Science 14: Chapter #5 - Heat & Heat Transfer. Baier's Science 14

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1 Science 14: Chapter #5 - Heat & Heat Transfer

2 5.1 - The Nature of Heat During the 1800 s, Robert Brown was using a microscope to observe grains of pollen in water. He noticed that although the microscope was quite still, the pollen bounced around. When he increased the temperature of the water the jiggling motion increased.

3 5.1 The Nature of Heat This jiggling motion became known as Brownian motion. He reasoned that: a) water must be composed of particles b) these particles are in constant motion c) the motion of the pollen grains must be caused by collisions between the pollen grains and the other unseen particles.

4 5.1 - The Nature of Heat

5 5.2 Heat vs. Temperature Kinetic energy is a measure of the amount of motion particles have. The more kinetic energy there is the more the particles are in motion. All particles have different kinetic energies. Like bumper cars, atoms and molecules collide with each other at different speeds. Brown noticed that heat energy or thermal energy is related to the jiggling motion (kinetic energy) of the particles or in a substance.

6 5.2 - Heat & Temperature Heat and temperature are related but they are not the same thing. Temperature Temperature is the average of all kinetic energies of all particles in an object. The particles in a hot substance will be moving faster than the particles in a cold substance no matter how much you have Heat (Thermal Energy) Heat is the sum of all kinetic energies of all particles in an object. The more of a substance you have the more heat there will be The hotter a substance is the more thermal energy you ll have

7 5.2 - Heat & Temperature 25ml of water at 30ºC 100ml of water at 30ºC The temperature of the water in both of these beakers is the same. The water in this beaker has more heat than the water in the beaker on the left.

8 5.3 Transfer of Heat The particles in a hot substance have a high average kinetic energy (temperature) and therefore vibrate quickly The particles in a cold substance have a low average kinetic energy (temperature) and therefore vibrate slowly. Heat flows from hot to cold. The flow continues until both objects are at the same temperature. During contact between hot and cold objects the particles of the hot mass collide with the slower moving particles of the cold mass. The collision results in a transfer of kinetic energy. The molecules of the cooler mass start to vibrate faster, gaining kinetic energy. The molecules of the warmer mass vibrate slower, having lost kinetic energy.

9 5.3 - Transfer of Heat There are three forms of heat transfer. Conduction Convection Radiation A) Conduction The transfer of heat by contact is called conduction.

10 5.3 Transfer of Heat B) Convection Heat is transferred, by conduction, from the heat source to the air molecules touching the heat source. The air molecules gain kinetic energy, vibrate faster, and get further apart. The warm air molecules are further apart than the cooler air, making the warmer air less dense. This air rises. Cooler air rushes in to take the place of the rising warm air. This flow is continuous until all the air is warmed.

11 5.3 - Transfer of Heat C) Radiation Radiation is produced by vibrating electrons. This vibration makes a wave called an electromagnetic wave or infrared radiation wave. These waves travel form the heat source and strike nearby objects transferring their heat energy.

12 5.4 - Heat Transfer in Nature Oceans moderate the climate of land areas near them They prevent the area from becoming too hot or too cold Sea Breeze A sea breeze is a cool wind that blows in from an ocean or a lake towards land usually on a hot summer day Sea breezes are caused by convection currents over areas where the land meets the sea. 1. Sea breezes are created during the day because solar radiation warms the land more than the water. 2. Air over the land is heated by conduction. 3. The heated air becomes less dense and is forced upward by cooler, dense air moving inland from the water. A convection current results. A temperature difference of about 10 C between the land and ocean is enough to create a fairly strong sea breeze. The typical speed of a sea breeze is about 12 m/s or about 43 km/h.

13 5.4 Heat Transfer in Nature Land Breeze A land breeze is a mild wind that flows from land to ocean. Often serve to carry land pollutants out to sea Land breezes usually occur in the evening Weaker than sea breeze because there is less temperature difference between the land and the sea during the evening Land breezes are also created by convection currents. 1. At night, land cools much more rapidly than water. 2. Air over land becomes cooler than the air over water. 3. The cool, dense air from the land moves out over the water, pushing the warm air over the water upward.

14 5.5 - Heat Transfer & Technology Knowledge of how heat is transferred is used to prevent overheating in technologies that use or produce heat Coolant mechanisms use heat transfer processes to protect engine components: Heat is conducted from the engine to the coolant Heat from the coolant is conducted into the air at the metal alloy radiator (good conductor) A fan or the motion of the machine force the air through the radiator to carry the heat away by convection Evaporative cooling is used by the human body and by technology to prevent overheating Liquid sweat on a surface will absorb heat from the surface until it evaporates The phase change removes further heat from the surface Similar heat conduction technologies are used in refrigerators and air conditioning systems