Thermal Physics

1. What do you see in these pictures?
2. What is meant be expansion?
   Is the difference between the original size of an object and its size when its cooled or heated.
   
3. Why things expand?
   

   When a solid is heated its particles vibrate more rapidly. As a result they collide and push each other further apart.

4.  

1. Explain what happen when solids are heated?
   When a solid is heated, its molecules gain kinetic energy and vibrate more vigorously.  As the vibration become larger, the molecules are pushed further apart and the solid expands slightly in all directions.
   
   
2. Explain what happen when solids are cooled?
   When a solid is cooled, its molecules lose kinetic energy and have less vibration.  As the vibration become lower, the molecules are pulled closer and the solid contracts slightly in all directions.
   
3. List the factors which affect the expansion of solids?
   Original length (direct), temperature (direct), material type
   
4. Compare between the expansivity of solids, liquids, and gases.
   Solids in general have the smallest expansivity while gases have the greatest expansivity and liquids lie in between.
   
5. List four real-life problems due to expansion of materials.

   On a hot day concrete runway sections in airport expands and this cause cracking.  To solve this problem we leave small gabs between sections.
   On a hot day concrete bridges expand. To solve this problem, we leave small gab at one end and support the other end with rollers.
   Telephone wire contract on cold days.  To solve this problem, we leave wires slack so that they are free to change length.
   On a hot day railway lines expand.  To solve this problem, gaps are left between sections of railway lines to avoid damage of the rails as the expand in hot weather.

6. What us meant by bimetal strip?
   A bimetal strip is a two-thin strips of different metals welded together.
   
7. Describe the action of a bimetal?
   

   When the bimetal strip is heated, the brass expands more than invar and this makes the strip bends with brass on the outside of the curve.  If the strip is cooled, instead of heated, it would bend the opposite way.

8. Explain how to build a fire alarm using bimetals.
   
   
9. What is meant by a thermostat?
   A thermostat is temperature operated electrical switch that uses the expansion properties of a bimetal strip.
   
10. Explain the structure and the action of thermostat?

    The figure shown to the right shows a thermostat.  The control knob sets the temperature at which the switch will turn off.  The bimetal strip provides the expansion and contraction which results closing or opening the electrical circuit of the system under control. When the bimetal strip is heated it curves and breaks contact then the temperature reaches a certain value.  Thus, switching off the current to the system under control.

11. List some applications to thermostat in industry

    electric irons fish tanks home heating/cooling systems Ovens, refrigerators,  fire alarms, car flashers

12.  

1. What do you see in the picture below?
   
   
2. Explain what happen when liquids are heated?
   When a liquid is heated, its molecules gain kinetic energy and vibrate more vigorously.  As the vibration become larger, the molecules are pushed further apart and the liquid expands slightly in all directions.
   
3. Explain what happen when liquids are cooled?
   When a solid is cooled, its molecules lose kinetic energy and have less vibration.  As the vibration become lower, the molecules are pulled closer and the solid contracts slightly in all directions.
   
4. List the factors which affect the expansion of liquids?
   temperature (direct), liquid volume (direct), liquid type
   
5. Compare between the expansivity of liquids and solids?
   Expansivity of liquids is much greater than the expansivity of solids.
   
6. Explain why when heating a liquid, its level initially decreases and then it increases to become more larger than the original level? 
   The liquid level drops due to the expansion of its container which initially absorbed all the heat.  After a while, the heat reaches the liquid it compensates for the expansion of the container and rises much more than the original level.
    
7. List some applications that uses the expansion of liquids?
   1. Thermometers
      
   2. The valve shown below operates based on the expansion of the oil.  As the room warms of the oil in the valve expands and pushes the piston down.  This shuts off the flow of hot water.  When the temperature drops the valve opens again. 
      
      
8. Explain the effect temperature on volume of water?
   The water volume decreases from 0ºC-4ºC.  After 4ºC, the volume of water increases gradually.  
   
9. When water has its minimum volume or higher density?
   Water has its minimum volume at 4ºC.
   
10. Explain the effect temperature on density of water?
    The water density increases from 0ºC to 4ºC.  After 4ºC, the density of water decreases gradually.  
    
    
11. Explain why filled bottles of water when placed in a freezer may burst when water is frozen?
    When water freezes it changes to ice and  its volume increases by about 9%. Since bottle is filled, then there is no space left for the increased 9%, thus a high pressure starts to builds up against the bottle walls from inside.  This high pressure burst the bottle. 
     
12. Explain why ice floats on water?
    When water freezes it changes to ice and  its volume increases by about 9%. The increase in volume means that the ice is less dense than water beneath it and it will floats. 
    
13. Explain why the surface of a lake freezes while the deepest water stays at 4C?

    When the weather gets cold, the temperature of water at the surface drops down and it becomes more dense than the water underneath.  The dense water starts to sink and warmer water is pushed up.  The water will keep circulating until the temperature of all water in lake become 4C.  When the water temperature at the surface drops below 4C its volume starts to increase and its density starts to decrease.  At this stage, the water is less dense than the water underneath and it stays on the surface.  In time, ice forms on the surface and the water temperature at the bottom of the lake will remain at 4C. Thus, fish can survive a severe winter by staying in the deep warm water.

14.  

1. Compare the expansion of gases to that of solids and liquids?
   The expansion of gases is much more larger than that of solids or liquids under the same rise in temperature.
   
2. Explain the effect of temperature on gas volume when pressure is constant?
   When pressure is constant, the volume of a gas is directly proportional to the Kelvin temperature (Charlie's Law).
   When the temperature of a gas is increased, the molecules move faster and the collisions become more violent thus they spread away from each other causing the volume to increase. 
   
   
3. Explain the effect of temperature on gas pressure when volume is constant?

   When volume is constant, the pressure of a gas is directly proportional to the Kelvin temperature (Pressure Law).  In gas, molecules are always on the move.  They travel fast, hitting each other and the sides of their container.  If the temperature is increased, the molecules move faster and the collisions become more violent thus the pressure increase.

4. Explain an experiment to show the study the effect of temperature on gas pressure?
   
5. What is the statement of "Charlie's law"?
   When pressure is constant, the volume of a gas is directly proportional to the Kelvin temperature.
   Mathematically: V₁/T₁=V₂/T₂
   
6. What is the statement of the pressure law?
   When volume is constant, the pressure of a gas is directly proportional to the Kelvin temperature. 
   Mathematically: T₁/P₁=T₂/P₂
   
   
7. What is the statement of the Boyle's law?
   When temperature is constant, the volume of a fixed max of a gas is inversely proportional to the pressure
   Mathematically: P₁V₁=P₂V₂
   
   
   
8. What is the general law of gases?
   When gas parameters change from P₁,V₁, and T₁ to P₂, V₂, T₂ then  P₁V₁/T₁=P₂V₂/T₂
   

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Created SEP 1, 2000
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Last modified: November 28, 2000