Chapter 15 Temperature, Heat, and Expansion Herriman High Physics

Temperature Scales Temperature is defined as a measure of the average kinetic energy of the molecules. This is often called the internal energy of an object. Temperature scales were developed using the freezing and boiling points of water at sea level as the standard reference points. Herriman High Physics

Temperature Scales There are three Temperature Scales used in Science Fahrenheit – Used primarily in the United States Celsius – the standard for the Metric System Kelvin – Also know as the “Absolute Zero” Scale. Herriman High Physics

Standard Reference Points ScaleBoiling PointFreezing Point Fahrenheit212 °F32 °F Centigrade100 °C0 °C Kelvin Herriman High Physics

Conversion Equations Fahrenheit to Centigrade C = 5/9 (F – 32) Ex: C = 5/9 (212-32) = 100 Centigrade to Fahrenheit F = (9/5 C) + 32 Ex: F = (9/5)(100) + 32 = 212 Herriman High Physics

Conversion Equations Centigrade to Kelvin K = C +273 Ex: K = = 373 Kelvin to Centigrade C = K Ex: C = = 100 Herriman High Physics

Heat Heat is a form of Energy Transfer Heat flows from areas of high energy to areas of lower energy Heat is transferred three way Conduction – requires contact Convection – mass movement of molecules Radiation – transfer over a distance Herriman High Physics

Conduction In order for conduction to occur the two objects must begin at different temperatures. Objects at the same temperature will not transfer heat and are said to be at thermal equilibrium. In order to understand conduction you must understand how heat is tranferred. Each material has an innate ability to absorb or give off heat – specific heat The amount of heat an object can transfer depends upon three things: The mass of the object, m (in Kg) The specific heat of the object, C p ( in J/g°C) The temperature change of the object, t (in ºC) Heat is transferred according to the equation: Q = mC p t The heat transferred is measured in a unit called a calorie. Herriman High Physics

Sample Problem How much energy is required to raise the temperature of 5 Kg of water from 0°C to 100°C ? (C p = 1 calorie/g°C) Q= m C p Δt = (5000 g)(1 calorie/g°C)(100°C) = 500,000 calories Herriman High Physics

Thermal Expansion Most objects tend to expand when their temperature rises and to contract when the temperature drops. Do you know what the one notable exception is? Herriman High Physics

Thermal Expansion Solids can expand linearly whereas liquids expand volumetrically! Each substance has a constant for which describes its ability to expand. Linear constants are denoted by the Greek letter alpha, α and the volumetric constant is denoted by the Greek letter beta, β. Herriman High Physics

Chapter 16 Heat Transfer Herriman High Physics

Convection Unlike conduction which transfers energy without transferring mass, convection requires a current – or mass movement of molecules in a fluid. Keep in mind that a fluid in physics is either a liquid or a gas Convection occurs because when fluids are heated they become less dense and rise past the colder, denser molecules – this creates the current which allows for heat transfer Herriman High Physics

Convection Wind is a form of convection Using the furnace in your house is a form of convection Boiling water is also a form of convection Herriman High Physics

Radiation Radiation is the transfer of energy via an electromagnetic wave. Electromagnetic waves include radio waves, microwaves, and light Packets of energy called photons are carried on a wave front Herriman High Physics

The Greenhouse Effect A car parked in the sun can get significantly hotter than the outside temperature, this is the greenhouse effect. It occurs because the glass windows let heat in as an electromagnetic wave (sunlight), but then don’t let the resulting heat waves back out. Herriman High Physics

Solar Power Solar Power is the rate at which the Earth receives energy from the sun. Solar power can be used by either passive solar heating (greenhouse effect) or by using photovoltaic cells, also called solar panels, to convert the solar energy to electricity. Herriman High Physics

Change of Phase Chapter 17 Herriman High Physics

Phases Changes Phase changes require that a substance absorb energy or release energy to occur. There is NO Change in Temperature associated with a phase change! Different words are used to denote direction when dealing with a phase change. Herriman High Physics

Vocabulary of a Phase Change Freezing – change from liquid to solid. Energy is released! Melting – change from solid to liquid Energy is absorbed! Both of these changes happen at the same point; 0° for water! Herriman High Physics

Vocabulary of a Phase Change Condensing – change from gas or vapor to a liquid. Energy is released! Boiling – change from liquid to gas or vapor Energy is absorbed! Vapor is the gas phase of any substance which is normally a liquid at room temperature! Herriman High Physics

Energy Required for a Phase Change Just like specific heat is a set amount of energy for each substance – the amount of energy required for a phase change is also substance specific. Heat of Fusion – ΔH f, is the amount of energy absorbed or released when a substance melts or freezes! Herriman High Physics

Energy Required for a Phase Change Heat of Vaporization – ΔH v, is the amount of energy absorbed or released when a substance boils or condenses! Mathematically: Q = mΔH f for Fusion or Q = mΔH v for Vaporization Herriman High Physics

Sample Problem How much heat is required to melt 5 kg of ice at 0° C? Solution Q = mΔH f = (5000 g)(80 calories/g) = 400,000 calories or 400 Kilocalories Herriman High Physics

Phase Diagram Ice Ice - Water Water Water- Steam Steam Herriman High Physics

Other Forms of Stored Energy: Chemical Energy Stored in the Chemical Bonds that make up a substance Often released by combustion (burning) Released as kinetic energy Heat Light Sound Herriman High Physics