Science 7 Chapter 6 Section 1

Transcription

1 Science 7 Chapter 6 Section 1 Processes of Transferring Heat Processes of Heat Transfer There are 3 processes responsible for heat transfer: 1) Conduction 2) Convection 3) Radiation 1

2 Conduction Conduction: the transfer of thermal energy that occurs when warmer particles come in contact with cooler particles and transfer energy to the cooler particles When particles on the surface of an object are heated by a heat source, they vibrate with more kinetic energy. This increased vibration causes the surface particles to collide with neighboring particles. The collisions transfer energy to those neighboring particles, which in turn vibrate faster and collide with their neighbors. This process continues with each particle colliding with other nearby particles until additional kinetic energy is transferred to all particles of an object. Conduction (cont...) Conduction works well in solids (particularly metals). There are some solids which will not conduct heat. In liquids and gases, conduction is less effective. There is less chance of collisions because particles are further apart. 2

3 Convection Convection: the process in which a warm gas or liquid moves from one place to another, carrying heat with it. Near a heat source, the particles of a liquid or gas move faster and further apart. This causes it to be lighter than the cooler particles. Hence, the warm particles rise. Cooler particles move into the empty space and those particles get warmed. The original warm rises, cools as it rises and sinks again. This process is known as a convection current. Convection (cont...) Convection currents: patterns of movement where a warm gas or liquid rises and cooler gas or liquid falls to replace it. There are several examples of convection currents in nature, such as winds and geothermal activity 3

4 Convection currents Radiation - Types Radiation: the transfer of energy in a wave-like form Specifically, radiation transfers heat and other energy forms in electromagnetic waves. There are many types of electromagnetic waves, such as: Radio waves: carries radio and TV signals Microwaves: carry information to and from satellites (also used in microwave ovens) Infrared waves: heat waves Visible light: ones you can see (colours) 4

5 Radiation - Types (cont...) Ultraviolet waves: come from the Sun and tanning beds; cause sunburns X-rays: used by physicians to see inside people Gamma rays: most powerful form of radiation; come from radioactive sources (nuclear power plants, atomic bombs) Radiation and Matter When radiation strikes an object, several things could happen, such as: Absorption Reflection Transmission 5

6 Radiation - Absorption Absorption: radiation is taken into an object. When this happens, the object gets warmer ex. black surfaces absorb heat radiation well Black surfaces also give off, or emit, heat as well ex. pavement on a summer day Radiation - Reflection reflection: radiation bounces off an object. When this happens, the temperature generally does not change. ex. white surfaces and shiny surfaces reflect well 6

7 Radiation - Transmission Transmission: radiation passes through an object. When this happens, the temperature generally doesn't change ex. clear objects transmit well Heating Homes The Past Early ways of heating homes involved using open fires (fireplaces) or contained fires (stoves) Radiant energy from the fire or stove heated close objects while convection currents helped spread heat in the room Room with the fire was warm, but other rooms were cold 7

8 Heating Homes The Present Most homes today have furnaces with ducts that run into all the rooms, or electric heaters Forced air heating happens when air is drawn into a furnace, is heated, and blown by fans through the ducts Some furnaces use water to transfer heat rather than air (hot water heating) Heating Homes The Future Because of rising fuel costs and environmental concerns, people are looking for new ways to heat their homes. A couple of new technologies that could be used are: Air-to-air heat pumps Geothermal heat pumps 8

9 Air-to-air heat pumps In the summer, the fluid absorbs heat from inside the house and pumps it outdoors. It does this by evaporating into a gas inside, being forced outside and a compressor condenses the gas back to a liquid Air-to-air pumps (cont...) In the winter, the pump is reversed. The fluid absorbs heat, changes to a gas, and this gas is pumped into the house where it condenses back to a liquid (releasing heat) 9

10 Geothermal heat pumps Geothermal pumps work like air-to-air pumps, except they exchange heat with the ground, not the air. More expensive, but more efficient. Temperature below ground is constant. Pumps go into the ground. In the summer, heat is pumped into the ground. In the winter, heat is taken from the ground Science 7 - Chapter 6 Section 2 Conductors and Insulators (pp ) 10

11 Conductors Conductor: a substance that allows heat to conduct through it Metals tend to be very good conductors, but some metals are better than others Uses of Conductors - Cookware Thermal conductivity: the rate at which a substance conducts heat. Knowing the thermal conductivity of metals allows people to make decisions about what ones to use when making pots, pans, etc. ex. copper is often used for bottoms as it is a really good conductor 11

12 Uses of conductors - radiators In homes, radiators are used to deliver heat (like electric heaters) In cars, radiators are used to get rid of heat from a car's engine Insulators Insulator: a substance that does not allow the transfer of heat Humans take advantage of the properties of insulators when it comes to keeping things at the correct temperature (hot or cold) ex. thermos bottles, homes, food 12

13 Insulators Thermos bottles A Thermos bottle uses a vacuum to prevent heat transfer Vacuum: an area where no particles are present The inside of the bottle is made of a double glass wall from which most of the air is removed and then the glass is sealed. The surfaces of the glass are silvered like a mirror to reduce radiation Insulators - Home Homes lose heat all the time. Preventing heat loss is important to lower heating cost and saving energy. Places to prevent heat loss: Walls: insulation Windows: sealant, special windows R-Values: resistance to heat transfer, used to compare how well certain materials stop heat transfer (higher R-values = better insulators) 13

14 Science 7 Chapter 6 Section 3 Temperature vs. Heat (pp ) Temperature vs. Thermal Energy Ex. Suppose you have a beaker with 100 ml of water. You measure the temperature and find it is 40 C. If you take that water and pour half of it into another beaker, what will the temperature of each beaker be? 14

15 Temperature vs. Thermal Energy (cont ) Answer: Both beakers will be 40 C. Temperature is the measure of the average kinetic energy of the particles of a substances, not the total amount. What is thermal energy? Thermal energy: the total kinetic energy of the particles of a substance. In the previous example, the original 100 ml of water had a specific amount of thermal energy. Each of the two beakers of 50 ml of water would have ½ of the thermal energy as the original. Thermal energy depends on the total number of particles in a substance. For two samples of a substance at the same temperature, the one with the most particles will have the most thermal energy. 15

16 Specific Heat Capacity Some substances require less energy to raise its temperature than others Ex. Equal amounts of water and alcohol will heat up at different rates. It takes more energy to raise the temperature of water compared to alcohol. Specific heat capacity: the amount of energy required to raise the temperature of 1.00 g of a substance by 1.00 C Substances with higher SHCs take more energy to raise their temperatures. Specific Heat Capacity and Water vs. Ground The SHC of water is very high compared to the land. This is why the temperature of the surface of the Earth is warmer than the land when sunlight hits it. However, heat is transferred more readily in the water because of convection (the heat is transferred through the ground by conduction 16