Lesson 2.5 Gases

  1. The molecules of gas in a container are in constant motion. They constantly collide with each other and with the inner surface of the container. The pressure on the inner surface of a container is caused by collisions of the gas molecules with the surface. The pressure increases when the rate of collision increases.
  2. The molecules of gas in a container are in constant motion. They constantly collide with each other and with the inner surface of the container. The rate at which they collide depends on their rate of movement and the number of molecules present. When more molecules are present in the container, the rate of collision is increased.
  3. The rate of movement of the gas molecules increases as the temperature is increased. An increase in the temperature will result in an increase in the rate of movement and an increase in the rate of collision with the surface of the container.
  4. The gravitational force exerted by 1 kg at the earth’s surface is 9.81 N. If this force is exerted on 1 m2, the pressure is 9.81 N/m2. (or 1 Pascal) The mass of air needed to create a pressure of 101300 N/m2 is therefore = 101300 / 9.81 = 10326 kg.
  5. A mercury barometer consists of a tube that is closed at one end, filled with mercury and inverted in a container of mercury. The tube is longer than 760 mm and a space is created between the upper surface of the mercury in the tube and the closed end of the tube. The height of the mercury in the tube depends on the atmospheric pressure on the surface of the container at the bottom of the tube.
    An aneroid barometer consists of a sealed container that changes in shape slightly as the pressure outside the container changes. A pointer is mechanically linked to the surface of the container in a way that small movements of the surface are easily observed and measured.
  6. The air pressure at the top of a high building will be slightly less than the pressure at the bottom of the building. This difference can be measured using a barometer and the barometer could be calibrated to indicate difference in height.
  7. In Question 4 above, we calculated the mass of air above a m2 on the earth’s surface to be 10326 kg. If 99% of this is contained in a column with a base of 1 m2 and a height of 30000 m, (0.99 x 10326)kg is contained in (1 x 30000) m3 of air.
    The average density is thus = (0.99 x 10326) / 30000 = 0.3408 kg / m3.
  8. If 50% of the earth’s atmosphere is below 5600 meters, what is the average density of the air at elevations between 0 and 5600 meters? In Question 4 above, we calculated the mass of air above a m2 on the earth’s surface to be 10326 kg. If 50% of this is contained in a column with a base of 1 m2 and a height of 5600 m, (0.5 x 10326)kg is contained in (1 x 5600) m3 of air.
    The average density is thus = (0.5 x 10326) / 5600 = 0.9219 kg / m3.
  9. The pressure of a sample of an ideal gas multiplied by its volume is constant if the temperature remains constant.
  10. P1V1 = P2V2 P2 = 2/1 x 101300 Pa P2 = 202600 Pa
  11. The pressure decreases as the bag travels upwards. The air expands as the pressure decreases and this increases the buoyancy of the bag. The cannon will speed up as it nears the surface and its momentum can cause it to collide with the bag as it breaks the surface and fall back down to the ocean floor.(and the divers)
  12. The lifting power of the balloon depends on the difference between tha mass of air displaced and the mass of the balloon.
  13. The mass of air displaced by the helium bag is = 10 x 1.18 = 11.8 kg.
    The mass of helium in the bag = 10 x 0.185 = 1.85 kg
    The mass of the bag is = 1 kg
    The lift created by the balloon is therefore equivalent to the weight of 11.8 kg – 1.85 kg – 1 kg = 8.95 kg
    This is equivalent to 8.95 kg x 9.81 N/kg = 87.8 N
  14. When the kinetic energy of the air increases, its pressure energy must decrease and visa versa.
  15. Yes.

 

Lesson 2.6 Heat Capacity

 

  1. Specific heat capacity of water = 4180 J/kg.ºC

  1. Energy traveling from one object to another object that is at a lower temperature is called HEAT
  2. If two objects are placed together in contact with each other and there is no net exchange of heat between them, the objects are in a state of THERMAL EQUILIBRIUM
  3. The quantity of energy needed to raise the temperature of one kilogram of a particular material by 1ºC (or 1K) is called the SPECIFIC HEAT CAPACITY of the material.
  4. The specific heat of fusion of a particular material is the quantity of energy needed to change 1 kilogram of the substance from SOLID to LIQUID at the same temperature.
  5. The specific heat of vaporization of a particular material is the quantity of energy needed to change 1 kilogram of the substance from LIQUID to VAPOR at the same temperature.
  6. Why is it called the specific heat of vaporization? In this context, the word "specific" refers to UNIT MASS. i.e. the heat of vaporization refers to 1 gram, 1 kilogram or 1 lb of material.
  7. Assume that the mass of the balloon is 1 kg.
    The potential energy of the water balloon = 1 x 9.81 x 30 = 294.3 J
    The specific heat capacity of water is 4180 J/kg.ºC.
    294.3 J will therefore increase the temperature of 1 kg of water by 294.3 / 4180 = 0.0704 ºC.
  8. The water increases its temperature by: (30ºC – 10ºC) = 22ºC.
    Using the equation:     H = m C D t , H = 0.3 x 4180 x 22 = 27588 J
    For the metal, 27588 = 0.5kg x C x (110 – 32)
    C = 27588 / (0.5 x 78) = 707.38 J / kg.ºC.
  9. If an electrically heated insulated container is rated at 300 Watts, how long will it take for 500 grams of ice (at 0ºC) in the container to be converted to steam at 100ºC?
    Energy needed to convert 0.5 kg of ice to water = 0.5 x 333000 = 166500 J.
    Energy needed to heat 0.5 kg of water from 0ºC to 100ºC = 0.5 x 4180 x 100 = 209000 J.
    Energy needed to vaporize 0.5 kg of water at 100ºC = 0.5 x 2280000 = 1140000 J.
    Total = 1515500 Joules.

 

 

Lesson 2.7 Expansion

  1. When a material is heated, the increase in energy causes the particles that make up a substance move faster and further. The movement of each particle has an effect on the adjacent particles. Increased movement of particles tends to drive the particles further apart.
  2. Yes. Because the forces of attraction between liquid particles are weaker than those between the particles that make up solids, they can move apart more easily.
  3. The forces of attraction between gas particles are much weaker than those between the particles that make up solids or liquids.
  4. The expansion of water as it cools from 4ºC to 0ºC is due to the formation of microscopic ice crystals in the water below 10ºC. Ice crystals are structured in such a way that the density of ice is less than that of water.
  5. Two metal strips joined together in such a way that it bends in one direction when the temperature is increased and bends in the other direction when the temperature is decreased.
  6. Large forces are created when metals expand and contract. Bending occurs because the coefficients of linear expansion of the two metals are not the same.
  7. The coefficient of linear thermal expansion of an object is the ratio of the change in length per ºC to its original length.
  8. If the coefficient of linear expansion of aluminum is 25 x10-6 ºC-1 and that of steel is 11 x10-6 ºC-1, will aluminum expand to a greater extent than steel when the temperatures of both substances are increased by the same amount? Yes.
  9. The coefficient of linear expansion of aluminum is 25 x10-6 ºC-1. If an aluminum pipe with a length of 5 meters at 15ºC is heated to 150ºC, by how much will its length increase?

. . . . . .D L
a = --------
. . . . L0 D T

= 25 x10-6 ºC-1

L0 = 5 m.

D T = (150 – 15) = 135ºC

D L = 25 x10-6 ºC-1 x 5m x 135ºC = 1.79 x10-2 m

10. The coefficient of volume expansion of gasoline is 950 x10-6 ºC-1. By how much will the volume of 1 cubic meter of gasoline increase if it is heated from 10ºC to 25ºC?

DV = 950 x10-6 ºC-1 x 1 m3 x (25ºC – 10ºC) = 1.425 x10-2 m3.

 

Lesson 2.8 Conduction & Convection

  1. Heat is energy in motion. Heat is often confused with internal energy but, by definition, heat isn’t heat unless it moves.
  2. Yes.
  3. Yes- with time.
  4. The copper bar because copper has a higher thermal conductivity than steel.
  5. Yes – in general.
  6. Heat is transferred by conduction when particles that make up the material vibrate or move faster and collide with or influence the adjacent particles. Metals atoms have outer electrons that are very loosely bound to the atoms. Because these electrons can move very easily, they transfer energy to adjacent particles more easily than the atoms would without the loosely bound electrons.
  7. Diamonds have a relatively high thermal conductivity and absorb energy rapidly from the fingers of the person holding the diamond. This makes a diamond feel cold.
  8. In general yes, but many solids are poorer conductors than many liquids.
  9. Mercury.
  10. Gases are invariably poorer conductors than liquids or solids.
  11. There is no difference.
  12. Natural convection occurs when the local differences in the densities of different parts of the fluid create buoyant forces that move less dense material upward and allow cooler, more dense material to move towards the source of heat.
  13. When a person’s finger is held above the candle, it is in the path of the natural convection current caused by the candle. Held towards the side of the candle, there is no conduction or significant convection current that will transfer the candle’s heat to the person’s finger.
  14. Surfing is best when there is an offshore breeze. Water has a much higher heat capacity than the solid materials on the earth’s surface. After sunset, and in the early morning, the air in contact with the water is less dense than the air in contact with the cooler land. As a result, the air moves from above the land towards the mass of water and creates an offshore breeze.
  15. The gas trapped between the fibers is a very poor conductor. The fibers prevent the formation of convection currents that would speed up the heat transfer process slightly.
  16. The fibers will conduct heat to some extent. Steel fibers are better conductors than the glass fibers and will thus have an effect on the quality of the material as an insulator.

 

 

Physics Lab 2.8 Vernier

Answers

  1. 3.7
  2. 6.4 (possibly 6.42 since the mark is just past the 4 but closer to the 4 than to the 5)