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sleepless nights

imagine a life so busy that two hours of sleep turns into a dream …

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“You have said more than you realize”, said Don Quijote, “for there are some,
who exhaust themselves in learning and investigating things which, once known and
verified, add not one jot to our understanding or our memory”. M. de Cervantes Saavedra, 1615

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Which way I ought to go?

“Would you tell me, please, which way I ought to go from here”, said Alice.

“That depends a good deal on where you want to get to”, said the Cat.

“I don’t much care where”,  said Alice.

“Then it doesn’t matter which way you go”, said the Cat. Lewis Carroll 1865

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How strong is your password?

In the online life, security is always a concern. The following link will show the amount of time a PC needs to find your password. The password cracking programs use sort of random generators but are clever enough to check for complete words. So never use a word which exists in the dictionary. As usual, the following factors are important for a safe password: 1) combination of lower and upper case, 2) better to be 16 character long, 3) use numbers and special characters.

http://howsecureismypassword.net/

Examples:

cghftkd         32 sec   (8 characters, only lower case)

cghf6tkd       3 hour   (9 char, lower case and number)

cghf+6tkd    2days     (10 char, lower case, number, and special character)

cGhf+6tkd_qrz     5 Million years    (13 char, lower/uppaer, number, special char)

cGhf+6&Vtkd_qPrz   193 Billion years  (16 char, upper/lower, number, special char)

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Solar battery chargers

Recently, there are quite a lot of solar battery chargers. Such instruments like the one in the following image are able, in theory, to recharge mobile devices, ipods, etc when they are charged, and charge themselves using the sun light.

In this particular example, the company provides the following specifications for the instrument: it has a charge capacity of 3.5 Ah (Ampere  hour) at a potential difference of 3.7 Volts. That means it contains 12.95 Watt h energy when it is fully charged. Just for comparison, a small auto battery has like 40 A.h at a potential difference of 12 V.  That means the energy stored in the device is like 2% of the energy stored in an auto battery, so it is quite a bit actually.

Needless to say, you can charge it via AC adapter or USB cable of your laptop and use it to charge your mobile phone when required. The life cycle according to the webpage is 500. Indeed, the typical Lithium-ion polymer batteries have a life cycle of 1000 or more. Perhaps due to irregular solar charging rather than the standard net charging, the life cycle will be shorter as expressed in the webpage.

Can it really charge itself via solar radiation? I try to simply evaluate how much solar energy it can absorb, according to the data given in the webpage. The solar constant, the energy that the unit area on the top of the earth atmosphere receives in a second is about S = 1400 Watt per square meter. On tropical latitudes, the received energy on the ground is larger than 1000 W/m2. On middle latitudes like central Europe, and in a summer day, S = 800 W/m2. In winter, it is usually about 500 to 600 W/m2 in a sunny day. When it is overcast and very dark, it drops to values comparable to 1W/m2.

Now let us calculate how long it takes to collect 12.95 Wh energy from the received solar radiation on an average day. The collecting area, according to webpage, is 11.5 x 6.0 cm2 , and the efficiency is 17%. If I take S = 300 W/m2 for a relatively bright day, then the rate of collecting energy is

Area  x  Solar constant  x  efficiency = 69 cm2 x 300 W/m2 x 0.17 = 0.35 Watt.

Therefore to collect 12.95 Wh, i.e. to recharge the battery via sun light,  one has to keep the instrument in sunshine for 37 hours. In tropical regions, this time can be a factor 2 or 3 shorter.

This simple calculation shows that if you have such an instrument, you should be patient to get it charged using its own solar panel. In practice, the charging time can be longer because of the cloudy sky, and a decline of the efficiency with aging. 

Do you save money if you buy it?

Perhaps not, specially if you live in high latitudes. The instrument can, in theory, work for 500 cycles of 12.95 Wh. That amounts to 6.5 kWh during its lifetime. The electricity price in expensive cases is like half a dollar per kWh. That means you collect in total like 3.2 US dollar while the instrument costs like 40+ dollars. This particular device is more a (big) spare battery than a solar charger.

There are better options. Consider a similar instrument with an area of 2-3 times larger. That efficiently reduce the recharging time. In addition, I guess the systems with rechargeable AA batteries have the ability to live longer: you can exchange those batteries when they are dead. The solar panels usually work much longer (like 10+ years).

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Recently, I tried to update an old Ubuntu 8.04 LTS system to 10.04 LTS.  Well, as easy as it could be, I clicked a few times, and it downloaded and installed a lot softwares, like 1500 or so. Then, it was time for a reboot, a one that turned into a very painful reboot !

After reboot, the boot menu was still the old one, exactly the old one. I pressed the normal boot. I knew that that kernel was perhaps removed. I was unfortunately right ! It asked for root password but could not even read key board !!! After another reboot, I tried recovery mode. This time, I had a terminal ! But soon I realized that the filesystem was mounted as read-only. That means I cannot change the menu.lst in the grub folder. I could see that the new kernel is there, I knew that I have to tell the system to boot via this kernel, but how?

After a couple of hours of nervous googling, I found the following trick: In the grub menu during boot, you can pres “e“, to edit the menu temporarily.  When you are done editing (press ENTER to finish editing), you can press “b” to boot with your altered settings. If you manage to boot into linux, you then have to alter the configuration permanently.  I changed the menu and put the correct name and rebooted. Not yet ! Again I edited the grub menu and this time put

modeset=1
at the end of the kernel line. After reboot, I enterred a filesystem check by force. After that, everything worked fine, exactly everything.

It had happened to me that I end up an upgrade with no graphics but having filesystem as read-only was really bad.

Never forget a backup before system upgrade !

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ABS or experience?

You might ask yourself is ABS breaking system more efficient than a normal system, or is it doing a better job comparing to an experienced driver.  The answer is NO.  As you see in the following plot, an experienced driver without ABS stops the auto faster than an amateur driver with ABS.

It should be expected since ABS avoids locking by reducing the break power.

Reference: Fundamentals of Physics

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