Personal possession of currency is prohibited in U. S. prisons, although, in practice, prisoners still exchange goods and services. American prisoners used cigarettes as a medium of exchange until smoking was banned in U. S. prisons in 2003. After 2003, prisoners looked to other items on the commissary menu to facilitate exchange. In 2008, the Wall Street Journal reported that energy bars had caught on as money in some American prisons. Outside of U. S. prisons, people use the U. S. dollar for day-to-day transactions. One important difference between using energy bars and using dollars as money is that have intrinsic value. U. S. dollars are an example of money.

One important difference between using energy bars and using dollars as money is that ENERGY BARS have intrinsic value. U.S. dollars are an example of FIAT money.

Explanation:

Energy bars have intrinsic value and therefore present a type of commodity inside the jail. This is called commodity money because energy bars have value besides the value assigned by the prisoners. They serve as a medium of exchange but they can also be eaten.

On the other hand, fiat money or official currency has value because the government backs it. For example, the US dollar has value because the US government stands behind them: "This note is legal tender for all debts, public and private." US dollars can only be used as money, since they cannot be eaten like energy bars or traded for another use besides being money.  

sound is invisible, but sometimes we can feel it. when a kettle-drum is thumped with a stick that time the tight drum skin moving up and down very quickly for some time and then it pumped sound waves into the air. loudspeakers also work in a similar manner.

components of a speaker:

electromagnet

in order to translate an electrical signal into an audible sound, an electromagnet is used in looudspeakers

  suspension

suspension is used to   center the voice coil in the gap of the magnet and it exerts a restoring force to keep it there. suspension is used to limits the maximum mechanical excursion of the diaphragm and voice coil.

voice coil

voice coil is a coil of wire which is attached to the apex of a loudspeaker cone. it provides the motive force to the cone by the reaction of a magnetic field to the current passing through it.

cone

in a   loudspeaker, a   thin, semi-rigid membrane attached to the voice coil, which moves in a magnetic gap due to which it vibrates   and produced sound. it is known as diaphragm and it can also be called a cone, though not all speaker diaphragms are cone-shaped.

surround

surround is also known as front suspension. surround is used to join the cone to the chassis. together with the suspension it controls the cone excursion, and it is also used to   determine how energy which is   travelling through the cone is absorbed, and how the speaker limits when it reaches the ends of its travel.

dustcap

dustcap is used to protect the voice coil from dust and dirt. it is a part of the cone.

how speaker work

the outer part of the cone is fastened to the outer part of the loudspeaker's circular metal rim. the inner part is fixed to   voice coil that sits just in front of a permanent magnet . when loudspeaker is hooked up to a stereo, electrical signals feed through the speaker cables into the coil. it will convert the coil into an electromagnet magnet . when electricity flows back and forth in the cables, the electromagnet either repels or attract the permanent magnet. this moves the coil back and forth, and it pull and push the loudspeaker cone. like a drum skin vibrating back and forth, the moving cone pumps sounds out into the air.in this way loudspeakers works.