Ch. 1.2 Pressure in Fluid Systems

States of Matter Matter can exist in four states: solid, liquid, gas and plasma. Atomic and molecular motion is different in each state. In a solid, molecules are constantly moving, but they do not move around in the solid – they vibrate about fixed positions. In a solid, molecules are constantly moving, but they do not move around in the solid – they vibrate about fixed positions.

States of Matter - 1 In a liquid, the molecules are not in fixed positions. In a liquid, the molecules are not in fixed positions. The molecules continue to vibrate, but they can now easily slide over one another and move about throughout the liquid. The molecules continue to vibrate, but they can now easily slide over one another and move about throughout the liquid. The shape of a liquid is not fixed – it takes the shape of its container. The shape of a liquid is not fixed – it takes the shape of its container.

States of Matter - 2 In the gaseous state, the molecules move freely about the container. In the gaseous state, the molecules move freely about the container. When a molecule in the gas hits another molecule or the side of the container, it bounces like a billiard ball hitting another billiard ball or a wall. When a molecule in the gas hits another molecule or the side of the container, it bounces like a billiard ball hitting another billiard ball or a wall. The gas also takes the shape of its container. The gas also takes the shape of its container.

States of Matter - 3

States of Matter - 4 A gas free electrons and ions due to the high temperatures at which it exists is called plasma. The stars, including our sun, and most intergalactic matter are in this state. The stars, including our sun, and most intergalactic matter are in this state.

Fluids A fluid is a material that can flow, has no definite shape of its own, and conforms to the shape of its container. Both liquids and gases are fluids. Fluid systems use liquids or gases to operate mechanical devices or to circulate fluids. The fluid used by the system is called the working fluid.

Fluids - 1 A hydraulic system uses a liquid as the working fluid and a pneumatic system uses a gas as the working fluid. A hydraulic system uses a liquid as the working fluid and a pneumatic system uses a gas as the working fluid. Pressure is the prime mover in a fluid system. Pressure is the prime mover in a fluid system.

Density The density of a material is the amount of matter per unit volume. Density is represented by the Greek letter  (rho). The density of a material is the amount of matter per unit volume. Density is represented by the Greek letter  (rho). The formula for density is Density(  ) = mass(m)/volume(v) The formula for density is Density(  ) = mass(m)/volume(v) In SI units, mass is measured in kilograms (kg), volume is measured in cubic meters (m 3 ). Thus, the unit for density is expressed in kg/m 3. In SI units, mass is measured in kilograms (kg), volume is measured in cubic meters (m 3 ). Thus, the unit for density is expressed in kg/m 3.

Density - 1 Density may also be expressed in units of grams(g) and cubic centimeters (cm 3 ) or milliliters (mL). Density may also be expressed in units of grams(g) and cubic centimeters (cm 3 ) or milliliters (mL). Density is sometimes written as a comparison of an object’s weight to its volume. This ratio is called weight density. Density is sometimes written as a comparison of an object’s weight to its volume. This ratio is called weight density.

Density - 2 The formula for weight density is: The formula for weight density is: Weight density (  w )=weight/volume(V) Weight density (  w )=weight/volume(V) In SI units, weight is measured in Newtons(N) and volume is measured in cubic meters (m 3 ). Therefore, the unit for weight density is N/m 3. In SI units, weight is measured in Newtons(N) and volume is measured in cubic meters (m 3 ). Therefore, the unit for weight density is N/m 3.

Specific gravity The density of substances is often described by stating the ratio of the density of the substance to the density of water. This ratio is called specific gravity. The formula for specific gravity is: The density of substances is often described by stating the ratio of the density of the substance to the density of water. This ratio is called specific gravity. The formula for specific gravity is: sg =  substance /  water sg =  substance /  water The density of water is 1000 kg/m 3. Specific gravity has no units. The density of water is 1000 kg/m 3. Specific gravity has no units.

Pressure Pressure is defined as the force (F) divided by the area (A) over which it acts. Pressure is defined as the force (F) divided by the area (A) over which it acts. The formula for pressure is The formula for pressure is Pressure (P) = force(F)/area(A) Pressure (P) = force(F)/area(A) The unit for pressure is N/m 2. However, we don’t use N/m 2 as the unit for pressure. 1 N/m 2 = 1 pascal (Pa), so we use pascals as the unit for pressure. The kPa is more commonly used and is equal to 1000 Pa. The unit for pressure is N/m 2. However, we don’t use N/m 2 as the unit for pressure. 1 N/m 2 = 1 pascal (Pa), so we use pascals as the unit for pressure. The kPa is more commonly used and is equal to 1000 Pa.

Buoyancy The upward force on an object immersed in a fluid is called the buoyant force. The upward force on an object immersed in a fluid is called the buoyant force. Archimedes’ principle states that: Archimedes’ principle states that: An object immersed in a fluid has an upward force exerted on it equal to the weight of the fluid displaced by the object.

Buoyancy - 1 You can also use densities to predict whether an object will sink or float when placed in a fluid. You can also use densities to predict whether an object will sink or float when placed in a fluid. If the object has a greater density that the fluid, it will sink. If the object has a lower density that the fluid, it willfloat. If the object has a lower density that the fluid, it will float.

Pascal’s principle Pascal’s principle states that: Pascal’s principle states that: A change in pressure at any point in a confined fluid is transmitted equally throughout the fluid. A change in pressure at any point in a confined fluid is transmitted equally throughout the fluid. Pascal’s principle is the basis Pascal’s principle is the basis of operation of hydraulic of operation of hydraulic lifts. lifts.

Atmospheric Pressure A barometer is an instrument used for measuring atmospheric pressure. A barometer is an instrument used for measuring atmospheric pressure. At sea level, the average atmospheric pressure is kPa. This pressure is also referred as one atmosphere. At sea level, the average atmospheric pressure is kPa. This pressure is also referred as one atmosphere. Absolute pressure is the total pressure measured above zero pressure (a perfect vacuum). Absolute pressure is the total pressure measured above zero pressure (a perfect vacuum).

Atmospheric Pressure - 1 Gage pressure is the pressure measured above atmospheric pressure. Gage pressure is the pressure measured above atmospheric pressure. Absolute, atmospheric, and gage pressures are related in a simple equation: Absolute, atmospheric, and gage pressures are related in a simple equation: Absolute pressure = gage pressure + atmospheric pressure

Equilibrium in Fluid Systems Liquid or gas moves in a fluid system when pressure differences exist between different points in the system. Liquid or gas moves in a fluid system when pressure differences exist between different points in the system. If there is no difference in pressure, there’s no movement and the system is in equilibrium. If there is no difference in pressure, there’s no movement and the system is in equilibrium.