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Physics
Chapter 06
The purpose of light bulb is to convert electrical energy into light energy. Similarly, the purpose of engine of car is to convert chemical Potential Energy in fuel to Kinetic energy of car. Both these objects transform energy into desired form but not 100%. Some part of this energy is converted into a form that is not useful. As light bulb converts some part of it’s energy into heat energy. Similarly heat engine also converts sum of its energy into heat energy and sound energy . Both these are not useful forms of energy.
The efficiency is defined as ‘. The ratio of useful output work done to input work ‘.
Efficiency= useful output work / input work
E = Wo/Wi
We can define efficiency in terms of energy also,
‘ The ratio of useful output energy to input energy is called efficiency’.
Efficiency= Output useful energy/ input energy
Efficiency= Eo/Ei
The efficiency is given by,
E = Wo/Wi
E = Joule / Joule
E = No unit
As efficiency is ratio of two same physical quantities so units cancel each other and efficiency has no unit.
As efficiency does not need any direction so it is a scalar quantity.
We know that efficiency is given by ,
E = Wo/Wi
As output useful work is always less than input work so efficiency is always less than 1 . It can not be greater than 1.
Examples:
The purpose of incandescent light bulb is to convert electrical energy into light energy. It does not convert all of the energy into light energy . It converts a lot of energy into heat energy when it is in use. In fact only 5% of energy in converted into light energy. Rest becomes waste thermal energy. We can say that light bulb is only 5% efficient.
Efficiency expressed in percentage is ratio of useful work done to input work multiplied by 100.
Efficiency % = (Wo/Wi) × 100
In terms of energy,
Efficiency % = (Eo/Ei)×100
Answer:
The efficiency of an engine can not be 100% because when a machine is operated , due to friction some amount of energy is converted into Heat energy that is why useful amount of energy is reduced. That is why output useful energy is Less than input energy. Therefore efficiency of an engine can not be 100% .
Given Data :
Input energy= Ei = 200 J
Output useful energy= Eo= 50 J
Required:
Efficiency= E = ?
Solution:
As we know that ,
Efficiency= Eo/Ei
Efficiency= 50 J/200J
Efficiency= 1/4
For efficiency Percentage multiply by 100,
Efficiency= (1/4)×100
Efficiency = 25%
So the Petrol engine is only 25% efficient.
Given Data:
Efficiency of fluorescent bulb = 20%
Input energy= Ei= 550 J
Required:
Output energy= Eo= ?
Solution:
As we know that ,
Efficiency= (Eo/Ei)×100
E = (Eo/Ei)×100
(E × Ei) /100 = Eo
Putting values ,
(20 × 550) / 100 = Eo
Eo= 110 J
So the output useful energy of 20% efficient fluorescent bulb with input energy of 550 J will be 110 J.
Given Data:
Efficiency of incandescent light bulb= 5%
Output energy= Eo= 35 J
Required:
Input energy= Ei= ?
Solution:
As efficiency is given by ,
Efficiency= (Output energy / Input energy) × 100
E = (Eo/Ei) × 100
(E/100) = Eo/Ei
Taking reciprocal ,
100/E = Ei/Eo
(100/E ) × Eo= Ei
Putting values
Ei= (100/5) × 35 J = 700 Joule
So the input energy of Incandescent light bulb of 5% efficiency of it gives 35 J output will be 700 J.
Efficiency has no unit because it is ratio of two same physical quantities.
The efficiency of an engine can not be 100% because friction lowers the efficiency of engine.