Heat Absorbed by Gas: An Expert Explanation

In summary, the conversation discusses two examples illustrating the concept of path dependence in the absorption of heat by a gas. The first example involves a slow expansion of a gaseous volume while maintaining a constant temperature, while the second example involves a free expansion of the gas. It is noted that in the second case, there is no heat absorbed or work done by the gas, resulting in a doubling of volume and halving of pressure. The conversation also raises questions about the state variables and applicability of the state equation for the first example, to which it is clarified that the temperature remains constant and the state equation is applicable. However, for an irreversible expansion like the second example, the pressure calculated from the state equation does not accurately describe the force
  • #1
Nikhil Rajagopalan
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Dear Experts,
While going through the explanation why heat absorbed by a gas is path dependent, there were two examples of comparison.
1. A gaseous volume of 2 liters expanding slowly to 4 liters , supplying heat from a heater keeping the temperature of the gas constant at 300K.
2. A gas allowed to free expand from 2 liters to 4 liters by bursting a membrane.

In the second case, there is no heat absorbed by the gas and no work done by the gas. Volume becomes twice and pressure becomes half. It seems easy to comprehend.
In the first case, how does the state variables change. Does the temperature of the gas decrease while expanding. Is the state equation PV = nRT applicable here?
 
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  • #2
Nikhil Rajagopalan said:
Does the temperature of the gas decrease while expanding.
The problem statement says the temperature stays constant.
Nikhil Rajagopalan said:
Is the state equation PV = nRT applicable here?
That law is always true (for ideal gases).
 
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  • #3
Nikhil Rajagopalan said:
Is the state equation PV = nRT applicable here?
For an irreversible expansion such as with the membrane, the pressure calculated from the ideal gas law does not describe the force per unit area on any real or conceptual surface. There is a "viscous" contribution to the force per unit area (for a typical Newtonian fluid gas like air).
 
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FAQ: Heat Absorbed by Gas: An Expert Explanation

1. What is the definition of heat absorbed by gas?

Heat absorbed by gas refers to the amount of thermal energy that is transferred to a gas, causing an increase in its temperature.

2. How is heat absorbed by gas measured?

Heat absorbed by gas can be measured using the formula Q=mcΔT, where Q is the heat absorbed, m is the mass of the gas, c is the specific heat capacity of the gas, and ΔT is the change in temperature.

3. What factors affect the amount of heat absorbed by a gas?

The amount of heat absorbed by a gas can be affected by its specific heat capacity, the mass of the gas, and the change in temperature. Other factors such as pressure and volume of the gas can also play a role.

4. Can heat absorbed by gas be negative?

Yes, heat absorbed by gas can be negative if the gas is losing thermal energy, resulting in a decrease in temperature.

5. How does heat absorbed by gas affect the gas's properties?

Heat absorbed by gas can cause changes in the gas's properties, such as an increase in temperature, volume, and pressure. It can also lead to changes in the gas's state, such as from a solid or liquid to a gas.

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