is trivial.

In the past this model may have been more questionable than today, since you might have actually been operating a motor close to or at its physical maximums. Today's motors, especially running off-road, that's pretty rare.
Old 05-06-2010, 04:34 PM
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Default RE: does voltage affect power in a battrey?

wow guys thanks this is all the info i need. and idont think your post was geeky flyer, but very informative  so you say that if the motor could take the amount of amp draw, so would the vxl motor be able to?? sorry for all the questions just a noob trying to figure it out ha
Old 05-06-2010, 04:41 PM
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Default RE: does voltage affect power in a battrey?


ORIGINAL: laextreme

wow guys thanks this is all the info i need. and idont think your post was geeky flyer, but very informative so you say that if the motor could take the amount of amp draw, so would the vxl motor be able to?? sorry for all the questions just a noob trying to figure it out ha
Good, because I tend to ramble off a little extra info, just in case anyone likes that stuff too.

I don't understand your question though. "If the motor can handle the amp draw the vxl motor be able to?"

Do you mean "if the motor can handle the amp draw, does the VXL ESC have to be able to handle it too?"

The answer to that would be yes, everything in the system has to work together. If the motor pulls a certain amount of amps (depending on the load put on it) the battery needs to supply that (so it has to be up to par) and the ESC is in between them, and has to be also able to handle the amp draw.

To put it simply, it is always better and safer to have a battery and ESC be able to handle MORE amps than what the motor's maximum rated amp draw is. This way, the bottle neck is not your ESC or battery, but whatever the motor can do, if you look at it that way.
Old 05-06-2010, 04:54 PM
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Default RE: does voltage affect power in a battrey?

sorry for the confusion i get what your saying please disregard that sadly asked question. and i see what your saying about the bottle neck theaory of how you will max out your motor but the esc and battery would be fine. thanks for all the info really appreciate. now i just need to rob a bank to pay for all this
Old 05-06-2010, 06:39 PM
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Default RE: does voltage affect power in a battrey?

ORIGINAL: laextreme
sorry for the confusion i get what your saying please disregard that sadly asked question. and i see what your saying about the bottle neck theaory of how you will max out your motor but the esc and battery would be fine. thanks for all the info really appreciate. now i just need to rob a bank to pay for all this
Just remember that amps are always 'pulled' through a system by the load, never 'pushed' by the battery.

And batteries and ESCs can't effectively restrict or bottleneck the flow of current. At least not the ones we use for RC.

These devices typically have a rating, one that cooresponds to a 'safe limit' or one that you need to try to stay under in your designs. For instance, you mentioned the VXL ESC and a lot of people have reported problems when attempting to run a 3.5 motor (10000+ KV) off it with 2s. While it's very rare to have a problem when run with the supplied (blue traxxas) motor.

The point is that ESC's, LiPo batteries, etc. are perfectly able to pass far more current than the safely rated limits. For instance a LiPo that can safely supply 80A can be made to supply far more, 200A, 300A, probably even 400A. Same with an ESC. But in doing so, they end up quickly overheating and/or destroying themselves.

The general ballpark limits you want to aim for are 25-35A for 1/18th scale and 100A for 1/10th scale. For 1/8th scale and up, 100A or more, at a higher voltage (no less than 4s).
Old 05-06-2010, 07:16 PM
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Default RE: does voltage affect power in a battrey?

Remember that the more current flows through a wire, the hotter it gets, and the hotter it gets, the higher its resistance is. This is why power lines are always running something like 50000v. Higher voltage on a lower kV motor, or on lower gearing(effectively lowers the amount of power the motor needs to draw to reach its peak torque/power) results in a cooler and more efficient motor. You didn't give enough information so we can't really tell you anything. With 3s with any buggy/truggy/whatever you will need to put heavy heavy oil in your center diff, or you will kill your diff by doing wheelies(front and back on ground, functions as normal diff, once the front end lifts off, it will begin "diff out" and reducing power to the rear... that kills your gears in no time), especially with the instant power of a brushless, if your battery can provide it, and your esc can support it, the motor will draw the max current that it requires to generate its max rpm. If there's something that has LESS rated amp capacity, that means the internals -or- externals(wires, pathing, used hardware) cannot support the current and will start heating up, which is unfavorable because the more hot it gets, the more resistance it has, and the more resistance it has, the hotter it gets, starting a constant heatup until power is reduced. It's my belief that anyone looking at electrics should read on ohm's law, and just general electrical theory.
Old 05-06-2010, 07:45 PM
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Default RE: does voltage affect power in a battrey?

ORIGINAL: rpg711
Remember that the more current flows through a wire, the hotter it gets, and the hotter it gets, the higher its resistance is. This is why power lines are always running something like 50000v. Higher voltage on a lower kV motor, or on lower gearing(effectively lowers the amount of power the motor needs to draw to reach its peak torque/power) results in a cooler and more efficient motor. You didn't give enough information so we can't really tell you anything. With 3s with any buggy/truggy/whatever you will need to put heavy heavy oil in your center diff, or you will kill your diff by doing wheelies(front and back on ground, functions as normal diff, once the front end lifts off, it will begin ''diff out'' and reducing power to the rear... that kills your gears in no time), especially with the instant power of a brushless, if your battery can provide it, and your esc can support it, the motor will draw the max current that it requires to generate its max rpm. If there's something that has LESS rated amp capacity, that means the internals -or- externals(wires, pathing, used hardware) cannot support the current and will start heating up, which is unfavorable because the more hot it gets, the more resistance it has, and the more resistance it has, the hotter it gets, starting a constant heatup until power is reduced. It's my belief that anyone looking at electrics should read on ohm's law, and just general electrical theory.
In practice this doesn't happen in RC though, it's not that uncommon to hear about an ESC toasted or a battery burnt up from overcurrent/overtemperature. And in these cases the wiring is typically fine. Sometimes people are 'saved' by poor solder joints on battery packs and things like that the solder joint is bad, it heats up, solder turns to liquid and the connection is broken. But that's about it.

Except in cases where the wiring leaves much to be desired, the ESC or battery is going to fail first. The potential for positive or negative feedback that you describe isn't typical at all.

"if your battery can provide it, and your esc can support it, the motor will draw the max current that it requires to generate its max rpm"
The motor draws current based on throttle input. Primarily when you accelerate. Hard acceleration is where your current is highest, just cruising along at max speed (max RPM) doesn't draw a lot of current unless you are on heavy terrain. This is just like an ordinary street car. Softer acceleration up to a constant speed is much more efficient than harder acceleration up to that same speed.

It's important to know when to apply theory and when to draw from experience. You apply theory when it's useful to simplify, model, or demonstrate experienced results. The statement "starting a constant heatup until power is reduced" can be very confusing. The increasing resistance (component, wiring, etc.) just isn't going to be enough of a factor to create any kind of negative feedback. I recognize that increasing MOSFET resistance may create a little bit of positive feedback here, since the ESC typically has much less resistance than the motor or anything else in the chain (so as that MOSFET resistance increases, current only drops slightly, power disipation increases -> positive feedback). The statement is only true in retrospect, the device heats up, it fails, and only then is power reduced, typically to zero (since a failure typically opens the circuit). There can be cases of partial failure, but they are rare. If you are lucky, you may end up simply thermaling the ESC (temperature sensor inside the ESC shuts it down or restricts usage to save itself). But these can never be counted on...
Old 05-06-2010, 08:29 PM
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Default RE: does voltage affect power in a battrey?

ORIGINAL: Access

ORIGINAL: rpg711
Remember that the more current flows through a wire, the hotter it gets, and the hotter it gets, the higher its resistance is. This is why power lines are always running something like 50000v. Higher voltage on a lower kV motor, or on lower gearing(effectively lowers the amount of power the motor needs to draw to reach its peak torque/power) results in a cooler and more efficient motor. You didn't give enough information so we can't really tell you anything. With 3s with any buggy/truggy/whatever you will need to put heavy heavy oil in your center diff, or you will kill your diff by doing wheelies(front and back on ground, functions as normal diff, once the front end lifts off, it will begin ''diff out'' and reducing power to the rear... that kills your gears in no time), especially with the instant power of a brushless, if your battery can provide it, and your esc can support it, the motor will draw the max current that it requires to generate its max rpm. If there's something that has LESS rated amp capacity, that means the internals -or- externals(wires, pathing, used hardware) cannot support the current and will start heating up, which is unfavorable because the more hot it gets, the more resistance it has, and the more resistance it has, the hotter it gets, starting a constant heatup until power is reduced. It's my belief that anyone looking at electrics should read on ohm's law, and just general electrical theory.
In practice this doesn't happen in RC though, it's not that uncommon to hear about an ESC toasted or a battery burnt up from overcurrent/overtemperature. And in these cases the wiring is typically fine. Sometimes people are 'saved' by poor solder joints on battery packs and things like that the solder joint is bad, it heats up, solder turns to liquid and the connection is broken. But that's about it.

Except in cases where the wiring leaves much to be desired, the ESC or battery is going to fail first. The potential for positive or negative feedback that you describe isn't typical at all.

''if your battery can provide it, and your esc can support it, the motor will draw the max current that it requires to generate its max rpm''
The motor draws current based on throttle input. Primarily when you accelerate. Hard acceleration is where your current is highest, just cruising along at max speed (max RPM) doesn't draw a lot of current unless you are on heavy terrain. This is just like an ordinary street car. Softer acceleration up to a constant speed is much more efficient than harder acceleration up to that same speed.

It's important to know when to apply theory and when to draw from experience. You apply theory when it's useful to simplify, model, or demonstrate experienced results. The statement ''starting a constant heatup until power is reduced'' can be very confusing. The increasing resistance (component, wiring, etc.) just isn't going to be enough of a factor to create any kind of negative feedback. I recognize that increasing MOSFET resistance may create a little bit of positive feedback here, since the ESC typically has much less resistance than the motor or anything else in the chain (so as that MOSFET resistance increases, current only drops slightly, power disipation increases -> positive feedback). The statement is only true in retrospect, the device heats up, it fails, and only then is power reduced, typically to zero (since a failure typically opens the circuit). There can be cases of partial failure, but they are rare. If you are lucky, you may end up simply thermaling the ESC (temperature sensor inside the ESC shuts it down or restricts usage to save itself). But these can never be counted on...
Mmhm, true true and true, but theory is useful to understand why some things happen that are unexplainable with just experience, in ideal circumstances at full throttle your motor will draw as much current as it requires to generate enough torque to move at its max capability. I know that the wiring statement is just trivial at best, but long term use with wires that are too low gauge to handle the current draw will just cause reduced performance. I took it by OP's posts that he had absolutely no idea how electronics worked, and so I took it from ground zero, wires and up. You try using servo 24awg wires to power a 100ah motor
But you're right, all is irrelevant to the question, which is does voltage effect power, and I think we've already answered that with textbook quality answers
I'm pretty sure 3s would rape the center diffs if they started to function as slippers, would it not?
Old 05-06-2010, 08:46 PM
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Default RE: does voltage affect power in a battrey?

ORIGINAL: rpg711
max capability. I know that the wiring statement is just trivial at best, but long term use with wires that are too low gauge to handle the current draw will just cause reduced performance. I took it by OP's posts that he had absolutely no idea how electronics worked, and so I took it from ground zero, wires and up. You try using servo 24awg wires to power a 100ah motor
What I've seen with insufficient wire gauge is actually scarrier than that, the insulation melts off the wires and once that happens, they are likely to short out.

And most people have no idea about electronics, other than perhaps how to operate some consumer devices.
Old 05-06-2010, 09:13 PM
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Default RE: does voltage affect power in a battrey?

I like the recommendation of reading up on Ohm' law for electric enthusiasts. It takes up all of 30 minutes to google and read up different examples and articles to have a basic understanding, at least for r/c purposes.

I didn't know WHY or WHAT caused heat in a motor/combo. Add to that, why more voltage caused more heat (without regearing etc etc). Then I found out about the magical Ohm's law and that amp draw increases with voltage, given the same resistance and load, and voila! Now I know why 2s is fine but 3s or 4s cooks motors without proper setup. And now I spread the gospel.

For that matter it took me a little while to understand that amp creates torque, not voltage itself, or voltage drop under load was common, etc. etc. Then again I don't think the general public is as geeky as me to sit and google all sorts of neat stuff.
Old 05-06-2010, 09:14 PM
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Default RE: does voltage affect power in a battrey?

imnot completely cluess, i have only basic knowledge like knowing running too smaal gauge a wire will decrease performance, and i agree you all have answered perfectly in a textbook manner. thanks for the info and i do plan on upgrading my tranny to atleast stand a small chance againdt a bl system
Old 05-07-2010, 03:33 AM
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Default RE: does voltage affect power in a battrey?

ORIGINAL: Slo-V Flyer
I didn't know WHY or WHAT caused heat in a motor/combo. Add to that, why more voltage caused more heat (without regearing etc etc). Then I found out about the magical Ohm's law and that amp draw increases with voltage, given the same resistance and load, and voila! Now I know why 2s is fine but 3s or 4s cooks motors without proper setup. And now I spread the gospel.
Just remember this is a dynamic situation. You have the ESC varying the duty-cycle (ohm's law is still usable here), and then you have a lot of components ohm's law can't model, induction, back-EMF, etc. Ohm's law and its derivatives work in concept at zero RPM, but once you get that rotor turning, Ohm's law starts to break down.
Old 05-07-2010, 08:35 AM
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Default RE: does voltage affect power in a battrey?


ORIGINAL: Access

Just remember this is a dynamic situation. You have the ESC varying the duty-cycle (ohm's law is still usable here), and then you have a lot of components ohm's law can't model, induction, back-EMF, etc. Ohm's law and its derivatives work in concept at zero RPM, but once you get that rotor turning, Ohm's law starts to break down.
That's probably right, my mind still can't wrap around induction or back emf even though I've read a few articles online already, but I'm getting there.
Old 05-07-2010, 09:25 AM
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Default RE: does voltage affect power in a battrey?

http://en.wikipedia.org/wiki/Faraday...w_of_induction
For starters, Certainly more complicated than a simple E = IR

And rather than wikipedia, I recommend this site
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
for explaining modern physics concepts in plain english.

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