Thermodynamics - example with answer

In summary, the conversation discusses the relationship between heat, work, and internal energy in the context of the first law of thermodynamics. The direction of work done by the system is dependent on its expansion or contraction and the direction of motion. The difference in temperature is not converted to Kelvin since it is a constant difference. Examples are given to clarify the concept of positive and negative work.
  • #1
manal950
177
0
Hi
Here is a question with answer , in fact I don't understand the answer fully ..
please can explain to me the answer .. pleas anyone clear out the answer

and here in answer I saw that work is negative is that because work done by the system ?

The question : -

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Answer

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  • #2
It is based on the 1st law of TD which relates that the energy of the system is conserved.
If Q is the (+) heat that it receives then this energy can be used to raise the internal energy of the system ΔU (+) and/or to do work W (+) thus

Q = ΔU + W

so that

ΔU = Q - W

that is the difference in energy between the received heat and the work done by the system is used to raise its internal energy. In this the aluminium cube does positive work since it is expanding against atmospheric pressure.
 
  • #3
Here - W (negative )
Is because the work done by the system during heating ?

and why delta T in C why not in kelvin (K)

thank you
 
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  • #4
The aluminium does positive work in expanding against the atmospheric pressure, that is the surface exerts an outwards force against the air as it expands. So the direction of movement (expansion) is in the same direction as the force it is exerting, so one inserts it as positive work in the formula

ΔU = Q - W

Weh ndifference of temperature are involved you need not change the temperature to kelvin since their is just a constant difference between the two 273 degrees.
 
  • #5
  • #6
The system will loose energy if it does work, like the expanding block pushing outwards against the air. Because the force that it is exerting against the air is directed outwards and the surface is also moving in this direction, outwards, so it is doing positive work. If the block was cooled down it would still need to support itself against the atmosphere, so it is still pushing back against it, but now the direction of motion is inwards as it is contracting while it is cooling, but now the work done by it is negative since the force and direction of motion are opposite to each other. Also since the atmosphere is pushing it inwards we get that the block will gain energy because the atmosphere is doing work on it. So positive work means it is losing energy and negative work means it is gaining energy.
 
  • #7
thanks a lot
 

FAQ: Thermodynamics - example with answer

What is thermodynamics?

Thermodynamics is the branch of science that deals with the relationships between heat, energy, and work.

What are the laws of thermodynamics?

The first law of thermodynamics states that energy cannot be created or destroyed, only transferred or converted from one form to another. The second law states that the total entropy of a closed system will never decrease over time. The third law states that the entropy of a perfect crystal at absolute zero temperature is zero.

What is an example of thermodynamics in everyday life?

An example of thermodynamics in everyday life is the operation of a refrigerator. The first law of thermodynamics explains how the refrigerant absorbs heat from the inside of the fridge, transferring it to the outside, thus keeping the inside cool. The second law explains why the fridge needs a constant supply of energy (usually in the form of electricity) to maintain this temperature difference.

How does thermodynamics relate to energy efficiency?

Thermodynamics plays a crucial role in determining the efficiency of energy conversion processes. The second law of thermodynamics states that some energy will always be lost as heat during these processes. Therefore, the higher the efficiency of a system, the less wasted energy it produces.

What are some practical applications of thermodynamics?

Some practical applications of thermodynamics include power generation, refrigeration and air conditioning, chemical and industrial processes, and even the functioning of living organisms. Understanding thermodynamics is essential for developing and improving these technologies and processes.

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