   Chapter 11, Problem 61AP

Chapter
Section
Textbook Problem

A class of 10 students; taking an exam has a power output per student of about 200 W. Assume the initial temperature of the room is 20°C and that its dimensions are 6.0 m by 15.0 m by 3.0 m. What is the temperature of the room at the end of 1.0 h if all the energy remains in the air in the room and none is added by an outside source? The specific heat of air is 837J/kg · °C, and its density is about 1.3 × 10−3 g/cm3.

To determine
How long does it take to boil away 2.00 kg of the liquid.

Explanation

Given Info: The mass of the liquid is 2.00 kg, boiling point of liquid helium is 4.2 K. latent heat of vaporization 2.00×104J/kg , rate of heat from electric heater is 10.0 W.

First, find out the total energy needed and then time taken.

Formula to energy needed is,

Q=mLv

• Q is the energy needed,
• m is the mass of the liquid helium.
• Lv is the latent heat of vaporization.

Substitute 2.00 kg for m and 2.00×104J/kg for Lv to find Q.

Q=(2.00kg)(2.00×104J/kg)=4.00×104J

Total Energy needed is 4.00×104J

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