Thermodynamics when work being negative or positive

In summary, based on the conversation, it is established that work done by a system is represented by a negative value, while work done on a system is represented by a positive value. In the given question, the work done is positive 400 J, which means that the system lost energy. This is in line with the first law of thermodynamics, where a decrease in internal energy is represented by a negative value.
  • #1
177
0
Hi

How are you

I have a question how we can know the work if - or +

also heat energy

please could see this question why in answer
take work as -
and heat as +

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  • #2
hi manal950! :smile:
manal950 said:
I have a question how we can know the work if - or +

it's really english rather than physics …

work done by a system is minus

work done on a system (or to a system) is plus :wink:

the question says "400 J of work is done by the system", so the work comes out of the system, so it's minus

(if the question said "400 J of work is done on the system", then the work would come into the system, so it would be plus)​
 
  • #3
Tim got it backwards for the sign convention you're using. You have the first law written as ##\Delta U = Q-W##. Assume for a moment that Q=0. If work is done by the system, the system is expending energy on its surroundings, so the system's energy U must decrease. For this to happen, you need W>0. Similarly, if the surroundings do work on the system, energy is being added to the system, so you want W<0 so that ##\Delta U>0##.
 
  • #4
thanks so much

thanks tiny-tim
thanks vela
 
  • #5
If the work was negative, that is -400J, then the answer would have been

ΔU = Q - W
= 1000 J - (-400 J)
= 1000 + 400
= 1400 J

So negative work means that the system gained energy or an external force did work on the system.
In this case the problem states that the work is positive 400 J. Which means that the system did work
and therefore lost energy indicated by a lower change in its internal energy.
 

1. What is thermodynamics?

Thermodynamics is the branch of science that deals with the study of energy and its transformations in systems. It includes the principles and laws governing the behavior of heat and its relationship with other forms of energy.

2. What is work in thermodynamics?

Work in thermodynamics refers to the transfer of energy from one system to another due to a force acting on a system. It can be either positive or negative depending on the direction of the force and the displacement of the system.

3. How do you determine if work is positive or negative in thermodynamics?

In thermodynamics, work is considered positive if the force and displacement are in the same direction, meaning the system is gaining energy. On the other hand, work is negative if the force and displacement are in opposite directions, indicating that the system is losing energy.

4. What is the significance of positive and negative work in thermodynamics?

The sign of work in thermodynamics is crucial in determining the direction of energy transfer and the overall change in the system's internal energy. Positive work contributes to an increase in the system's energy, while negative work results in a decrease in the system's energy.

5. How can negative work be useful in thermodynamics?

Negative work can be useful in thermodynamics for processes such as refrigeration and heat pumps, where the system needs to absorb heat from a colder reservoir and release it to a hotter reservoir. This is achieved by doing negative work on the system, which results in a decrease in the system's internal energy and the absorption of heat from the surroundings.

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