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Question

A perfect gas at 27°C is heated at constant pressure till its volume is doubled. The final temperature is

The correct answer is

327°C

Perfect Gas Temperature Change

This problem involves understanding the relationship between the volume and temperature of a perfect gas when heated at a constant pressure. This scenario is perfectly described by Charles's Law, a fundamental principle in gas dynamics.

Gas Problem Analysis

Let's first identify the given information and what we need to determine in this perfect gas problem:

  • Initial Temperature (\(T_1\)): The gas starts at \(27^\circ\text{C}\). For gas law calculations, temperatures must always be converted to the absolute Kelvin scale.
  • Process Condition: The gas is heated at constant pressure. This is a crucial detail that points us to a specific gas law.
  • Volume Change: The volume of the gas is doubled. If the initial volume is \(V_1\), then the final volume \(V_2\) will be \(2V_1\).
  • Goal: We need to find the final temperature (\(T_2\)) of the gas in degrees Celsius.

Charles's Law Application

Since the heating process occurs at a constant pressure, Charles's Law is the appropriate gas law to use. Charles's Law states that for a fixed amount of gas at constant pressure, the volume is directly proportional to its absolute temperature.

Mathematically, Charles's Law can be expressed as:

\( \frac{V_1}{T_1} = \frac{V_2}{T_2} \)

Where:

  • \(V_1\) is the initial volume.
  • \(T_1\) is the initial absolute temperature (in Kelvin).
  • \(V_2\) is the final volume.
  • \(T_2\) is the final absolute temperature (in Kelvin).

Temperature Calculation Steps

Let's proceed with the step-by-step calculation to find the final temperature.

  1. Convert Initial Temperature to Kelvin:

    The initial temperature \(T_1 = 27^\circ\text{C}\) must be converted to Kelvin by adding \(273\) (or \(273.15\) for higher precision, but \(273\) is commonly used in such problems).

    \( T_1 = 27^\circ\text{C} + 273 = 300 \text{ K} \)

  2. Set Up Charles's Law Equation:

    We are given that the final volume \(V_2\) is double the initial volume \(V_1\), so \(V_2 = 2V_1\). Now, substitute the known values into Charles's Law equation:

    \( \frac{V_1}{T_1} = \frac{V_2}{T_2} \)

    \( \frac{V_1}{300 \text{ K}} = \frac{2V_1}{T_2} \)

  3. Solve for the Final Temperature (\(T_2\)) in Kelvin:

    We can cancel \(V_1\) from both sides of the equation, as it appears on both numerator sides:

    \( \frac{1}{300 \text{ K}} = \frac{2}{T_2} \)

    Now, cross-multiply to solve for \(T_2\):

    \( T_2 = 2 \times 300 \text{ K} \)

    \( T_2 = 600 \text{ K} \)

  4. Convert Final Temperature Back to Celsius:

    The options are given in degrees Celsius, so we convert the final temperature from Kelvin back to Celsius.

    \( T_2 (\text{in }^\circ\text{C}) = T_2 (\text{in K}) - 273 \)

    \( T_2 (\text{in }^\circ\text{C}) = 600 - 273 \)

    \( T_2 (\text{in }^\circ\text{C}) = 327^\circ\text{C} \)

Final Temperature Result

The final temperature of the perfect gas after being heated at constant pressure until its volume doubles is 327°C.

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Important Questions from Thermodynamics System and Processes

  1. In a polytropic process described by $PV^n = C$, if the polytropic index $n$ is equal to zero, then the process is characterized by constant
  2. Why do particles in liquid water at 0°C have more energy as compared to particles in ice at the same temperature?

  3. Choose the INCORRECT option for the process and its work done (W) and heat transfer (Q) relations.

  4. For a closed system. identify the processes where the following quantities are zero.

    1. Heat

    2. Work done

    3. Internal Energy

  5. Identify the CORRECT statement with respect to the magnitudes of different quantities for different thermodynamic processes.

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