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Transcript
Chapter 6 Thermal Energy Section 1 Page 158-162 Temperature and Heat • What do we observe about hot and cold? – Can we see hot? – We can compare hot and cold by touch. • Hot and cold are subjective – For me, 72% is cold, for my wife it is hot – Swimming • If a hot object and a cold object are placed side by side in an enclosed container, they come to the same temperature. Particles in Constant Motion Modern Theory Kinetic Theory of Heat Page 476 • Matter is made up of tiny particles, atoms and molecules • These particles are in constant random motion – Move in all directions until they strike something • Kinetic energy = ½ mv2 – More mass of the particle, more energy, more heat – Greater speed, more energy, more heat • Potential energy – Particles attract each other so when separated they have potential energy. Heat/Thermal Energy • Thermal Energy is the total kinetic and potential energy of the particles making up an object. • Heat is thermal energy that flows from an object at a higher temperature to an object at a lower temperature. • Heat always flows from higher temperature to lower temperature. Temperature • The temperature of an object is a measure of the average kinetic energy of the particles in an object. – Higher temperature, more kinetic energy – Lower temperature, less kinetic energy • Temperature is measured by comparing characteristics of a substance that changes with temperature. – The volume of most substances change with temperature Heat • Heat is thermal energy that flows from an object at a higher temperature to an object at a lower temperature. • Heat always flows from higher temperature to lower temperature. • Since energy is conserved, heat that flows from one object must be gained by other objects. Heat Quantity • At noon on the beach, how does the temperature of the air, the water and the sand compare? • The more heat that is added to an object, the higher the temperature rise. • The amount of heat required to raise the temperature of an object depends on the material. • The amount of heat (Joules) required to raise the temperature of 1 kg of material by 1oC is called the specific heat. (Page 161) Quantity of Heat • • • • • • • • • Q = quantity of heat, measure in units of Joules m= mass, measured in kilograms (kg) Tf = final temperature, units of oC Ti = initial temperature, units of oC C = specific heat (c- used in most textbooks) Q = m (Tf - Ti )C ∆T = (Tf - Ti ) or Q= mc ∆T Specific Heats (P161) Substance Aluminum Gold Concrete Copper Diamond Ethyl Alcohol Glass Iron Water Specific Heat (J/kg Co) 899 129 880 386 518 2400 837 447 4186 Definitions • Temperature – (measured with a thermometer) – a measure of the average kinetic energy of the particles that make up an object. • Thermal Energy – the sum of the kinetic and potential energy of the particles in an object. • Heat – thermal energy that is transferred. • Specific heat – the amount of heat that is needed to raise the temperature of 1 kg of a substance by 1 oC • Q = m (Tf - Ti )C or Q= mc ∆T • The heat lost by one object must be gained by some other object. Transfer of Thermal Energy • Conduction – thermal energy is transferred by the collision of particles with high kinetic energy with particles of lower kinetic energy. – Metals are good conductors • Convection – the transfer of thermal energy in a fluid (liquid or gas) by the movement of warmer and cooler fluid from place to place • Radiation- the transfer of thermal energy by electromagnetic waves (light, infra-red, ultraviolet, microwaves) Controlling Thermal Energy Transfer • Reflective surfaces reflect radiation and do not let the radiation pass. • Vacuum prevents both convection and conduction. • Air spaces prevents conduction. • Examples – Attic insulation – Thermos bottle – Clothing Heating Systems • Source of Heat – Burning Fuel (propane, fuel oil, coal, wood) – Electrical Resistance heating – Solar Heat – absorbing heat from sun light • Transfer of Heat – – – – Radiant Heat – old fashioned fireplace or stove Force Air – Fan blows air through ducts and vents p172 Circulating Hot Water or Steam – pump and radiators Heat pumps – as a gas expands it absorbs heat as it contracts it gives off heat Solar Energy • Passive solar – designing buildings to absorb and store heat directly from sunlight. – Orientation – South facing windows – Materials the store heat • Active solar – systems that use sunlight to heat fluid (usually water) these fluids are then pumped to areas needing heat. Thermodynamics • Thermodynamics – movement of heat • Study of the relationships between work, thermal energy and heat. • First Law of Thermodynamics – In ordinary chemical and physical reactions, energy cannot be created nor destroyed. – The increase in thermal energy of a system must equal the work done on the system and the heat transferred to the system Laws of Thermodynamics • Second Law of Thermodynamics – Energy always tends to go from a more usable form to a less usable form – It is impossible for heat to flow from a cooler object to a warmer object unless work is done on the cooler object. Converting Work to Heat (p175, 177) • Pushing the bicycle pump handle compresses air. • Pushing the pump handle also generates heat. – Friction of the pump – Compressing the air • Friction from the parts of an automobile produce friction which generates heat. • Heat is usually a wasted byproduct of work – Second Law of Thermodynamics Refrigerator p178 • Second Law of thermodynamics (Page 175) • It is impossible for heat to flow from a cool object to a warm object unless work is done on the cool object. • A refrigerator takes heat from the cold inside of the refrigerator and transfers it into the hot room. • Impossible to move heat from a cold object to a warmer object unless we do work on the colder object. Refrigerators and Heat Pumps • Special materials – freon • Freon changes to a gas at a low temperature if the pressure is high. • Freon absorbs heat as it is changed into a gas. • The heat is absorbed from the surrounding air. • A pump (compressor) then compresses the Freon and it turns back to a liquid, but in the process gives up heat. • These two processes take place at low temperatures. Human Heating and Cooling Human Coolant • Evaporation of a liquid to its gaseous form (vapor) requires energy. • Evaporation of sweat on the body takes heat energy from the body and uses this to convert the sweat to water vapor. • Heat is conducted to the surface of the skin by blood vessels opening up to allow more blood flow. • In cold weather, the blood vessels close down to prevent heat loss. • We shiver to generate more heat.