# First Law of Thermodynamics (Eint = Q-W) question. I am only stuck on part (d)

• zag4life
In summary: And, you can do it again to get Q for bf. Then, if you apply Eint_i + Q_i = Eint_f, you can solve for Eint_f.
zag4life
When a system is taken from state i to state f along path iaf in the figure, Q = 60 cal and W = 10 cal. Along path ibf, Q = 65 cal.

(a) What is W along path ibf? (b) If W = -19 cal for the return path fi, what is Q for this path? (c) If Eint,i = 11 cal, what is Eint,f?(d) If Eint,b = 27 cal what is Q for path ib?(e) For the same value of Eint,b, what is Q for path bf?

## Homework Equations

Eint= internal energy of the system; Q= heat; W= work
I have used Eint= Q - W to solve most of the problem.

## The Attempt at a Solution

I only am marked wrong on part d which is very frustrating. So far I have:
(a)Wibf = 15 cal because W= Q-E (b) Qfi= -69 cal because Q= -(E)+W (c) Eint,f= 61 cal because Eint,f= Eint+Ei (d) Qib= ? (e) Qbf= 34 because Eint,f =E-Eint,b
Any thoughts?

Last edited:
Hi, zag4life. Welcome to PF.

How does the work for i $\rightarrow$b compare to the work for i$\rightarrow$b$\rightarrow$f?

Well, from b to f, because the volume does not change, work is equal to zero. The work for i,b,f would be the same as the work for i,b I think.

Good. So, use that to get Q for ib.

For part (d), we can use the First Law of Thermodynamics to solve for Qib:

Eint,b = Qib - Wib
27 cal = Qib - 15 cal
Qib = 42 cal

So, for path ib, Q = 42 cal.

For part (e), we can use the same equation to solve for Qbf:

Eint,b = Qbf - Wbf
27 cal = Qbf - (-19 cal)
Qbf = 8 cal

So, for path bf, Q = 8 cal.

## 1. What is the First Law of Thermodynamics?

The First Law of Thermodynamics, also known as the Law of Conservation of Energy, states that energy cannot be created or destroyed, only transferred or converted from one form to another. This means that the total energy in a closed system remains constant.

## 2. What does the equation Eint = Q-W represent?

This equation represents the internal energy (Eint) of a system, which is the total energy contained within the system. Q represents the heat added to the system and W represents the work done by the system.

## 3. How is the First Law of Thermodynamics applied in real-life situations?

The First Law of Thermodynamics is applied in various real-life situations, such as in the operation of engines, power plants, and refrigeration systems. It is also used in understanding the energy transfer and conversion in chemical reactions and biological processes.

## 4. What is the significance of the First Law of Thermodynamics in the study of thermodynamics?

The First Law of Thermodynamics is the cornerstone of thermodynamics and provides the fundamental understanding of energy and its conservation. It serves as the basis for studying other laws and principles in thermodynamics and is essential in analyzing and predicting the behavior of various systems.

## 5. How does the First Law of Thermodynamics relate to the concept of entropy?

The First Law of Thermodynamics and the concept of entropy are closely related. The First Law states that the total energy of a closed system remains constant, while the Second Law of Thermodynamics, which is related to entropy, states that the total entropy of a closed system tends to increase over time. This means that while energy is conserved, the quality of energy tends to decrease over time due to irreversible processes.

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