What does temperature have to do with energy? What three temperature scales are commonly used? What makes things feel hot or cold?

Transcription

1 Heat and Temperature Section 1: Temperature What does temperature have to do with energy? What three temperature scales are commonly used? What makes things feel hot or cold? 1

2 Intro: Discussion A person from Seattle tells his friend from Florida that the weather in Seattle is somewhat warm. When the friend arrives for a visit, he finds that he is uncomfortably cool wearing the shorts he packed. What would be a more effective way for the person from Seattle to explain the weather? Temperature -The degree of hotness or coldness of an object, -related to the average kinetic energy of an object s atoms or molecules What makes something hot? Particles that make up matter are in constant motion They have kinetic energy When you heat something the particles move faster So, what kind of energy does temperature measure? 2

3 Kinetic Energy transferred as particles collide Particles are always in motion no matter what state they are in. What is happening to the coffee? What else is being affected? Kinetic Energy Temperature Measuring Temperature Thermometers-mechanical or electrical device for measuring temperature. Early thermometer was invented by Galileo. How thermometer works: Most materials expand when heated. Liquid thermometers have a large bulb hooked to a narrow tube. When the liquid expands it rises up the tube. Won t work if too hot or cold 3

4 Measuring Temperature Metals expand when heated Different metals expand different amounts Hook two metals together to make a thermometer that turns Bimetallic Thermometers Used in thermostats Iron Copper Cold Hot Limits of Temperature -Upper Limit - No upper limit exists. - Plasma found in starts= millions of degrees C -Lower Limit - Definite limit called absolute zero. - Molecules will slow down SO much, they will essentially stop moving - Out of energy, so they can t get any colder. 4

5 Temperature Scales Three different scales: 1. Fahrenheit-the one we use 2. Celsius- metric standard 3. Kelvin-starts at absolute zero but same degree size as Celsius Fahrenheit scale. In the US we know at 32 ⁰F water freezes and 212 ⁰F water boils. Measuring Temperature - The absolute temperature scale is called the Kelvin scale. -Absolute zero is K. -The melting point of ice is 273 K, and the boiling point of water is 373 K. -There are no negative numbers on the Kelvin scale. 5

6 ⁰C 32⁰F 273 K 1⁰C 212⁰F 373 K ⁰C 32⁰F 273 K 6

7 1⁰C 212⁰F 373 K ⁰C 32⁰F 273 K -273⁰C -492⁰F K Notice anything else about the scales? Heat, T and Energy Video 7

8 Converting Temperature Fahrenheit to Celsius C = ( F 32) 1.8 Celsius to Fahrenheit F = (1.8 x C) + 32 Celsius to Kelvin K = C Water freezes at 32 F, what is this in Celsius? C C = ( F 32) 1.8 C = (32 F 32) 1.8 In Kelvin? K = C K = C K Practice Problems: Temperature Scales 2. Water boils at 1 C. What is this in Fahrenheit? F = (1.8 x C) + 32 F = F F = (1.8 x1 C) + 32 In Kelvin? K = C K =1 C K 8

9 Practice Problems: Temperature Scales 3. Lead melts at 6 K, what is this in Celsius? K = C C= C C= K In Fahrenheit? F = (1.8 x C) + 32 F = (1.8 x327 C) + 32 F = F Practice Problems: Temperature Scales 4. Body temperature is 98.6 F, what is this in Celsius? 37 C In Kelvin? K = C Methanol boils at 75 C, what is this in Fahrenheit? F = (1.8 x C) + 32 In Kelvin? K = C C = ( F 32) 1.8 C = (98.6 F 32) 1.8 K =37 C K =75 C K F = (1.8 x 75 C) F 348 K 9

10 Heat We know that temperature is the hot or cold nature of something (based on the kinetic energy of its molecules), So what is heat? Heat is the thermal energy transferred from one thing to another due to a temperature difference. Always moves from high temperature to low temperature. Faster molecules (high temperature) hit slower molecules (low temperature) and speed them up Heat - If you touch a hot stove, thermal energy enters your hand because the stove is warmer than your hand. When you touch a piece of ice, thermal energy passes out of your hand and into the colder ice. Clip 1

11 Section 14.2 Energy Transfer How does energy transfer happen? What do Conductors and Insulators do? What makes something a good conductor of heat? Energy Transfer Heat can be transferred 3 different ways: 1. Conduction 2. Convection 3. Radiation 11

12 Energy Transfer: Conduction Transferred of energy by direct contact Works well in some solids, then liquids, and least in gases. Conductors- materials that allow heat to pass through them Most metals Insulators- materials that don t let heat pass through them well Rubber, plastics, glass, air Energy Transfer: Convection Transferring energy by moving fluids Liquids and gases are fluids When heated they expand, become less dense They rise, replaced by cooler denser fluids Make a circular flow called a convection current Clip 12

13 Convection Current Hot Air Rises Cool Air Sinks Same in water Clip Convection Current 13

14 Why is it windy at the seaside? The third method of heat transfer How does heat energy get from the Sun to the Earth?? There are no particles between the Sun and the Earth so it CANNOT travel by conduction or by convection. RADIATION 14

15 Energy Transfer: Radiation Energy transferred by electromagnetic waves Ex: infrared radiation, visible light, ultraviolet rays Can travel through empty space When wave hit object they make the molecules move faster. Radiation Radiation travels in straight lines True/False Radiation can travel through a vacuum True/False Radiation requires particles to travel True/False Radiation travels at the speed of light True/False 15

16 Specific Heat Some materials heat up easily Others require a large amount of energy to change their temperature Specific heat measures the amount of energy required to raise the temperature 1 kg of a substance by 1 degree Kelvin Table on page 485 (Learn to use it) Water has a high specific heat of 4186 J/kg K Specific Heat The amount of energy required to change the temperature of substance identifies type of conductor. Metals have a low specific heat 16

17 Calculating Specific Heat Energy = Specific heat x mass x change in temp Energy (Q) = cm T mass= kg Q energy= Joules Temperature= K c= specific heat c m T Specific heat is the price to change 1 kg of a substance. Practice Problems: Specific Heat 1. How much energy must be transferred as heat to 2 kg of water in a bathtub to raise the water s temperature from 25 C to 37 C? energy = cm T energy = 4186 x 2 kg x 12 k T= 37 C- 25 C T= 12 K energy = 1,, J or 1. x 1 7 J m= 2 kg c= 4,186 J/kg x K energy =? 17

18 Practice Problems: Specific Heat 2. How much heat does it take to change the temperature of 3 kg of water by 75 K? energy = cm T T= 75 K energy = 4186 x 3 kg x 75 k energy = 9 J or 9. x 1 5 J m= 3 kg c= 4,186 J/kg x K energy =? Practice Problems: Specific Heat 3. How much energy is needed to increase the temperature of.755 kg of iron from 283 K to 43 K? energy = cm T T= 43 K -283 K T= 12 K m=.755 kg c= 449 J/kg x K energy =? energy = 449 x.755 kg x 12 k energy = 4,7 J 18

19 Temperature vs. Time Adding energy either raises T or changes state, not both at the same time. Section 3 Using Heat Laws of Thermodynamics First Law of Thermodynamics: the total Energy used in any process is conserved whether that energy is transferred as a result of work, heat, or both. Second law of Thermodynamics: energy transferred as heat always moves from an object at a higher temperature to an object at a Lower temperature. 19

20 1 st Law of Thermodynamics -The word thermodynamics stems from Greek for movement of heat. First Law of Thermodynamics: When thermal energy transfers as heat, it does so without net loss or gain. The energy lost from one place is gained by the other. Whenever heat flows into or out of a system, the gain or loss of thermal energy equals the amount of heat transferred. (You can t get something from nothing because energy and matter are always conserved). 2 nd Law of Thermodynamics Energy transfers as heat always moves from a hot to cold objects. (You can t break even; you can t return to the same energy state because entropy always increases). Entropy is the tendency for natural systems to become disorganized over time. Any system that is left to itself will fall apart. This means Entropy increases. Usable energy decreases with every transfer or energy transformation. 2

21 Thermodynamics Work increases average kinetic energy Mechanical Process is the process in which energy is transferred by work. Bow drill Heat Engines Heat engines convert chemical energy to mechanical energy through the process of combustion. Internal combustion engines Fuel burns inside the engine A carburetor is the part of the engine in which liquid gasoline becomes vaporized. 4 strokes: Intake, Compression, Power, and exhaust Automobile Cylinder Animation 21