chap. 9 transfer of heat

• Thermal energy refers to the energy possessed by a system due to its temperature
• A hotter system will have more thermal energy than a colder one
• Heat is defined as the thermal energy that flows from a hotter region to a colder one
• Objects gain heat when placed in a region of higher temperature
• Objects lose heat when placed in a region of lower temperature
• When an object is at the same temperature as its surroundings, there is no net heat flow between the object and its surroundings
• The S.I. unit for heat and thermal energy is the Joule (J)
• Temperature is a measure of the average kinetic energy of the particles in a system
• When particles of a system move faster, they have more kinetic energy, resulting in a higher temperature
• When heated, objects generally expand and become less dense
• When cooled, objects generally contract and become denser
• Solids, liquids, and gases expand and contract to different extents
• For any state, the size of the particles is unchanged when the object is heated or cooled
• The spaces between the particles of gases increase the most when heated, followed by liquids and then solids
• During cooling, the spaces between the particles of gases decrease the most while the spaces between the particles of solids decrease the least
• Expansion and contraction have various applications in daily life
• Different materials expand or contract by different amounts
• Bridges have expansion gaps or rollers to allow for movement during expansion and contraction
• Expansion and contraction of a liquid are used in thermometers
• In real life, mercury or alcohol is used in thermometers because they change their volumes more than water for the same temperature change
• As a substance expands, its volume increases, leading to a decrease in density
• As a substance contracts, its density increases
• Water exhibits unusual expansion and contraction properties
• Water contracts as it is cooled to 4 °C, but further cooling below 4 °C causes water to expand
• Ice has a larger volume than water, explaining why ice floats on water
• A bimetallic strip is made up of two different metals securely fastened to each other
• When heated, one metal expands more than the other, causing the strip to bend one way
• Bimetallic strips are used in thermometers and thermostats to control temperature
• Bimetallic strips bend towards the metal that expands more when heated and contracts more when cooled
• There are three main ways to transfer thermal energy: conduction, convection, and radiation
• Conduction is the transfer of heat through a medium without visible movement of the medium itself
• Solids are better heat conductors than liquids and gases
• Metals are better thermal conductors than most non-metals
• Convection is the transfer of heat due to the movement of the medium arising from density differences
• Radiation is the transfer of heat in the form of waves without the need of a medium
• Factors affecting the rate of radiation include the nature of the surface, surface area of the object, and surface temperature of the object
• Applications of radiation include using different surfaces to gain or lose heat quickly
• A vacuum flask is designed to keep liquids warm or cold for a long time by reducing heat transfer through conduction, convection, and radiation
• The design of the vacuum flask includes materials and structures that prevent heat transfer effectively