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Suppose in a football game a player always makes foul. So the other good players cannot concentrate in the game and referee also. The foul making player breaks the discipline of playing. So the total game cannot be conducted properly.

Similarly increase or decrees of oxygen level in the environment both are difficult for human being.  In that two cases are disorderliness or entropy increases. Disorderliness of any system is called entropy. In other words entropy is a measure of disorderliness.

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The branch of physics where relation between heat and work done is discussed called thermodynamics. In thermodynamic process firstly we have to know about thermodynamic system. What is thermodynamic system? The system is a closed area where thermodynamic substance observed. Two types of system are i) close system and ii) open system.

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Amplifier is an electronic device. The device which increases the value of a signal is called amplifier. In input a weak signal is applied, in output a strong, same shape signal is gained. Amplify means to increase the value of something.

Transistor is used in amplifier. It can amplify a signal many times. In two ways a transistor can amplify a signal

(i) by using high value of output resistance compared to the input resistance,

(ii) by controlling the collector current with the help of base current.

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What is transistor? Transistor is an electronic device. It is made with p and n type semiconductor. When a semiconductor is placed in middle between same type semiconductors the configuration is called transistor. We can say that a transistor is the combination of two diodes connected back to back.

Bardeen W.H. Brattain of Bell telephone laboratories, USA first invented transistor in 1948. After the invention of transistor revolution occurred in electronic world. Before the invention of transistor big size circuits were used. But transistor turned them into a tiny size.

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An atom has electron, protons and neutrons. According to the Bohr’s atomic model theory nucleus of an atom contains protons and neutrons. Neutrons has no charge and protons have positively charged electrons moves around the nucleus in different orbits far away from this nucleus.

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Definition of solar cell

 Solar cell is that cell which converts light energy to electrical energy. Working principal of solar cell is related to emission of light or absorption. It is an electronic device. Actually solar cell is p-n junction device which convert solar energy to electrical energy.

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There are two types of semiconductors

(i)  Intrinsic or pure semiconductor

(ii)  Extrinsic or impure semiconductor

(i)  Intrinsic or pure semiconductor: The conduction band of silicon and germanium is empty and the valence band is fully filled up with electrons at very low temperature. Germanium and silicon have four valence electrons. Each atom of germanium silicon shares one electron with its neighboring atom. Thus covalent bond is made. So, there is no free electron in germanium and silicon. For this reason there is no conduction of electricity in them. Such pure semiconductors are known as intrinsic semiconductors. If pure semiconductors are heated at high temperature due to thermal agitation electrons of pure semiconductors becomes free by breaking the bonds. The electrons can pass forbidden gap if the energy of their electrons is very large and transferred into conduction band. When an electron goes into conduction band from valence band there a vacancy occurs. The vacancy makes a hole and this hole is equal to a positive charge.

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 1.   Insulator: Through these materials electricity cannot pass is called insulators. Plastic, glass, wood etc are the examples of insulators. The valence band of those material remains full of electrons. The conduction band of those material remains empty. The forbidden energy gap between the conduction band and the valence band is widest. The difference is more than 10ev. Crossing the forbidden energy gap from valence band to conduction band large amount of energy is needed.

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