Unit A: Mix and Flow of Matter

Transcription

1 Unit A: Mix and Flow of Matter Science 8 1 Section 3.0 THE PROPERTIES OF GASES AND LIQUIDS CAN BE EXPLAINED BY THE PARTICLE MODEL OF MATTER. 2 1

2 Viscosity and the Effects of Temperature Topic Viscosity How quickly fluids flow is a property called. It is determined by a fluid s or that keeps it from flowing. Consider the particle model of matter: The greater the friction or rubbing between particles in any fluid, the higher the viscosity. Fluids with (honey) do not flow as easily as fluids with a (water). 4 2

3 Temperature and Viscosity A fluid with a high viscosity has a large amount of internal resistance or friction. of a liquid = of a liquid = This means that 5 Temperature and Viscosity (continued) Recall: In the particle model, a liquid is made of particles that can slide and roll over each other. When energy or heat is added to the liquid, the particles slide and roll more quickly. As a result, The reverse is also true. As the temperature of the liquid drops, the particles 6 3

4 Homework! Check and Reflect Page 41 # 1, 2, 3, 4, 5 7 Density of Fluids Topic

5 Density is the amount of matter in a given volume. The density of a fluid or any other kind of substance depends on the particles it is made of. The greater the density of the fluid, the heavier it is and the lower it will sink in a column. Lighter density fluids will float above heavier density fluids. 9 Calculating Density Density is the mass per unit of volume, which can be measured in ml or cm 3. Mass Density = or d = m Volume V The units for the density of liquids and gases are usually g/ml or kilograms per litre kg/l. The units for the density of solids are usually g/cm

6 Homework! Check and Reflect Page 46 # 1, 2, 4, 5 11 Density, Temperature, and Buoyancy Topic

7 Density and Temperature The particle model of matter states that for each substance, the number of particles in a given volume remains constant if the temperature is kept constant. Density does not change as long as 13 Density Changes Think about swimming in a lake on a hot summer day. The water on the surface of the lake is noticeably warmer than the water below it. The warm water floats on the cold water because it has a lower density than the cold water has. In other words, 14 7

8 Different Temperature, Different Density One substance can have different densities depending on its temperature. What happens to a substance as it is heated? It changes state: solid to liquid to gas. 15 Salt vs. Fresh Water Do you think it would be easier to float in salt water or fresh water? Recall from the particle model of matter that dissolving one substance (salt) in another (water) in a given volume. By adding more particles, you of the water solution. That means denser objects can float in the solution now than could in the distilled water. So, 16 8

9 Buoyancy When an object is in a liquid, the force of gravity pulls it down. The liquid, however, exerts an opposite force, called the buoyant force, that pushes the object upward. Buoyancy is 17 Floating Objects What happens when the density of the liquid is greater than the density of the object? The buoyant force of the liquid on the object is 18 9

10 Sinking Objects What happens when the density of the object is greater than that of the liquid? The force of gravity acting on the object will be 19 Buoyancy Applications: Plimsoll Line Because of density variations in the world s oceans and rivers, all cargo ships have what is known as a Plimsoll line painted on their hulls. The Plimsoll line shows how heavily a ship can be The marks on the left indicate where the waterline should be in fresh water. The marks on the right indicate where it should be in salt water

11 Buoyancy Applications: Hot Air Balloons As the air in the balloon is heated, it becomes less dense than the surrounding air. The buoyant force pushes the balloon up into the air. The balloon stops rising when the buoyant force equals the force of gravity. That s the point when the balloonist stops adding heat to the air in the balloon. 21 Homework! Check and Reflect Page 52 # 1,

12 Compression of Fluids Topic Compressibility Objects under compression tend to in shape. For example, when you kick a soccer ball, the force of your foot compresses the ball and temporarily deforms it. In this example, your foot is actually compressing the fluid (air) that fills the ball

13 Compressibility in Gases Gases can be compressed much more than liquids can. Example: Think about squeezing a sealed plastic bottle when it s full of juice and then when it s empty. Gas particles have Very little compression occurs in liquids. Materials in a liquid state are said to be ; that is, they cannot be compressed easily. This property of liquids is very useful. 25 Homework! Check and Reflect Page 56 # 1,

14 Pressure in Fluids Pascal s Law Topic Pressure Pressure is the. It is measured in (Pa). A pascal equals the force of 1 N (newton) over an area of 1 m 2. 1 Pa = 1 N 1 m 2 The more force you can apply to a given area, the the pressure

15 Calculating Pressure You can write this relationship as an equation: p = F A where p is pressure, F is force, and A is area. Note: 1 kpa = 1000 Pa (1 kilopascal = 1000 pascals) Pressure can also be measured in N/cm Pressure and Depth The greater the depth of water, the greater the pressure at that point. What do you think would happen if you put holes in the barrel at different depths? 30 15

16 Pascal s Law Pascal s law states 31 Applications of Pascal s Law Pascal s discovery of this law led to the invention of many different types of hydraulic and pneumatic devices

17 Hydraulic Devices A hydraulic device is used for lifting cars. You may have noticed these in car repair garages. Such a device uses two pistons of different sizes to create pressure and to lift the car. A piston is a disk that moves inside a cylinder. The small piston is the input piston, which pushes down on the liquid to create pressure. This pressure is then transmitted through the liquid where it pushes up on the large piston, which is the output piston. Recall that pressure equals force divided by area (p = F ). You A can see that the output piston has a much larger area than the input piston does, but the pressure is the same everywhere in the system. So, because p = F, the force of the larger piston is A greater than the force of the smaller piston. 33 Hydraulic Devices The area of the output piston in this example is 16 times larger than the area of the input piston. The result is an output force 16 times greater than the input force a force strong enough to lift a car! One of the benefits of a hydraulic system is that it can multiply force. However, to move the large piston, the small piston must move much farther that the large piston does

18 Pneumatic Devices Pneumatic devices use compressed air to do tasks. Dentists drills, jack hammers, paint sprayers, and air brakes on trucks are all examples of pneumatic devices. Reasonable cost and safety are two advantages of pneumatic systems. Compressed air is cheap and safe, as the devices do not create sparks within the system. Pneumatic devices are also free of electrical hazards, which is one reason that dentists drills are pneumatic. 35 Homework! Check and Reflect Page 60 # 1, 2, 3, 4 Section Review Page 61 #