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Moment of truth
The fastest man-made object that ever leave the Earth is the Pinoeer 10 spacecraft, launched in 1972, and attained a speed of more than 32000 mph.

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dalton Chem.
Abbreviated as Da. See atomic mass unit.

Dalton's law of partial pressure Chem.
A gas law which states that the total pressure exerted by a mixture of gas is the sum of partial pressure of each constituent gas. The partial pressure being that each gas would exert if it alone occupied the whole volume. The law is valid only if the gases do not react chemically.

Daniell cell Chem.
A wet cell consists of a zinc electrode (negative) in contact with zinc sulfate solution and a copper electrode (positive) in contact with copper sulfate solution. It has a standard cell potential of +1.08V. The working of the cell is due to the difference of electrochemical reactivity between the two metals. Zinc is more electropositive and thus tend to dissociates to form cations, Zn2+, leaving electrons at the electrode, while copper cations (Cu2+) tend to accept electrons to form metallic copper, leaving electrons in solution. Hence, when a circuit is completed, electron flows from Zn electrode to Cu electrode.

The cell was developed in 1836 by John Frederic Daniell and was regarded as the first reliable battery that can supply continuous steady current for a long period of time.

dark energy Astron.
Mysterious, exotic form of repulsive energy, sometimes refer to 'antigravity force'. It is thought to exert influences across large distance and is responsible for the expansion of the Universe, at an increasing rate. It is also believed to made up of 73% energy-mass of the Universe. The existence of such force is not yet confirmed, as scientists do not know what it is and where to look for. Even as early as 1917, Albert Einstein has to add arbitrarily a mathematical constant (lambda) to his general theory of relativity to account for a mysterious force that acts against the attractive gravitational force.

dark matter Astron.
A collective name for the mysterious, yet-to-detect, matter that account for the 'missing mass' in the Universe. It is thought to make up of 90% of a galaxy. The whole universe may contain about 23% of the dark matter and only 4% of the ordinary matter (the mysterious dark energy, about 73%, is thought to made up of the rest of the universe). Dark matter is believed to exist because, without it, and based only on all visible matter, calculations show that the gravity will be too weak to hold objects together in a galaxy and would move apart as they spun round. They do not emit electromagnetic radiation and hence cannot be detected by normal means such as telescopes. Possible candidates for the dark matter are MACHOs, WIMPs and neutrinos.

data path Comp.
The maximum size of data that can be transported into a processor in an operation. This is measured in bits, such as 16, 32 or 64-bit. A data of 32-bit length would take twice as long to transfer to a processor with 16-bit data path than a processor with 32-bit data path.

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