a 10-car subway train in New York's system requires a jolt of three to four megawatts of power for 30 seconds to get up to cruising speed — that's enough energy to power 1,300 average U.S. homes."
For how long?
For 30 seconds, more or less, if a home is ~ 2-3 kW.
This isn't the place!
You could try http://krugman.blogs.nytimes.com/
Not a flame war for me, anyway. But it does seem like a lot of the pro-Mac anti-Windows comments are aimed at Win XP or even Win 98. I'm a Linux guy, but I do see Win 7 as pretty respectable in terms of GUI quality, stability, security, etc. A resource hog, sure, but that's what we have cheap RAM and CPU cycles for.
Mac OS X and Mac hardware are fine, too. For me, they're worth a premium, but not 2X compared to Linux-friendly laptops.
I'm retired and willing to spend time doing some system stuff on Linux. If your time is at a premium, that would tilt the scales. My grad student daughter just upgraded her Mac, so there you are. (If was a grad student again, I'd be wary of spending time on
A few more simple rules: Don't buy the cheapest, and don't buy the most expensive. The best value is usually somewhere in the middle. If I buy a laptop, it's generally in the $500-700 range, and it might be last year's model on close-out. Oh, and don't buy Apple... unless "cool" is worth a ~100% tax to you.
You have to read the instructions carefully. The current wants to flow in the proper direction. Don't hook it up backwards, or the warranty is void! I wonder how many
You might also want to look into the pre-charged dielectric cable. It needs a (premium - of course) DC supply to be sure the dielectric is operating in the linear range.
These things really do work. Just ask the users who spend $K on their system wiring!
There are many ways to look at a university. It's about research, it's about federal funding, it's about raising money from alumni, it's about patent licensing and athletics, it's about recruiting faculty, promoting faculty, about running a big physical plant, internet pipes, etc. etc. Oh, and there are students, too. Teaching is just one of many things that happen there, not the most visible - even at places that claim to emphasize the student experience. Students do get to pay up to around $200K for the privilege of becoming alumni and then being asked to contribute cash. But a big research university actually loses money on every student. They'd be better off financially if they stopped admitting.
Universities, most of them, are non-profit corporations. You're right they aren't charities in the moral sense, but the US tax laws treat them as if they were. Pity the legislator who suggests repealing the their charitable tax deduction.
Wrote a few seminal comments, and now I have these shiny new mod points.
What I've also wondered is how big of an antenna would we need to detect a communication from a near star, say 50ly. And how much power would it take to send a message that far? If we can't even see planets, how can SETI expect to receive a transmission from one? I've asked some astronomy majors about this and received only blank stares. Do they teach this kind of thing in astronomy? What are the calculations?
It's a fair question. Fortunately, an intentional transmitter can be much brighter than the star or planet -- in a narrow bandwidth and pointing straight at you. (And stars aren't very bright at microwave frequencies.) There's no problem communicating to nearby space if you know each other's frequency and direction. You can concentrate megawatts of power into
Interferometry (increasing the spatial resolution of the receiving antenna) actually doesn't help you much, except to discriminate against the diffuse galactic background. You need all the collecting area you can afford, but that's not the same thing.
...dedicated output power of 48KW per hour.
48 kW period. (Physics: A Watt is a unit of power. A Watt-hour is a unit of energy. A Watt per hour is a rate of increase of power.)
You won't be getting a "quick charge" at home without an expensive service upgrade. Overnight probably means up to 12 hours for a charge. That gets you about 288 kW-hr of energy if you draw 100 A on a 240 V circuit. In very rough terms, you can think of a kilowatt as a horsepower (1.3 actually). So you could run your high performance car at full power for 1 hr on an overnight charge. Your typical commute needs a lot less energy, but YMMV.
There's another reason that switching batteries is good. If a 62 kW supply is required to charge a battery in 30 minutes, you would need 360 kW+ to charge it in 5 minutes. That's a phenomenal power level. If your charging efficiency is 90%, that means you will be dissipating 36 kW in your car as heat while charging. That's pretty close to explosive.
The service station and the power utility would have an interest in leveling their load, so charging an inventory of batteries relatively slowly is a good thing. Even so, each recharging station might need a flywheel energy storage unit (or comparable) to even their load on the utility.
Not really. It depends on the current drawn through the wire. For power P (constant for the computer, more or less) required at a voltage V, you need I=P/V amps. You're not going to distribute 3 V or 5 V, which is what your ICs want, I hope! You could distribute 120 V DC with the same size wiring you use for the usual AC connection. You could send around 1,200 V (DC or AC) and use 1/100 the copper. (Power lost to heating goes as I**2.) The high voltage limit is set by safety and cost of DC-DC converters.
The system is arguably less secure. You have 50 systems depending on one AC-DC converter (or a small number of them), and that introduces a single point of failure.
When our data center installed a nice shiny big UPS system to help us solve some problems about unreliable AC supply, I (correctly) predicted the next data center outage would be from the UPS.
Another crack at this:
15 Mb/s is 162 GB/day or about 5000 GB/month
So at 250 GB cap is 5% of theoretical maximum transfer.
Or you could say the (inverse) cap is 4 months / TB - the time it would take to fill up my hard drive.
Congratulations! You are the one-millionth user to log into our system. If there's anything special we can do for you, anything at all, don't hesitate to ask!
