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sebuah bola bermassa m dan berjari-jari R terletak pada lantai licin sebuah mobil memiliki percepatan horizontal a1 ke kanan. Berapakah percepatan bola a2? anggap bola menggelinding tanpa tergelincir.diketahi percepatan grafitasi adalah g.

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Sebuah mesin carnot dengan reservoir suhu tinggi 640K menyerap kalor sebesar 1250 kaloridengan efisiensi 20%. Berapakah kalor yang di serap  pada reservoir suhu tinggi bila efisiensi mesin naik menjadi 50%? Mohon di jawab dengan langkah2nya. Terima kasih
 

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Penulis Topik: Vitamin C dan plastik  (Dibaca 3040 kali)

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Vitamin C dan plastik
« pada: Maret 04, 2007, 05:03:35 PM »
http://www.nsf.gov/news/news_summ.jsp?cntn_id=108123&org=NSF&from=news

Vitamin C and Water : Healthy for Plastics

Two new laboratory breakthroughs are poised to dramatically improve how plastics are made by assembling molecular chains more quickly and with less waste. Using such environmentally friendly substances as vitamin C or pure water, the two approaches present attractive alternatives to the common plastic manufacturing technique called free radical polymerization (FRP).

"The methods both present novel and complementary ways to dramatically improve efficiency, product control, and cost for the polymer industry," said Andy Lovinger, the National Science Foundation program director who oversees funds for the two projects. "Each of these approaches could have a very significant impact on polymer manufacturing."

Plastics are polymers, long, potentially complex, molecule chains crafted from an array of smaller chemical units.  Using FRP, chemical engineers can create the right plastic for a range of applications, such as a specific trim for a car door or soft foam for a pillow.

For some plastics, the building-block molecules do not easily link together.  To surmount this problem, researchers from Carnegie Mellon University in Pittsburgh, Pa., devised a process called atom transfer radical polymerization (ATRP), which provides creative ways to coax the chemical subunits into chains.  However, this method comes with certain costs, such as the need for a copper catalyst that can become unwanted waste.

Now, the Carnegie Mellon researchers have discovered that adding vitamin C, glucose, or other electron-absorbing agents to the ATRP process can reduce the amount of copper catalyst by a factor of 1000. Because the catalyst often needs to be removed from the end products, less copper means far less waste and drastically reduced removal costs. Mass manufacturing could become more affordable for a range of items such as advanced sensors, drug delivery systems, paint coatings, and video displays.

At the University of Pennsylvania (UPenn), researchers are using a different approach to improve FRP. Called single electron transfer-living radical polymerization, the new method relies upon relatively low-energy reactions, uses elemental copper (copper metal, as opposed to copper in a chemical solution) as a catalyst to limit byproducts and allows manufacturers to use one of the most environmentally friendly solvents in the arsenal, water.  The entirely new method of polymerization builds upon existing mechanisms to craft large molecules very quickly.



 

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