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A pressure cooker reduces cooking time for food, becuase
Thermal properties of matter refer to the characteristics that describe how substances respond to heat. These include thermal expansion, specific heat capacity, and thermal conductivity. Understanding these properties is essential for analyzing heat transfer processes, phase changes, and the behavior of materials in different temperature conditions in both natural and engineered systems.
Class 11 Physics Chapter 10 Thermal Properties of Matter discusses heat transfer methods such as conduction, convection and radiation. It examines concepts like thermal expansion, specific heat capacity, calorimetry and latent heat. This chapter emphasizes the practical applications of these principles in everyday life and their significance in understanding various thermal phenomena and scientific principles.
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A pressure cooker cuts down significantly the cooking time of food primarily because it raises the boiling point of water by raising the pressure inside the cooker. This phenomenon can be explained using the concept of vapor pressure and the Clausius-Clapeyron equation.
Key Concepts:
1. Boiling Point and Pressure: The boiling point of a liquid is the temperature at which its vapor pressure equals the external pressure surrounding the liquid. A liquid boils at a 100 degrees Celsius in standard atmospheric pressure, 1 atm (101.3 kPa), but the boiling point increases with the pressure.
2. Pressure Cooker Mechanism: In a pressure cooker, the environment within the sealed vessel creates steam; due to this, the internal pressure builds up. The boiling point of water will rise with the increase in pressure.
Clausius-Clapeyron Equation:
The Clausius-Clapeyron equation governs how pressure and temperature for phase changes are related to each other. It is given as:
dP/dT = L/(T ΔV )
Where,
– dP is the change in pressure
– dT is the change in temperature
– L is the latent heat of vaporization
– T = absolute temperature
– ΔV = change in volume
This equation is more complicated in its use, but the general principle shows that with increased pressure, the temperature needed to boil is increased as well.
Example Calculation:
1. Standard Boiling Point: At 1 atm, water boils at 100°C (373 K).
2. Increased Pressure: In a standard pressure cooker, the pressure will reach about 2 atm (approximately 202.65 kPa).
3. Application of Ideal Gas Law: In order to find the new boiling point, you can apply the above in a rough approximation, though detailed calculations would be more complex modeling as given below:
P1/T1 = P2/T2
Where,
– P1 = 1 atm
– T1 = 373 K or 100°C
– P2 = 2 atm
– T2 = boiling point at 2 atm
T2 = T1 * (P2/P1) = 373 K * (2/1) = 746 K ≈ 473°C
Thus, at 2 atm, the boiling point of water can be around 120°C (approximately), which is significantly higher than the boiling point at normal atmospheric pressure.
Conclusion:
Because of this effect of the boiling point increase on the temperatures used, it increases food to higher cooking temperatures for fast cooking times. It is on account of these grounds that the answer for which one states “a pressure cooker shortens the cooking time” as: the boiling point of water used for cooking increases
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