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Heat, Temperature, and Internal Energy

Heat, Temperature, and Internal Energy. Starter. If a Fahrenheit thermometer registers a 10 degree temperature increase in air temperature, what increase would a Celsius thermometer register?. Note: D F o / D C o = 180/100 Why?. 10/ D C = 9/5 D C = 10(5/9) = 5.55.

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Heat, Temperature, and Internal Energy

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  1. Heat, Temperature, and Internal Energy

  2. Starter If a Fahrenheit thermometer registers a 10 degree temperature increase in air temperature, what increase would a Celsius thermometer register? Note: DFo/DCo = 180/100 Why? 10/DC = 9/5 DC = 10(5/9) = 5.55

  3. Internal Energy E • The total kinetic energy of all the molecules in the substance. • The units are Joules. E = S KE Example: A gas sample consists of 100 particles, each with a kinetic energy of 2 Joules. What’s the internal energy? Answer: E = 2 x 100 = 200Joules.

  4. Temperature = Average KE • Units depend of what scale you use • T = E/N where N = # particles and E = internal energy Example: A gas sample consists of 100 particles, each with a kinetic energy of 2 Joules. What’s the internal energy? What’s the temperature? Answer: E = 2 x 100 = 200Joules. T = E / 100 = 2 degrees.

  5. Heat = energy transferred due to a temperature difference • Heat flows only from a warmer object to a cooler one. • Units are Joules or calories. • Q is the symbol for heat. A gas sample consists of 100 particles, each with a kinetic energy of 2 Joules. If 300J of heat are added, what’s the new temperature? Answer: E = 200J + 300J = 500J. T = E / 100 = 5 degrees. Note: the temperature increases if heat is added, as expected.

  6. Heat Transfer Processes Heat energy can be transferred from one body to another in three different ways. • Conduction: Energy is transferred when two objects are in direct contact. Molecules of the hotter object bump into molecules of the colder object and cause them to speed up, warming the colder object. • Convection: Energy is transferred from one body to a cooler one via currents in a fluid (a gas or liquid). • Radiation: All objects, at any temperature, radiate electromagnetic radiation (light of visible and invisible wavelengths). Unlike conduction & convection, no medium (matter of any type) is necessary for heat transfer through radiation. Objects absorb radiation as well. At thermal equilibrium it will absorb as much as it radiates.

  7. Sense of Touch • Sensations of cold or hotness are not reliable temperature readings. • Sensations of cold or hotness tell you about heat flow. If you put one hand in hot water, the other in cold, and then both in room temperature water, each hand tells you something different. Which hand feels cool, and which feels warm?

  8. Connection How is convection important in cooking?

  9. Starter Compare an iceberg to a cup of hot coffee. a. Which has the greatest internal energy? b. Which has the greatest temperature?

  10. Temperature Scales Fahrenheit: water freezes at 32 °F; boils at 212 °F Celsius: water freezes at 0 °C; boils at 100 °C So, DF / DC = 180 / 100 = 9/5 = 1.8 This means, a one degree step in Celsius is a 1.8 degree step in Fahrenheit. OR, a one degree step in Fahrenheit, is a .56 step in Celsius. F = 32 + (9/5)C C = 5/9 (F – 32)

  11. There’s a problem with the Celsius and Fahrenheit Scales, and our definition of temperature as Average KE.What’s twice as hot as 40 degrees C?What’s twice as hot as zero degrees C?What’s twice as hot as -10 degrees C?

  12. Absolute Zero & the Kelvin Scale The Kelvin scale is setup so that its zero point is the coldest possible temperature--absolute zero, at which point a substance would have zero internal energy. This is -273.15°C, or -459.69°F. Absolute zero can never be reached, but there is no limit to how close we can get to it. Scientists have cooled substances to within 10-5Kelvin of absolute zero. How do we know how cold absolute zero is, if nothing has ever been at that temperature? The answer is by graphing Pressure vs. Temperature for a variety of gases and extrapolating. P gas A A gas exerts no pressure when at absolute zero. gas B gas C T(°C) 0°C -273.15°C

  13. Starter Compare an iceberg to a cup of hot coffee. a. Which has the greatest internal energy? b. Which has the greatest temperature?

  14. Heat Units - Calories and Joules Calorie - the amount of heat that will raise the temperature of 1 gram of water by 1 degree C. 1 calorie = 4.184 J Systems of Units Used: Calories and grams, Joules and grams or Joules and kg.

  15. Specific Heat c When heat is added to a substance its temperature rises. How much will it rise? Q = heat added DT = temperature change M = mass of the substance C = the specific heat of the substance Q = mcDT

  16. Some Specific Heats

  17. Example How much heat must be added to 500g of aluminum to take it from 25 degrees Celsius to 125 degrees Celsius? c = .215 cal / g oC DT = Tf – Ti = 125 – 25 = 100 oC m = 500g Q = mcDT = 500(.215)(100) = 10705cal

  18. Example If 500cal of heat are added to 100g of lead at 25 degrees Celsius, what will its final temperature be? c = .031 cal / g oC DT = Tf – Ti = Tf – 25 m = 100g Q = mcDT , 500 = 100(.031)(Tf – 25) Tf = ((500)/(100)(.031)) + 25 = 161 + 25 = 186oC

  19. Latent Heat The word “latent” comes from a Latin word that means “to lie hidden.” When a substance changes phases (liquid  solid or gas  liquid) energy is transferred without a change in temperature. This “hidden energy” is called latent heat. Q = mcDT doesn’t work during a phase change.

  20. Latent Heat

  21. Latent Heat Formula Q = mLfor Q = mLv Q = thermal energym = massL= heat of fusion or vaporization Example: Lf (the latent heat of fusion) for water is 80 cal/g. Ice melts at 0ºC. 50 grams of ice at 0ºC will not become liquid until additional heat is added. The amount of heat needed is: Q = mLf = 50(80) = 4000 cal In other words, it takes 4000 calories of heat to turn 40 grams of ice at zero degrees C into 40 grams of water at zero degrees C.

  22. Latent Heat For Water Lf = 80 cal / g = 3.35 x 10 5 J/kg Lv= 540 cal / g = 22.6 x 10 5 J/kg

  23. Find the heat required to turn 100g of ice at -20 degrees C to 100 g of steam at 120 degrees C. 1st : Get the ice to its melting temp, 0 degrees C. Q = mcDT = 100 ( .5) ( 0 – (-20)) = 1000 cals 2nd: Melt the ice Q = mLf = 100(80) = 8000 cals 3rd: Heat the water to 100 degrees Q = mcDT = 100 ( 1) ( 100 – 0 ) = 10000 cals 4th: Turn the water into steam Q = mLv = 100(540) = 54000 cals 5th: Heat the steam Q = mcDT = 100 ( .48) ( 120 – 100) = 960 cals GRAND TOTAL = 1000 + 8000+ 10000 + 54000 + 960 = 73960 cal

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