How does it relate to temperature? How much is stored in a material?

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1 Heat What is it? How does it relate to temperature? How does it move? How much is stored in a material?

2 Heat If molecules are moving fast, they have more kinetic energy. Averaged over billions of molecules, we feel the kinetic energy as temperature. The temperature of an object is proportional to the average kinetic energy of the molecules in the object. Gas flash animations are from Solid flash animation is from

3 Celsius Temperature Scale 0 Water Freezes 100 Water boils Upper limit????? Lower limit 273 C. At 273 C the kinetic energy of the molecules is zero. They are no longer moving. Kinetic energy can not go any lower, so temperature can not go any lower. As a side note...you might think there is an upper limit to temperature. Nothing can go faster than the speed of light, so there should be an upper limit to kinetic energy based on K.E. = 1/2 m v. It turns out that near the speed of light, the kinetic energy equation changes. The new equation is: 2 In this equation, m is the mass of the molecule at rest, v is the velocity of the particle, and c is the speed of light. If you look at this equation, you will notice that K.E. can become infinitely large the closer the velocity gets to the speed of light. As a second side note, so say that the molecules have no kinetic energy and no motion at absolute zero is not technically correct, but it is close enough to correct for now. It would be correct to say that at absolute zero the kinetic energy of the molecules is as small as it can be.

4 Celsius Temperature Scales Kelvin 100 Water Boils Water Freezes Absolute Zero 0 T = T +273 K C What is room temperature (25 C) in Kelvin?

5 Examples of speeds of molecules Oxygen molecule at room temperature (25C) Oxygen molecule outside in winter (0C) Steam molecule (100C) Cesium atom in atom trap (10 K) km/h 1660 km/h 2600 km/h 1 cm/s (36 m/h) As a side note, below is a description of how to calculate the speed of a molecule in a gas. This information is copied from Velocity Explained The root mean square velocity of a molecule can be obtained by using the formula v rms = (3RT/M) Example: Calculate the root mean square velocity of oxygen molecules at room temperature, 25oC. M is the molecular mass of oxygen which is g/mol; the molar gas constant is J/mol K, and the temperature is K. The molecular mass must be divided by 1000 to convert it into a usable form, therefore vrms = (3(8.314)(298.15)/( ))1/2 = m/s So an oxygen molecule travels through the air at m/s which is 1726 km/h, much faster than a jetliner can fly and faster than that of most rifle bullets. The very high speed of gas molecules under normal room conditions would indicate that a gas molecule would travel across a room almost instantly. In fact, gas molecules do not do so. If a small sample of a very odorous (and poisonous) gas, H2S is released in one corner of a room, our noses will not detect it in another corner of the room for several minutes unless the air is vigorously stirred by a mechanical fan. The slow diffusion of gas molecules which are moving very quickly occurs because the gas molecules travel only short distances in straight lines before they are deflected in a new direction by collision with other gas molecules.

6 Thermal Energy Transfer Heat transfer (Q) caused by a temperature difference. Hot Q (measured in joules) If Q is negative, heat left the object. If Q is positive, heat entered the object. Cold

7 Heat Transfer Conduction Convection Example movie in attachments. Radiation Convection movie is from frankfurt.de/~schmelin/what is convection.html Sun picture from

8 Specific Heat The property of a material that tells how much heat can be stored in the material. Specific heat (C) = heat added to raise temperature of 1kg by 1 degree Units are Q = C m T How much energy must be added to 10kg of aluminum to raise the temperature by 5C? For aluminum, C = 903 J/(kg K) Q = C m T Q = (903 J/(kg*K)) (10 kg) (5 K) Q = J

9 Q = C m C = 235 J/kg*K for silver. C = 4180 J/kg*K for water T How much will 0.03 kg of silver heat up if you add 1000 J of heat? How much will 0.03 kg of water heat up if you add 1000 J of heat?

10 Assume that all of the kinetic energy in a falling piece of lead heats up the lead when it hits the ground. How high do you have to drop a piece of lead so it will heat up 2 C when it hits the ground? For lead, C = 130 J/kg*K.

11 Calorimeter Measures changes in thermal energy Thick insulating walls prevent heat from entering or leaving the calorimeter. The heat from the object goes into the water, raising the temperature of the water. Measuring the temperature of the water lets you measure the heat lost by the object.

12 Water m =1 kg C = 4180 J/kg*K Start temp = 25 C Metal m = 0.50 kg C =? Start temp = 100 C Final temp = 30 C Qw + Q m = 0

13 Qw + Q m = 0 Q = C m T Cwmw T w + C mmm T m = 0 Cmmm T m = C wmw T w Cm = C wmw T w mm T m

14 Water m =1 kg C = 4180 J/kg*K Start temp = 25 C Final temp = 30 C Metal m = 0.50 kg C =? Start temp = 100 C Cm = Cm = C wmw T w mm T m ( 4180 J/kg*K) (1kg) (5K) (0.50kg)( 70K) Cm = 600 J/kg*K

15 Water m =0.50 kg C = 4180 J/kg*K Start temp = 25 C Final temp = 30 C Metal m = 0.50 kg C =? Start temp = 150 C Cm = C wmw T w mm T m

16 If 50 g Iron at 100C goes into a calorimeter filled with 30 g of water at 20C, what will be the final temperature when equilibrium is reached? Ci = 452 J/kg*K Cw = 4180 J/kg*K

17 C1m1 T 1 + C 2m2 T 2 = 0 C1m1 (T end T 1start ) + C 2m2(T end T 2start ) = 0 C1m1Tend C 1m1T1start + C 2m2Tend C 2m2T2start = 0 (C 1m1 + C 2m2)T end (C 1m1T1start + C 2m2T2start )=0 (C 1m1 + C 2m2)T end = (C 1m1T1start + C 2m2T2start ) Tend = (C 1m1T1start + C 2m2T2start ) (C 1m1 + C 2m2)

18 If 50 g Iron at 100C goes into a calorimeter filled with 30 g of water at 20C, what will be the final temperature when equilibrium is reached? Ci = 452 J/kg*K Cw = 4180 J/kg*K Tend = (C 1m1T1start + C 2m2T2start ) (C 1m1 + C 2m2) Tend = 32.2C

19 50 g Iron at 100C 30 g water at 20C Tend = 32.2C

20 Homework: 1. Liquid nitrogen boils at 77K. What is this temperature in Celsius? 2. How much heat is needed to raise the temperature of 50.0 g of water from 4.5 C to 83.0C? 3. A 0.5 kg block of metal absorbs 5016 J of heat when its temperature changes from 20.0C to 30.0C. What is the specific heat of the metal? 4. A 100 g mass of tungsten at 100C is placed in 200 g of water at 20.0C. The mixture reaches equilibrium at 21.6C? Calculate the specific heat of the tungsten. 5. A 10.0 kg piece of zinc at 71.0C is placed in a container of water. The water has a mass of 20.0 kg and has a temperature of 10.0C before the zinc is added. What is the final temperature of the water and zinc. 6. The kinetic energy of a typical compact car traveling 100km/h is 2.9x10 J. To get a feel for how much energy 5 is required to heat water, calculate what volume of water (in liters) would 2.9x10 J of energy warm from room temperature (20C) to boiling (100C). 5

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22 Attachments molecular KE high.swf molecular KE low.swf molecular KE medium.swf eqilibrium v1.swf idealgas.jnlp convection1.mpg heat experiment.swf