2.9.6 Read the text, translate it and choose the right form from brackets. Expansion of Gases

Just as we can cool a gas by (make, making) it do work, so we can heat it by (do, doing) work upon it. Suppose, instead of letting the gas push the piston we (apply, appling) power to the piston and make it push the gas. This speeds up its molecules and makes it hot. It follows, then, that if we (compress, compressing) a gas it becomes hotter. The (better, best) example of this is seen in a bicyclе pump, which becomes very (warm, warmest) when a tyre is inflated. Yon might think this was due to the friction of the piston, but if you try working the pump without a tyre, you will find it does not heat up (noticeable, noticeably).

The expansion of a compressed gas is used in driving steam-engines, petrol-engines, hot-air-engines, etc.

Gases expand very (larger, largely) when they are heated. We saw that a cubic foot of steel expanded by 5 cubic inches when heated from 0 °C to 78 °C, while a cubic foot of alcohol expanded 150 cubic inches over the same range. A cubiс foot of air when heated from 0 °C to 78 °C expands by no less than 493 cubic inches. An interesting thing is that all gases expand to exactly the same extent when heated under the same conditions, which is by no means true for liquid or solids. When a liquid or solid is heated and expands there are two forces at work. The molecules are speeded up and so tend to swing in (bigger, biggest) orbits or to get further from each other. This effect is the same for all solids or liquids. But the attraction of the molecules opposes this effect; consequently substances whose molecules attract each other (strong, strong­ly) will expand little and vice versa. But the molecules of gases are too far from each other to attract each other appreciably, so the effect of heating them is simply to increase the speed and energy of the molecules and make them bounce off each other (hard, harder) and so fly farth­er apart. As the same rise of temperature means the same increase of energy, all gases expand (equal, equally).

The expansion of gases is very large, but it is not very useful for measuring temperatures because they expand and contract not only when the temperature alters but also when the air pressure alters. The expansion of a gas is sometimes used to measure rather high or very (low, lower) temperatures and also for very accurate work. An air thermometer is rather a difficult affair to handle, and it is used only in the laboratory.

2.9.7 Try to explain your choice grammatically.

2.9.8 Read the text. Find the definitions of Brownian motion and specific heat of a gas. Summarize the text into 8 main sentences. Kinetic Theory of Gases

Brownian Motions. — The simplest and most direct evidence for the existence of molecules was first noted by an English botanist by the name of Brown. With a microscope he observed very fine particles held in suspension in water and noted that these fine particles are con­stantly in motion. The smaller the particles the more freely do they move. The motion of these particles is caused by the incessant bombardment of the molecules of the water or other liquid in which they are suspended. This bombardment of the water molecules is not the same on the different sides of the particles. Hence they are driv­en hither and thither. An approximate picture of the behavior of such small particles is obtained by projecting on a screen the shadows of finely divided glass particles that are set in motion by rapidly boiling mercury.

Perrin and others who have made careful studies of these motions have found that the distribution of these particles, their velocities, and their mean free paths are precisely what should be expected from the kinetic theory of gases. From these observations it is possible to de­termine the number of molecules in 1 cu cm of a gas under standard conditions. The fact that the number of molecules per cubic centimeter in a gas as determined in this way is in good agreement with the number derived from the methods involving the kinetic theory of gas­es shows that the motion of these particles obeys the same general laws as the motion of molecules.

Basic Assumptions. — To explain the physical properties of gases, three basic assumptions are necessary:

1. The molecules of a gas are extremely small, perfectly elastic spheres. This assumption implies that when molecules of gas collide with other molecules or with the walls of the containing vessel, the total kinetic energy of the molecules is not diminished in any way.

2. The molecules move with changing velocities through the space occupied by the gas. Between collisions, their paths are straight lines. This assumption implies that the forces acting on the molecules are neg­ligible except at collision.

3. The time occupied in a collision between two molecules or in a collision of a molecule with the wall is small compared with the time between collisions. This assumption implies that a collision is nearly instantaneous.

Specific Heats of Gases. — The specific heat of a gas depends on whether the gas is heated at constant volume or at constant pressure. These two specific heats are known as specific heat at constant pres­sure and specific heat at constant volume.

Specific Heat at Constant Volume.—When heat is supplied to a gas in which the volume is kept constant, the pressure increases, and all the energy which is supplied to the gas is used to increase the kinetic energy of the molecules. There is no external work done by the gas. When the temperature of 1 g of the gas is raised through l°C, the gas will absorb C_v units of heat, and this quantity of heat is its specific heat at constant volume.

Specific Heat at Constant Pressure. — In heating a gas 1 °C, at constant pressure the heat required to increase the speed of the molecules will be the same as it was in case the gas was heated an equal amount at constant volume. In addition to this heat, it is necessary to supply a certain amount of heat to do external work while the gas is expanding. For example, if the gas is expanding in a cylinder closed by a moving piston, the molecules after colliding with the piston will rebound with less energy than that with which they reached the piston. Additional energy must be supplied to make up this decrease. Consequently, the specific heat at constant pressure must exceed the specific heat at constant volume by an amount which is just equal to the thermal equivalent of the work which is done when unit mass of gas is heated through 1 °C at constant pressure.

The ratio of the specific heat of a gas at constant pressure C_p to the specific heat at constant volume C_v is

C_p / = 1.41 for air

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k = C_v /= 1.66 for mercury vapor.