Why are gases described as fluid

In microgravity, a liquid forms a ball inside a free surface. Regardless of gravity, a liquid has a fixed volume. In the gas phase the molecular forces are very weak. A gas fills its container, taking both the shape and the volume of the container. Liquids and gases are called fluids because they can be made to flow, or move. In any fluid, the molecules themselves are in constant, random motion, colliding with each other and with the walls of any container.

The motion of fluids and the reaction to external forces are described by the Navier-Stokes Equations , which express a conservation of mass , momentum , and energy. The motion of solids and the reaction to external forces are described by Newton's Laws of Motion. Any substance can occur in any phase. Under standard atmospheric conditions , water exists as a liquid.

But if we lower the temperature below 0 degrees Celsius, or 32 degrees Fahrenheit, water changes its phase into a solid called ice. Similarly, if we heat a volume of water above degrees Celsius, or degrees Fahrenheit, water changes its phase into a gas called water vapor. Liquids and gases are considered to be fluids because they yield to shearing forces, whereas solids resist them.

We can understand the phases of matter and what constitutes a fluid by considering the forces between atoms that make up matter in the three phases. Connections: Submicroscopic Explanation of Solids and Liquids Atomic and molecular characteristics explain and underlie the macroscopic characteristics of solids and fluids.

This submicroscopic explanation is one theme of this text and is highlighted in the Things Great and Small features in Conservation of Momentum.

See, for example, microscopic description of collisions and momentum or microscopic description of pressure in a gas. This present section is devoted entirely to the submicroscopic explanation of solids and liquids.

PhET Explorations: States of Matter—Basics Heat, cool, and compress atoms and molecules and watch as they change between solid, liquid, and gas phases. Click to download the simulation. Run using Java. Conceptual Questions 1. What physical characteristic distinguishes a fluid from a solid? Which of the following substances are fluids at room temperature: air, mercury, water, glass?

The enthalpy of dry air measured from 0 o F has to be added to the enthalpy of the moisture water vapor or steam content of the air measured from 32 o F. The specific heat of air is 0. Example What is the enthalpy, measured from 0 o F, of 1 lb of Aluminum at o F? The specific heat of aluminum is 0.

The example states that the enthalpy of aluminum is to be considered as zero at 0 o F. Work measurement was established arbitrarily and by tradition in the same way as length, weight, time, heat, temperature and pressure. In I-P units a unit of work is done when 1 lb mass is lifted vertically against gravity through a distance of 1 foot. The unit is called foot-pound ft-lb. Similarly, when 1 kilogram mass is lifted vertically against gravity through a distance of 1 meter then the work done is 1 kilogram-meter.

If you lift this person off the ground by 1 foot then you have done ft-lbs of work. Work done is Force x Distance. Force is pressure per unit area. In Figure?? So work has been done. The process is not usually a straight line. Energy is defined as the capacity to do work. So energy can exist in many forms such as nuclear, chemical, electrical, fossil fuel, solar, wind and geothermal.

The basic forms of energy are potential, kinetic, mechanical, heat, internal, and electrical. Potential energy is due to position or state. For example a mass raised to some height above a reference level such as the earth's surface can be made to do work by letting it fall.

A compressed spring possesses potential energy because it can do work when it is released. Mechanical energy is possessed by any mass which is in motion. It is also referred to as kinetic energy. Kinetic energy is the energy possessed by a body in motion.

Electrical energy is obtained from electrical generators and batteries and consists of the flow of electrons in an electrical circuit. Heat energy is the result of the kinetic energy possessed by the atoms and molecules which make up the mass.

Internal energy of a gas is a function of temperature only and is independent of changes in pressure and volume and the symbol used is U. This is known as Joule's Law. In general for solids, liquids and gases, a change in internal energy dU is related to the change in heat content dQ and the change in work done dW. When one mass loses X btu of heat then the heat content of other masses must increase by the same X btu. There is no such thing as heat or any form of energy just disappearing into nothing.

Energy is transferred from one object to another or just stays where it is. The heat energy from fossil fuel can be converted into mechanical energy to drive turbines. The mechanical energy of turbines can be converted to electrical energy using generators. Electrical energy can be converted to mechanical energy using motors or converted to heat energy as in the case of resistance heaters.

This is the basis of the First Law of Thermodynamics which states that heat and work are interconvertible. If any system goes through a process during which work energy or heat energy is added or removed from the system, then none of the energy added is destroyed within the system and none of the energy removed is created within the system.

In other words energy is indestructible and the total energy in the universe remains the same. Another observed phenomenon is that heat will not of it's own accord flow up the gradient of temperature. For example if one object is at o F and another at 10 o F then heat will flow from the object at o F to the object at 10 o F and not the other way around.

It can be made to do so by applying some form of external energy such as mechanical energy as in the case of refrigeration equipment. However, external energy is not required for the transfer of heat from an object at a higher temperature to an object at a lower temperature as in the case of heating equipment.

This is basis of the Second Law of Thermodynamics which states that heat will not flow up the gradient of temperature. Neither of the Laws of Thermodynamics can be proved, but since no exceptions to the two laws have ever been observed, they are accepted as true.

The main use of Energy is used to do work. For example the heat energy from petrol is converted to mechanical energy to move an automobile. The process is reversible. Mechanical energy can be converted to heat energy. For example the mechanical energy applied to a rotor churning a liquid in a barrel is converted to heat energy and this will raise the temperature of the liquid.

The adiabatic process is essentially one where no heat is added or removed. In the P-H chart of Figure?? The process is instantaneous and there is no means for heat gain from the surrounding space or loss to the surrounding space. The compression process from 2 to 3 can also be considered adiabatic. The piston compresses the gas in the engine cylinder in a fraction of a second.

However, the gasrefrigerant does increase in enthalpy as shown in the P-H chart. The gain in enthalpy is the heat equivalent of the mechanical energy applied to the piston to compress the gas. The isothermal process occurs at constant temperature. In the evaporator, the liquid refrigerant absorbs heat from the space being cooled and is converted to gas.

The process consists of latent heat gain that vaporizes the liquid which occurs at constant temperature. In the condenser, the refrigerant gas is cooled and condenses into liquid. Latent heat is removed at constant temperature to convert gas to liquid. Note that the Pressure-Volume compression and expansion processes are not straight lines.

The same amount of work can be done over different time periods. Moving lbs through a distance of feet in hours is not the same as moving the same lbs through the same distance of feet in 1 hour. Power is defined as work done per unit time or the rate at which work is done. A unit of work commonly used in I-P units is the Horse-Power HP which is defined as 33, foot-pounds per minute or foot-pounds per second. This is used in mechanical engineering calculations. The electric HP is equivalent to 33, The metric HP is 32, However the unit of power commonly used in S-I units is the watt which is derived from electrical units.

He is currently a Research Professor with the Ph. Academic: B. Honors in Mathematics from St. Registered Professional Engineer P.

All our energy modeling courses are video based. What better way to learn energy modeling software than screen-casts of exactly how things are done? Hirsch respectively. Skip to Content Skip to Navigation. Become an Instructor Blog About. Create new account Request new password. Sign in with LinkedIn. Figure There is an explanation for why the Atlantic and Pacific Oceans are at slightly different levels and "locks" have to be used to move ships up and down through the Panama Canal.

Absolute Temperature T : The relationship between the volume and temperature of a gas is linear. The volume of a gas Figure - If the lines representing the linear relationship between gas temperature and gas volume, for any element or compound in the gaseous state, then at a certain low temperature the theoretical volume of the gas will become zero. Figure - Solids and liquids can be considered incompressible. Heat Q Heat is a form of energy just like nuclear, chemical, potential, kinetic and electrical energy.

Specific Heat Sh Specific Heat Sh of a substance is the amount of heat required to raise 1 lb of the substance through 1 o F. Figure- Specific heat can vary with temperature. Measuring Temperature Solids, liquids and gases expand with increasing temperature and contract with decreasing temperature. Measuring Heat There is no instrument for measuring the quantity of heat directly.

All heat quantities would be positive if heat was measured from absolute zero temperature o C or o F Example: 10 lbs of aluminum with a specific heat of 0. Figure - Dry Air: The term "Air" refers to a mixture of gases which can include small quantities of steam H 2 O , water that evaporates into the air under various conditions , carbon dioxide CO 2 , breathed out by animal life , and artificially added pollutants such as carbon monoxide CO, emitted by automobiles and other processes.

Atmospheric Air Pressure The term "air" when referred to in comfort air-conditioning consists of water vapor and dry air DA which consists of the rest of gases in the mixture of gases.

Figure - Table - Gauge Pressure Every surface or object on the earth has the weight of the atmosphere resting on it. Figure - Measuring Pressure Figure - The instrument used to measure atmospheric pressure is called a barometer. Water Vapor Steam and Vapor Pressure Water vapor is a term used to describe the existence of steam in air. This is because water vapor steam can condense into a liquid water within the earth's range of atmospheric pressures and temperatures, whereas all the other gases in the air mixture will remain a gas Table-x shows that steam can exist in air and water can exist as water over a very wide range of temperatures depending on the partial pressure of the steam in the air mixture and the pressure under which the water exists.

Table - Dalton's Law of Partial Pressures Dalton's Law states that the pressure exerted by a mixture of gases including vapors is equal to the sum of the pressures of each gas in the mixture. Water Vapor in Air The relative weight of water vapor in 1 lb of air is extremely small.

Steam Besides air and water, the medium that is used extensively in building environmental control is steam. Figure - Table - Figure?? Gas can be compressed much more easily than a liquid or solid. Think about a diving tank — L of gas is compressed into a 3 L cylinder. Right now, you are breathing in air — a mixture of gases containing many elements such as oxygen and nitrogen.

Water vapour is the gaseous form or state of water. Unlike ice or water, water vapour is invisible. We exhale water vapour whenever we breathe out. We cannot see the water vapour as we exhale, but if we hold our eyeglasses or smartphone to our mouths, we can see the water vapour condensing becoming liquid on these objects. One state is plasma, which naturally occurs in lightning, and we create it in fluorescent light bulbs and plasma TVs. Another state of matter is Bose-Einstein condensate, but this state only occurs with super-low temperatures.

Science knowledge changes as we discover new evidence. Technology helps us find this evidence. To learn more about plasmas and Bose-Einstein condensates, read these two articles that look at these science ideas and concepts. States of matter Matter in our world. Use the interactive Moisture sources in our homes to find out how moisture enters our homes and how we can minimise and remove it.