# Calculating the energy required to change temperatures

1. How much energy (In kcal) is required to raise the temperature of 1 L of water from 20 to 100C, and then turn it to steam at 100C?

2. A 2 kg rod of aluminum (c = 0.9 kJ/kg-C) at 90C is dropped into 10 L of water at 10C. What is the final temperature of the mixture?

3. A 0.2 kg block of an unknown metal at 50C is immersed in 1L of water at 4C. The equilibrium temperature of the mixture is 35C. What is the specific heat of the metal, in kcal/kg-C?

4. A liter of gas, initially at a pressure of 500Pa, is compressed from 1.00 L to 0.25 L. During the compression process, heat is dissipated to maintain a constant temperature. What is the final pressure?

5. A radioisotope thermoelectric generator (RTG) generates electricity by absorbing heat from a core containing a radioactive material (Plutonium 238 is typical) and discharging it into the environment. An RTG installed on an unmanned sonobuoy, tethered to the ocean floor, has a core temperature of 150 C, and discharges heat into sea water at 10 C. What is its theoretical efficiency?

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#### Solution Preview

1. Specific heat for H2O is 1 cal/(g.celsius), 1 liter of water is 1000 g.

For water from 20 to 100C, energy needed: 1000*1*(100-20) = 80*10^3 cal = 80 kcal

Heat of Vaporization of Water Hv = 2260 J /g = 2260/4.2 cal/g

For 1000g ...

#### Solution Summary

The expert calculates the energy required to change temperatures.