# Solving Thermodynamics Ideal Gas Probs: Q, W, ΔU w/Help | Physicsforums.com

In summary, the conversation revolves around finding the work, heat, and internal energy for different processes involving an ideal gas using various equations and graphs. The equations mentioned include the ideal gas law, laws of thermodynamics, and an equation for calculating work during an isothermal process. The individual is seeking clarification on which equations to use and the procedures for solving the problems. They also ask if there are any important equations they may be missing.
I'm sorry I cannot conform to the default format Physicsforums.com; it is because I do not even know the first step to solving these sorts of problems, I don't know which equations to use which is a major problem. Here are the types of questions I require understanding.
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An ideal gas goes through three processes (A>B>C>[A]) (Triangular form) (PV Chart)
How would I figure out The Q, W, and ΔU (internal energy) for A to B, B to C, C to A?
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On another graph using variables but this time with numerical values for P and V on the axis, how would I find the work done by a monatomic ideal gas as it expands from point A to point C along the path shown in the figure? Also, how much heat would be absorbed BY the gas during this process?
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Finding the net work, heat, and ΔU in another PV Graph with data on the axis-es?
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Calculating temperature, work, and/or internal energy in another PV Graph?
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Whether or not W, Q, or ΔU is positive(gained) or negative(released) in an ideal gas system as well as how those three (Q,W,..U) are related?
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I really wish to work on the problems myself, so I have only asked what procedures I should take.
Here is a list of equations I have scavenged.

ΔU = Won + Q

ΔU = (3/2)nRΔT

Won = -PΔV

P1V1 = P2V2

(P1V1)/T2 = (P2V2)/T2

PV = nRT

Is there any important equation that I am missing?

Last edited:

I think that since you are working with idel gasses, you can dervie most of the stuff from 3-4 equations, namely:
The ideal gas law : pV = nRT
The Laws of thermodynamics.(2-3 laws is useful).

If you want to find the Work done during a isothermal(T constant) step e.g.:
$W = -\int\limits_{V_1}^{V_2}\! p\,\text{d}V = -\int\limits_{V_1}^{V_2}\! \dfrac{nRT}{V}\,\text{d}V = -nRT\left(\ln(V_2)-\ln(V_1)\right) = -nRT\ln\left(\dfrac{V_2}{V_1}\right)$
I hope that I understood your question.

## What is the ideal gas law and how is it used to solve thermodynamics problems?

The ideal gas law is a fundamental equation in thermodynamics that relates the pressure, volume, temperature, and number of moles of an ideal gas. It is represented by the equation PV = nRT, where P is the pressure, V is the volume, n is the number of moles, R is the gas constant, and T is the temperature. This equation can be used to solve for any of the variables when the others are known, making it a useful tool in solving thermodynamics problems.

## What is the difference between Q, W, and ΔU in thermodynamics?

Q, W, and ΔU are all terms used in thermodynamics to represent different quantities. Q represents the heat transferred into or out of a system, W represents the work done on or by the system, and ΔU represents the change in internal energy of the system. Q and W are both energy transfer terms, while ΔU is an energy state term.

## How can I determine the sign of Q, W, and ΔU in a thermodynamics problem?

The sign of Q, W, and ΔU can be determined by considering the direction of energy transfer or change in energy. If energy is entering the system, Q will be positive and W will be negative. If energy is leaving the system, Q will be negative and W will be positive. The sign of ΔU will depend on the direction of the change in energy - if the internal energy of the system increases, ΔU will be positive, and if it decreases, ΔU will be negative.

## How do I calculate the work done in a thermodynamics problem?

The work done in a thermodynamics problem can be calculated using the equation W = -PΔV, where P is the pressure and ΔV is the change in volume of the system. This equation is based on the fact that work is equal to the force applied multiplied by the distance over which it is applied. In thermodynamics, the force is represented by pressure and the distance is represented by the change in volume.

## What are some common mistakes to avoid when solving thermodynamics problems?

Some common mistakes to avoid when solving thermodynamics problems include using the wrong sign for Q, W, or ΔU, forgetting to convert units, and not considering all of the energy transfers and changes in energy that may be occurring in the system. It is important to carefully read the problem and consider all relevant equations and variables before solving. It can also be helpful to check your final answer using the laws of thermodynamics to ensure it is physically possible.

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