Follow Slashdot stories on Twitter


Forgot your password?

Comment: Re:Premature (Score 1) 481

by Mr Z (#49794997) Attached to: How Tesla Batteries Will Force Home Wiring To Go Low Voltage

I used 5v as an example as the linked article spoke specifically of running 5V and 12V everywhere. I agree that you really want a higher voltage for distribution. 48V goes a long way, although it still requires quite a lot more copper than 110V or 240V for the same power carrying capacity. (About 5x if I did my math correctly.)

Now, if those in-wall adaptors could store some charge locally (small capacitor bank), and you didn't have to wire for peak current, only sustained current, maybe you could get away with smaller wiring that way. I'm skeptical.

Comment: Re:Premature (Score 1) 481

by Mr Z (#49793659) Attached to: How Tesla Batteries Will Force Home Wiring To Go Low Voltage

I see what you mean. Let's put some numbers to that for everyone's benefit.

According to the table I linked previously, the OOOO gauge wire is 0.16072 ohms per 1000m. So, for a 20m run, that's about 0.00321 ohms. The voltage drop incurred by 330A across that resistance would be just over 1.06 volts.

For a 5V run, that's pretty significant, really. And you'd be dissipating over 350W in that wire alone. Yow! At 330A, you'd be burning 20% of your power just in that cable if you used OOOO gauge cabling.

Now the same numbers for 10 gauge wire, 15A, 110V, 20m. That's 3.276392 ohms per 1000m, or 0.0655 ohms for 20m. Voltage drop at 15A is 0.983V. Peak power dissipated in the wire is 15A * 0.983V = 14.7W. (RMS power is only ~10W.)

Comment: Re:Premature (Score 1) 481

by Mr Z (#49792429) Attached to: How Tesla Batteries Will Force Home Wiring To Go Low Voltage

I'd hate to think of just how much extra copper I'd need in my walls to distribute ample power throughout my house and not suffer resistive losses. A 15A 110V circuit would become a 330A 5V circuit. The wire that carries the 15A @ 110V circuit is about the thickness of a pencil lead (about 1/10"). The wire required to carry 330A would have conductors the size of a garden hose (about 1/2"). (Note: The table linked above only goes up to 302A with a thickness of 0.46"; 330A would be larger still.)

Comment: Re:Tesla enables Edison to win the endgame? (Score 3, Informative) 481

by Mr Z (#49792287) Attached to: How Tesla Batteries Will Force Home Wiring To Go Low Voltage

There's two main sets of losses, as I understand: Resistive losses and radiative losses. You can get into other issues, such as power factor and phase error related losses. The two biggies that hit you almost before you get started are resistive and radiative losses, though, if you just consider a single transmission line driving a resistive load.

You combat resistive losses by going up in voltage, so you can send more power with less current. Since resistive losses are proportional to the square of current, each doubling of voltage reduces your resistive losses by a factor of 4. That's why long haul transmission lines are high voltage.

Radiative losses are different. Whenever you accelerate a charged particle, you generate an electromagnetic wave. With respect to wires carrying current, that corresponds to changing the amount of current. (Current measures the rate at which electrons flow, so changing current means accelerating or decelerating electrons.) That's how radio transmitters works, for example.

In an AC system, that current is continuously changing, so those transmission lines are continuously radiating away some amount energy. But that's not all. If there are any conductors nearby, those E-M waves can induce a current in those conductors, and the resulting E-M waves from that induced current can drag on the AC line further. This mutual induction is how transformers work. But, along an AC transmission line, unwanted coupling results in transmission losses. So, an AC system has a built in, inherent source of losses in the alternating current itself.

In a DC system, with a fixed, perfectly resistive load, the current doesn't change, so there's no radiative losses. In the real world, though, the loading on the system is continually changing, so the actual current demand on the DC system will vary over time, and some energy will be radiated away. To some extent that can be filtered, but that's limited by the amount of storage you can put near the ends of the transmission.

The reason AC won out over DC in the early days is that we didn't have practical means to step DC voltages up and down. But, we had just invented the first practical transformers, and those can step AC to higher and lower voltages trivially.

HVDC is practical now since we've had 100 years to develop better technology for converting DC voltages on the grand scales required.

Comment: Re:Russian rocket motors (Score 1) 61

by Bruce Perens (#49787045) Attached to: SpaceX Cleared For US Military Launches

Russia would like for us to continue gifting them with cash for 40-year-old missle motors, it's our own government that doesn't want them any longer. For good reason. That did not cause SpaceX to enter the competitive process, they want the U.S. military as a customer. But it probably did make it go faster.

Also, ULA is flying 1960 technology, stuff that Mercury astronauts used, and only recently came up with concept drawings for something new due to competitive pressure from SpaceX. So, I am sure that folks within the Air Force wished for a better vendor but had no choice.

Comment: Context (Score 3, Informative) 61

by Bruce Perens (#49782349) Attached to: SpaceX Cleared For US Military Launches

This ends a situation in which two companies that would otherwise have been competitive bidders decided that it would cost them less to be a monopoly, and created their own cartel. Since they were a sole provider, they persuaded the government to pay them a Billion dollars a year simply so that they would retain the capability to manufacture rockets to government requirements.

Yes, there will be at least that Billion in savings and SpaceX so far seems more than competitive with the prices United Launch Alliance was charging. There will be other bidders eventually, as well.

Comment: Soverign debt (Score -1, Troll) 742

I'd like to hear what the economists here think should be done about Greece.

"Soverign debt is not like personal debt!"

That's what the economists on this very blog say, when discussing US debt. It doesn't matter how far into debt the US is, anyone can see this by comparing our debt to our GDP: the latter number is really big, while the debt is really small.

See? You can't just say getting into debt is bad, because the two are entirely different.

I'd like to hear what the Slashdot economists think should be done about Greece.

Comment: Re:32MB? (Score 1) 225

by Okian Warrior (#49760509) Attached to: Google Developing 'Brillo' OS For Internet of Things

32MB? Bah. I remember the days when you could fit a whole OS in a hundred K! And 640K was enough for anyone!

Pfft, I remember running 40 users on terminals on a machine with 16K that probably had less than 1/10,000th of the cpu power a laptop has today.


I remember putting the boot card at the front of my deck, placing it in the card reader, and pressing the "load" button on a system with 4K of core memory(*).

Who'd have thought 40 years ago, we'd all be sitting here drinking chateau de chatillon.

(*) That part's actually true. I started on an IBM-1130, predecessor to the IBM-360.

Comment: Proctored voting (Score 3, Insightful) 103

A lot of people think online voting is the next big thing, but the problem is actually very hard to do online.

To do it right requires a "proctored" setting where the person is guaranteed to be alone, and unobserved (including video recording).

If you can't guarantee that the person is alone, then they can be coerced into voting a specific way. If you can't guarantee that the person isn't observed, then the person can sell their vote.

Video recording hasn't been addressed yet, but with the current system a voter can record their vote as proof of how they voted, and so vote selling is possible. It's functionally the same as being observed, just time shifted.

Add in the requirements for recounts and verification, and physical ballots in a proctored environment is the simple solution.

I've seen mathematical solutions that make tampering statistically impossible. The system injects a large portion of non-human votes in a cryptographically secure way such that it doesn't change the actual outcome, but it's impossible for a hacker to change votes due to the statistical likelihood that he'll change one of the non-human votes and be detected.

Even with these systems, you still need a proctored environment that guarantees anonymous and unobserved voting.

Comment: Re: Apple ][ was a great product (Score 1) 74

by cpt kangarooski (#49745473) Attached to: In 1984, Jobs and Wozniak Talk About Apple's Earliest Days

Though there was a good reason for the original compact Macs to discourage users from opening them up -- there were exposed high voltage monitor electronics in there which could give you a hell of a zap of not properly discharged.

The later all in one Macs of the 90s were better in that regard. Their user suitable parts (motherboard, drives) all were easy to get at, but the monitors and power supplies were fully enclosed.

Comment: My response to Elon Musk (Score 1) 494

by Okian Warrior (#49740555) Attached to: The Brainteaser Elon Musk Asks New SpaceX Engineers

And on the subject of interviewing companies, here's my response to Elon Musk:

The North Pole.

You lay a rifle on the surface of a [perfectly] spherical planet with no atmosphere. Firing the rifle, due to the curvature of the planet the bullet goes some distance and then falls to the ground. As you increase the muzzle velocity of the bullet, the point if impact gets further and further from the rifle.

If the planet has an acceleration of 10m/(s^2), what velocity must the bullet have to go around the planet and hit the gun in the stock?

(NB: This is a trick question, but Elon Musk is an actual rocket scientist.)

Comment: And for the record (Score 1) 494

by Okian Warrior (#49740393) Attached to: The Brainteaser Elon Musk Asks New SpaceX Engineers

Oh, and for the record, the puzzle that Elon Musk asks is:

1) Older than dirt
2) The answer is common knowledge (hence, not a good puzzle to ask)
3) Has more than one correct answer
4) Is being asked wrong.

The actual full text of the puzzle should read something like: "A hunter walks [South... West... North... ends up in the same spot] and sees a bear. What color was the bear? (This version has only one answer.)

Here's one that *you* can ask during an interview.

You need to order weights for a 2-pan balance to weigh objects. The objects all weigh integral ounces (ie - no fractions), and the weights are all integral ounces.

What is the minimum *number of weights* you can use that lets you weigh anything up to 100 ounces?

(NB: the answer isn't 7.)

Comment: I hate puzzles in interviews (Score 1) 494

by Okian Warrior (#49740303) Attached to: The Brainteaser Elon Musk Asks New SpaceX Engineers

I really, *really* dislike hearing brain teasers in an interview.

Not because I don't like puzzles (I do), not because it's not a good way to judge the candidate (it is, in a sense), but because it shows up the deficiencies of the interviewer and the company.

Most of the time, the interviewer isn't into puzzles. They just looked something up on the internet, got a list of "here's a puzzle to ask the candidate", and mindlessly ask the question(*).

And when this happens, I answer the puzzle and then ask the interviewer my own puzzle, and see how they react.

Invariably, the answer is "I don't know. What's the answer?" within 3 seconds.

I don't want to work for someone like that, I don't want to work for a *company* that would hire someone like that, life's too short to spend time working amid thinkless drones.

A really bad company is when the VP or someone sticks his head in the door with a "hey, just wanted to see how it's going. Can you answer this question for me?" thing. I keep a chinese block puzzle in my pocket (that I invented) for this exact situation: I write down his answer on the whiteboard, hand him the puzzle, and say "if you can't disassemble this and reassemble it before the day is over, I don't want to work here".

Polite and reasonable interviews don't get this level of response, but turnabout is fair play. Ask me about my experience, ask me to solve a typical problem from the job description, get a feel for how well I work with others... these are reasonable.

But ask me why sewer caps are round, and you'll have to prove why you're company is good enough for me to work there. While you're interviewing me, I'm also interviewing *you*.

If everyone was more aggressively responsive to these types of games, companies wouldn't play them.

(*) Once, just once, I got into a real discussion of puzzles with the interviewer, I've got no problem with that. So long as it's not mindless bingo-card checkmarking, it's OK.

Comment: Re:Energy underlies all economic activity (Score 1) 256

So, in a world where the best EROEI available is 20, only 4% of all that societies efforts need to be devoted to obtaining energy. In a world where the best EROEI is 5, 20% of all the work is devoted to getting the energy to power civilization. At EROEI of 2, fully half of all our efforts as a civilization are devoted to energy extraction/production and everything else (agriculture, industry, medicine, art) has to fit into the remaining half of our time and resources.

So as EROEI drops, sometime before you hit 1, you're left with a nation of nothing but farmers and people working in solar panel factories, and any further decrease in EROEI means choosing between food on the table and power when you hit a light switch. At that point (if not before) civilization collapses.

That's an insightful answer, and is probably what people are talking about when they describe EROEI issues.

But there's a logical flaw in that argument, which is that it assumes that the energy needs of civilization must exactly fit the leftover energy. It does not take into account the magnitudes involved, it does not count leftover "unused" energy, and it compares a linear system with an exponential.

Take a concrete example from your numbers. Suppose EROEI is 2 as described. One half of our energy goes into making more energy, one half of the remainder (1/4 of the total) lets us live comfortably, and the remainder can be invested in even more energy production.

Or to put it another way, suppose a solar panel will produce the energy equivalent of 4x it's energy investment over 20 years (which is about right with today's technology). This is an exponential rise in energy output which triples every 20 years! At some point in the future, the solar panels produce enough energy to replace themselves as they get worn out, and have enough leftover energy to power our civilization.

Or a counter example, on a distant planet suppose EROEI for fossil fuels is 100 but the maximum flow is limited by geology. How much civilization can there be when EROEI is high, but the total energy is only enough to power [the equivalent of] a small U.S. state?

So far as I can tell, no one has addressed this logical flaw in the EROEI argument.

From a mathematical perspective, EROEI appears to makes no sense.

The trouble with doing something right the first time is that nobody appreciates how difficult it was.