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THE UBIQUITOUS HAIR DRYER

THE UBIQUITOUS HAIR DRYER. P. A QUICK OVERVIEW. We are going to look at: Briefly how it works and mention some of the physics we have covered in class. These will be highlighted in bold for your reference. The emphasis is on the process of drying your hair. P. THE HAIRDRYER.

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THE UBIQUITOUS HAIR DRYER

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  1. THE UBIQUITOUS HAIR DRYER P

  2. A QUICK OVERVIEW • We are going to look at: • Briefly how it works and mention some of the physics we have covered in class. These will be highlighted in bold for your reference. • The emphasis is on the process of drying your hair. P

  3. THE HAIRDRYER You can find a hair dryer like this one in almost any drug or discount store. This model has two switches, one to turn it on and off and one to control the rate of airflow. Some models have an extra switch that also lets you regulate the temperature of the airflow. P

  4. HAIRDYER – How it works. • It takes energy in the form of electricity • Which drives a motorized fan and a heating element. • The fan causes air to flow at speed in one direction through a tube. P

  5. HEATING THINGS UP • As it pulls air at room temperature through a large opening the fan forces the air through a heating element warming the air that comes out through a smaller outlet (nozzle) at a much higher temperature. P

  6. HAIRDYER – How it works. • Forced convectionoccurs when the motion of the fluid (air) is maintained by some external agency such as a fan or pump. • The essential ingredients of forced convection heat transfer analysis are given by Newton's Law of Cooling, • A convection coefficient, h, can again be defined such that Q=h * A * delta T • where A is the area of the body and  T the temperature difference between body and fluid.  However hdepends on many factors such as shape and orientation of surface; density, viscosity, specific heat and thermal conductivity of fluid. P

  7. HAIRDYER – Safety Check A bimetallic switch is made out of sheets of two metals. Both metals expand when heated, but at different rates. When the temperature rises inside the hair dryer, the strip heats up and bends because one metal sheet has grown larger than the other. When it reaches a certain point, it trips a switch that cuts off power to the hair dryer.

  8. DRYING YOUR HAIR The warmed air coming from the hair dryer is where we want to focus and that focus is on the physics of: EVAPORATION S

  9. EVAPORATION • Four factors affect the rate of evaporation. These are: • temperature, • humidity, • wind speed (flow of air) • the surface area of the liquid. S

  10. First Some Definitions • temperature - the degree of hotness or coldness of a body or an environment. The average kinetic energy of atoms & molecules. • evaporation - to change from a liquid into the gaseous state • condensation - to change from a gas into the liquid state. • liquid - the state of matter in which a substance exhibits an ability to flow. • gas - the state of matter distinguished by relatively low density, an ability to flow • heat - a form of energy associated with the motion of atoms or molecules S

  11. EVAPORATION • HEAT • Heat energy is the end point of many (and, eventually, all) physical processes. • Important: You must understand that although heat is a form of kinetic energy, molecules in motion, it is random kinetic energy. Thus a flying bullet has a lot of kinetic energy, but the motion of the molecules is hardly random. • Heat is the total random kinetic energy of the molecules in an object. S

  12. EVAPORATION • Heat MeasurementA temperature reading gives us the heat intensity of a substance and not the actual quantity of heat.Heat quantity is measured in "KILOCALORIES" (KCAL). One KCAL is the most amount of heat required to raise the temperature of one kilogram of water by one degree Celsius (at sea level). P

  13. EVAPORATION • Evaporation is the process whereby atoms or molecules in a liquid state gain sufficient energy to enter the gaseous state. It's the opposite process of condensation. • Evaporation is a critical component of the water cycle, which is responsible for clouds and rain. Solar energy drives evaporation of water from oceans, lakes, moisture in the soil, and other sources of water. • So the sun and wind is the Earth’s Hair Dryer!! S

  14. EVAPORATION – Whats happening. • The molecules in a liquid are in motion -- they bounce around at different speeds and directions (Kinetic Energy) -- some faster, some slower than average.That average depends upon the temperature of the liquid.If the temperature is high enough, some of the molecules near or at the surface will be kicked so hard that they end up in the space above the liquid or solid. P

  15. EVAPORATION – Whats happening. • There will be other molecules in this space that will collide and move the molecule further from the liquid or solid boundary, or bounce the molecule back into the liquid. The number of molecules in this gas (also called vapor) depends on this average temperature. P

  16. EVAPORATION • What is Humidity To be specific we must say relative humidity (usually meant when the term humidity alone is used). • The ratio of the actual water-vapor content of the air to its total capacity at the given temperature. Humidity expressed as a %age of the saturation density. S

  17. Evaporation: Relative Humidity First, the warmer air is, the more water vapor it can "hold." Relative humidity is a measure of the amount of water in the air compared with the amount of water the air can hold at the temperature it happens to be when you measure it. Amount of water vapor air can hold at various temperatures: 30 C: 30 grams per cubic meter of air 20 C: 17 grams per cubic meter of air 10 C: 9 grams per cubic meter of air Note: Dew point is a measure of how much water vapor is actually in the air. S

  18. EVAPORATION • How we express Relative Humidity (RH) • RH = Humidity / Saturation Density X 100% P

  19. EVAPORATION • How does relative humidity affect evaporation? The rate of evaporation decreases as the moisture content of the air increases and approaches saturation. In addition, the saturation point (moisture-holding capacity of the air) increases rapidly as the temperature of the air rises. S

  20. Lets Experiment • You have two strips of absorbent paper one marked A and the other marked B. • Apply a droplet of water to each strip. • Hold them both up in front of you one in each hand A in your left hand and B in your right. • Gently blow on the strip marked B in your right hand for a couple of minutes. • WHAT HAPPENS? P

  21. EVAPORATION • Simply then: • EVAPORATION - is the process that converts liquid, in this case water, into water vapor. The vapor is further diffused into the atmosphere. In this way water is lost from the surface, no matter whether it is from open water (lakes, rivers) or from YOUR HAIR. S

  22. EVAPORATION • The End • (Drippy and Questions) P

  23. Evaporation: Natures Grand Scale Evaporation Pond in The Salton Sea, California

  24. EVAPORATION • For Future Teachers here is a glimpse of Drippy. That explains the water cycle.

  25. EVAPORATION

  26. EVAPORATION For more on Mr Drippy please visit this clickable link here. http://www.kimballmedia.com/Drippy/DrippysWorldTrialStories/ToMountainsAndBack/Page1.htm

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