Question

It is NOT possible to completely convert
A. heat into internal energy
B. mechanical energy into heat
C. internal energy into mechanical energy
D. internal energy into heat

Answer 1

C. internal energy into mechanical energy.

Answer 2

thanks

Answer 3

Can you explain . I think it should be all the 4options because effeciency can never be 100% for any energy related conversion process. Correct me if I am wrong @experimentX

Answer 4

yeah .... thats correct.

Answer 5

@experimentX , see this http://answers.yahoo.com/question/index?qid=20100307133158AAvFoUv

Answer 6

@experimentX Looks like you are right :)

Answer 7

Infact The answer should be A.

The 2nd form of the 2nd law - heat cannot be completely converted into other forms of energy - places some practical restrictions on the efficiency of, for example, internal combustion and steam powered engines.

Source: http://theory.uwinnipeg.ca/mod_tech/node80.html

Answer 8

second law of thermodynamics ...

Answer 9

Yes. I meant that :D

Answer 10

Well it is A because heat can be converted but not completely as stated in the second law of thermodynamics. So A fits. You are right @shivam_bhalla

Answer 11

@shivam_bhalla
Isn't heat basically same as internal energy?

Answer 12

during adiabatic process heat is converted to internal energy,
dQ = dU + PdV

Answer 13

adiabatic= no heat=>dQ=0
so
dU=-PdV
dU=-W

Answer 14

sorry, it must have been ... isochoric process

Answer 15

dV = 0

Answer 16

@Aditya790 , http://en.wikipedia.org/wiki/Internal_energy

Answer 17

@shivam_bhalla
I've read the article you've mentioned. Internal energy is more of less the sum total of kinetic and potential energies of the constituent particles of the system. It is the total energy contained by the system.

Heat is energy in transit and flows spontaneously from higher to lower temperature. It changes the potential and kinetic energy of the body it flows into, thereby changing its internal energy.

So, heat transfer always causes change in internal energy of the system. Option A in incorrect as heat transfer causes a proportional increase in internal energy. That is, in fact, the statement of the first law of thermodynamics when no work is done.
<<dU=dQ>>

Option B is totally possible too, as in friction.

Option C isn't possible. It would violate second law.

Option D isn't possible as well since all of internal energy cannot be converted to heat. Heat transfer cannot take place with difference in temperature.

To sum up, C and D are correct answers.

I put considerable thought on this one and read enough upon the topic to tell you that they are indeed the correct answers.

Answer 18

@Aditya790 ,According to me

a)completely convert heat into internal energy--> not possible
The 2nd form of the 2nd law - heat cannot be completely converted into other forms of energy
In simple words it is the conversion of heat into other forms of energy that turns out never to be 100% efficient, even in principle
Simple example would be the heat engine
http://theory.uwinnipeg.ca/mod_tech/node80.html

b)completely convert Mechanical energy to heat -->Possible
-->Refer option D

c) internal energy into mechanical energy-->Not certain

mechanical energy = sum of potential energy and kinetic energy present in the components of a mechanical system
internal energy = sum of all energies possessed by thermodynamic system

essentially,They mean the same . Now If somebody can cite an example of such a conversion,then it would great. So not sure about this one

D) internal energy into heat --->Possible

Just read this statement, you will get your answer(read the Note)
"The second law makes the very surprising statement that some of the heat energy must always be lost, so that the conversion from heat to work is never 100% efficient. Note, however, that this form of the 2nd law places no restriction on converting other forms of energy into heat - it is the conversion of heat into other forms of energy that turns out never to be 100% efficient, even in principle."

So this is possible too

So according to me the answer should be option A.
I am not sure about option C

Answer 19

Ok my answer goes like this..
Give me an example of anything with internal energy=zero... If you can't believe the fact that internal energy can never be "completely" converted to any other forms of energy.. So, C and D is the correct answer according to me..
Regarding A and B, think of ideal conditions... stop thinking about heat engines.. they are not ideal.. In ideal conditions conversions mentioned in A and B are possible..

Answer 20

Well, there is no mention to ideal conditions

Answer 21

in the question

Answer 22

I know.. But you always suppose ideal conditions in physics.. Else even F=ma is incorrect..

Answer 23

And I am saying that No energy can be converted into heat by 2nd law of thermodynamics

Answer 24

I meant completely

Answer 25

give me the statement of second law of thermodynamics..

Answer 26

Claussius statement
No process is possible whose sole result is the transfer of heat from a body of lower temperature to a body of higher temperature

Answer 27

Kelvin statement
No process is possible in which the sole result is the absorption of heat from a reservoir and its complete conversion into work.

Answer 28

Claussius only talks about temperatures.. Not a problem.. Now regarding kelvin... where do you find reservoirs??

Answer 29

The 2nd form of the 2nd law - heat cannot be completely converted into other forms of energy

Answer 30

You asked for the statement of 2nd law of thermodynamics, so I just stated it. The one which is relevant to this context is stated above

Answer 31

Buddy, the statement of kelvin is made thinking about heat engines.. when you absorb heat from a reservoir and convert it into mechanical energy some energy is lost in sink..
But now think of one closed system.. and adiabatically expanding one. where internal energy (heat) is converted to mechanical energy..

Answer 32

OK I agree to your point of view that kelvin's statement is made thinking of heat engines. In ideal context, your point of view appears right. :)

Answer 33

What do you think about option C ?? I have no idea about it ?? What do you mean by internal energy to mechanical energy. Essentially they mean the same. Somebody can differentitiate them??

Answer 34

internal energy means sum of KE, vibrational energy, rotational energy and lots others.. you can't find exact value of internal energy but you can find the difference in internal energies of a body at two different set of conditions by just measuring the amount of energy released or absorbed when the body goes from one set of condition to other..

Answer 35

OK. I got the answer myself. Sharing it for benefit of others
Mechanical energy = KE + PE in mechanical system
Internal Energy = KE + PE in thermodynamic system

http://en.wikipedia.org/wiki/Mechanical_system
http://en.wikipedia.org/wiki/Thermodynamic_system