Thermal ppt
-
Upload
praveen-kombucha-mushroom -
Category
Science
-
view
117 -
download
7
description
Transcript of Thermal ppt
What did we learn yesterday?
ConvectionLearning Outcome• Describe convection in fluids (liquids and gases) in
terms of density changes.
ConvectionConvection is the transfer of thermal energy by means of convection currents in a fluid (liquid or gas), due to a difference in density.
Conduction vs Convection
• In conduction, thermal energy is transferred from one particle to another.
• In convection, thermal energy is transferred through the actual movement of the heated particles from the warmer to cooler parts of the fluid.
Convection in Liquids• When the water at the bottom of the
flask is heated, it expands.
• The expanded water is less dense than the surrounding water and rises.
• Since the upper region is cooler, it is denser and therefore sinks.
• The difference in the densities of water in the different regions sets up a convection current.
• This is shown by purple streams rising from the bottom and sinking at the sides.
Convection in Gases• When the air above the candle is
heated, it expands.
• The expanded air is less dense than the surrounding air and rises out of chimney B.
• Since the surrounding air is cooler, it is denser and therefore sinks into chimney A.
• The difference in the densities of the air at the different chimneys sets up a convection current.
How are Land and Sea Breezes Formed?
Discuss with you partners!
Land & Sea Breeze• In the day, the land heats up faster than the
sea. The air above the land is heated, expands and rises.
• Cool air above the sea is denser and moves in to replace the warmer air. This sets up a sea breeze.
• At night, the land cools faster than the sea.
• The air above the land is now cooler than the air above the sea.
• A convection current is set up in the opposite direction. This is called a land breeze.
Radiation
Learning Objectives• explain energy transfer of a body by radiation• state the factors that affect the rate of energy
transfer by radiation
RadiationRadiation is the transfer of thermal energy in the form of
electromagnetic waves without the aid of a medium.
• All bodies emit infrared radiation • Infrared radiation does not require a medium to be
transmitted. This is how the Earth is warmed by the Sun
Electromagnetic Waves
Effect of Colour and Texture onEmission Rate of Infrared Radiation
Both tins are filled with water, heated to the same
temperature and connected to a data logger
Effect of Colour and Texture onEmission Rate of Infrared Absorption
The pieces of foil need to be positioned at equal distances
from the source of heat in order for the experiment to
be fair
Other Factors affecting Rate of Infrared Radiation Absorption/Emission• Surface areaFor two objects of identical mass and material, the object with the larger surface area will emit or absorb infrared radiation at a higher rate.• Surface temperature
The higher the temperature of an object relative to its surrounding temperature, the higher the rate of emission of infrared radiation.
Applications of Transfer of Thermal Energy
Learning OutcomeAt the end of this section, you should be able to:
• describe how the concept of thermal energy transfer by conduction, convection and radiation applies to everyday life.
Conductors
A material through which heat can move easily is called a conductor.
Iron, silver, aluminum, stainless steel, and copper are all examples of metals that are good conductors.
Insulators
A material that heat does not move through easily is an insulator.
Plastic, wood, rubber, and glass are all good insulators.
Many cooking utensils and handles are made out of plastic, wood, or rubber.
Air is a good insulator.
Geothermal Power Station
• Explain how water is used to generate electricity in geothermal power stations.
• How could the system be improved to make it more efficient?
Geothermal Power Station
Plastic StopperVacuum Flask
• Using the components given, explain how the vacuum flask manages to keep the water at a steady temperature. If possible, also explain how the material of the components affect the heat transfer.
Vacuum Flask
Black Chrome PlateInsulating Material
Outlet Connection
Inlet Connection Glass Cover
ss
s
sss
Collector Housing made from aluminium alloy
Flow Tubes
Solar Panel
• Using the components given, explain how heat transfer occurs in a solar panel. If possible, also explain how the material of the components affect the heat transfer.
Solar Panel
Solar Water Heater
Dddddddddddddddddd dddddddd
Dddddddddddddddddd dddddddd
Dddddddddddddddddd dddddddd
• Using the components given, explain how heat transfer occurs in a solar water heater. If possible, also explain how the material of the components affect the heat transfer.
Solar Water Heater
f
Household Water System
f
f
• Using the components given, explain how the water system functions.
Household Water System
A saucepan with a thick copper base contains water and is placed on a flat electric hot plate.
(a)State the process by which thermal energy is
(i) transferred from the hot plate to the water;
(ii) transferred through the water.
(b)The sides of a saucepan are often polished, but the base is usually dark-coloured. Why is this so?
Saucepan
freezing unit
Refrigerator• The freezing unit of the refrigerator is
placed at the top.
• Why this this the case? Draw arrows to show the convection current within the refrigerator
What have we learnt today?
• How conductors and insulators can aid or reduce the rate of heat trasnfer
• How conduction, convection and radiation can all play a part at the same time to affect the transfer of heat