PhysicsLAB CP Workbook
Thermodynamics

Refer to the following information for the next four questions.

A mass of air is contained so that the volume can change but the pressure remains constant. This would be an example of an isobaric process.
 
Table I shows air volumes at various temperatures when the air is heated slowly.
 
Data Table
Isobaric Process
 
Plot the data in Table I on the EXCEL graph provided. State the equation of the trendline. 

The graph shows how the volume of air varies with temperature at constant pressure. The straightness of the line means that the air expands uniformly with temperature. From your graph, you can predict what will happen to the volume of air when it is cooled. Extrapolate (extend) the straight line of your graph to find the temperature at which the volume of the air would become zero. Estimate this temperature and state your EXCEL graph's correlation coefficient, R2

Although air would liquefy before cooling to this temperature, the procedure suggests that there is a lower limit to how cold something can be. This is the absolute zero of temperature. Careful experiments show that absolute zero is ____ ºC. 

Scientists measure temperature in Kelvin instead of degrees Celsius, where the absolute zero of temperature is 0 K.
 
 
If you relabeled the temperature axis on the graph in Question 1 so that it shows temperature in Kelvin, would your graph look like the one shown above? 

Refer to the following information for the next eight questions.

 
A major puzzle faced scientists in the 19th century. Volcanoes showed that the earth is molten beneath its crust. Penetration into the crust by bore-holes and mines showed that the earth's temperature increases with depth. Scientists knew that heat flows from the interior to the surface. They assumed that the source of the earth's internal heat was primordial, the afterglow of its fiery birth. Measurements of the earth's rate of cooling indicated a relatively young earth - some 25 to 30 million years in age. But geological evidence indicated an older earth. This puzzle wasn't solved until the discovery of radioactivity. Then it was learned that the interior was kept hot by the energy of radioactive decay. We now know the age of the earth is some 4.5 billion years - a much older earth.
 
All rock contains trace amounts of radioactive minerals. Radioactive minerals in common granite release energy at the rate 0.03 J/kg/yr. Granite at the earth's surface transfers this energy to the surroundings practically as fast as it is generated, so we don't find granite any warmer than other parts of our environment. But what if a sample of granite were thermally insulated? That is, suppose all the increase of internal energy due to radioactive decay were contained. Then it would get hotter.
 
 
How much? Let's figure it out, using 790 J/kg/K as the specific heat of granite.
Sun's radiation
How many joules are required to increase the temperature of 1 kg of granite by 1000 K? 

How many years would it take radioactivity in a kilogram of granite to produce this many joules? 

How many years would it take a thermally insulated 1-kilogram chunk of granite to undergo a 1000 K increase in temperature? 

How many years would it take a thermally insulated one-million-kilogram chunk of granite to undergo a 1000 K increase in temperature? 

Why does the earth's interior remain molten hot? 

Rock has a higher melting temperature deep in the interior. Why? 

Why doesn't the earth just keep getting hotter until it all melts? 

True or False? The energy produced by earth radioactivity ultimately becomes terrestrial radiation.
 




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