Physical Science. Thermal Energy & Heat

Transcription

1 Physical Science Thermal Energy & Heat

2 Sometimes called internal energy Depends on the object's mass, temperature, and phase (solid, liquid, gas) TOTAL potential and kinetic energy of all the particles in an object Thermal Energy

3 Heat vs. Temperature Heat n The spontaneous flow of THERMAL energy from a hot object to a cold object Temperature n a measure of how hot or cold an object is compared to a reference point n A measure of the AVERAGE kine;c energy of the par;cles in an object

4 Let s compare Temp, Thermal Energy, & Heat!! Temperature a measure of the AVERAGE kinetic energy of the individual particles of a substance. Thermal energy TOTAL energy of all of the particles Heat THERMAL energy moving from a warmer object to a cooler object, trying to reach thermodynamic equilibrium. Which of these can we measure?

5 Thermal energy vs. temp Now remember that thermal energy, unlike temperature, depends on mass Compare a cup of tea and a teapot full of tea, both at the same temperature Is the average kinetic energy the same in both? YES! (that s why they have the same temperature) Which has more thermal energy and why? The teapot because it has more particles (mass)!

6 Thermal energy varies with temperature Compare a cup of hot tea with a cup of cold tea Assume each cup is the same size and has the same mass Does each cup have the same amount of particles? Yes Which has the higher average kinetic energy? Hot tea So, which has the greater thermal energy? Hot tea

7 Thermal energy Which has more thermal energy? Pitcher of cool lemonade or cup of hot tea? Pot of boiling water or an iceberg? Eureka - Temperature vs. heat

8 So, how do thermometers work? Thermal expansion and contraction Particles move faster and expand as they warm So, as the temperature increases, the alcohol in the tube expands and its height increases

9 Absolute zero- that temperature where the individual particles contain no more energy. The particles (atoms and/or molecules) cease vibrating. No movement occurs. Absolute Zero -459 o -273 o 0 o

10 Warm-up Section 16-1 Review p.478 In which direction does heat flow on its own spontaneously? Name 2 variables that affect the thermal energy of an object. Why is it necessary to have regularly spaced gaps between sections of a concrete sidewalk?

11 Calorimeter Measures changes in thermal energy Uses the principle that heat flows from a hotter object to a colder object until both reach the same temperature. Read p.478 in textbook

12 Thermodynamics

13 Heat Transfer n Heat moves in only one direc;on: from a warmer object to a cooler object n Two drinks in the same room: n The hot coffee to room temp because... n The iced tea to room temp because...

14 Hot coffee cools to room temp because the heat of the coffee is transferred to the cooler temperature of the room. A cold glass of Iced tea soon warms up to the surrounding room temperature because the warmer temperature of the room s surroundings is transferred to the colder glass of iced tea thereby warming it up.

15 Heat Transfer n Heat is transferred in one of three ways: Conduction Convection Radiation n Specific Heat Video

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17 Conduction Conduction heat is transferred from one object to another by direct contact. Examples include: a metal spoon in hot water gets hot or a pot gets hot as it sits on an electric stove. Fastest in solids. Much slower in gases. Why? (think about collisions)

18 Conductors vs. Insulators Conductor a material that transfers heat well: metal, tile, glass Insulator a material that does not transfer heat well: air, carpet, wood, wool

19 n Why does a ;le floor feel colder than a wood floor, even though they're both at room temperature? The ;le feels colder because it is a bejer conductor and transfers thermal energy rapidly away from your skin.

20 Why does a thick wool sweater keep you warm?

21 Convection Convection the transfer of thermal energy when particles of a fluid move from one place to another.the particles transfer the heat. Examples include: a pot of boiling water sets up convection currents to move the hot water at the bottom of the pot being heated to the cooler water at the top of the pot, convection currents in a heated room keep the temperature uniform.

22 Convection of warm air n Air is heated by sunlight n The temperature of the air Increases n The air Expands n The less dense air Rises n The denser air Sinks

23 Radiation Radiation transfer of energy by electromagnetic waves. Examples include: the Sun s energy traveling through space and heating up the Earth w/out heating space itself, Heat lamps used at fast food restaurants, and the radiator of a car dissipating the heat of an engine.

24 Radiation n All objects radiate energy: a fire, the sun, your body n As temp goes up the rate at which it radiates goes up

25 What type of heat transfer is represented by each arrow?

26 Laws of Thermodynamics 1 st Law- energy is conserved 2 nd Law- heat energy can only go from a cold to a hot object if work is done Refigerator A heat engine converts heat into work One consequence of the 2 nd law is that the efficiency of a heat engine is always less than 100% Waste heat must be lost for the heat engine to work!!! 3 rd Law- absolute zero can t be reached

27 Laws and Principles Boyle's Law - for a gas at a constant temperature, if you increase the pressure on the gas, the volume will decrease, and vise versa (ind. prop.) Ex.: -step on a Pringle's can -squeezing a balloon

28 A whoopie cushion utilizes Boyle s Law.

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30 Charle's Law For a gas at a constant pressure, if you increase the temperature, then the volume will increase, and v/v. Ex.: Hot air balloon, car tires

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32 Charles Law is similar to what other concept which we have discussed this chapter?

33 Pascal s Principle Pascal s Principle states that pressure exerted on a fluid is transmitted equally throughout the fluid.

34 As a downward force is applied to the piston, the increased pressure is transmitted throughout the entire system. P = F/A and P = F/A 10 Pa = 50 N/5 m 2 and 10 Pa = 500N/50 m 2

35 Examples: -squeezing a tube of toothpaste -The Blob -Pressurizing the cabin of an airplane

36 Carlos made a balloon rocket as shown in the picture. According to Pascal s Principle, what would happen if the rocket had an opening on both ends instead of just the one. Explain.

37 Bernoulli's Principle If you increase the speed of a fluid moving over the surface of a material, the pressure will decrease as a result (i.e. fluid velocity and pressure are indir. prop.) Ex.: Airplane wing, frisbee, fan blade, treading water

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41 Archimede's Principle The buoyant (upward) force on an object submerged in a fluid is equal to the weight of the fluid displaced by that object. Buoyancy is the ability of an object to rise or float in a fluid.

42 Ex.: underwater handstand, lifting a submerged rock, finding your weight in a vacuum.

43 Let s say you accidentally fill your bathtub all the way full. So you know when you get in, some of the water will have to overflow.

44 If you were to collect all of the water that overflowed and weigh it (in Newtons), then that would be the amount of buoyant force pushing up on you in the tub.

45 So if you weight 668 N (about 150 pounds), and the water which you caused to overflow (displaced) weighed 620 N (about 139 pounds), then your weight as you sit in the tub would be about 48 N (11 pounds), if you were sitting on a scale on the bottom of the tub.

46 Buoyant Force h - Weight i Net Force

47 So if you lifted a 200 pound rock underwater, which displaced 160 pounds of water, how much it would make the rock seem to weigh?

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49 Heating Systems Central heating systemsuse convection to distribute heat Radiators-steam, and boilers- water Put heat into the air it rises, circulates, then falls Controlled by a bimetallic thermometer

50 Electric Baseboardheating element similar to a oven is used to create radiation and convection Space heaters are small examples Forced-Air: use fans to circulate hot air, vents on floor

51 Cooling Systems Heat pumps- reverse the flow of heat energy Refrigerant- fluid that vaporizes absorbing heat and condenses when giving off heat Must do work to reverse flow of heat energy

52 Refrigerator Puts cold air into warm Heat coil beneath fridge releases heat produced by the work

53 Air Conditioners Heats the outdoor air Warm air flows from inside the house into the AC, then as heats is removed from coil cold air is created

54 Ideal Gas Law The constant value in the ideal gas law depends on the number of par;cles. PV=nRT n number of moles R = 8.31 Pam 3 /molk In general, the ideal gas law predicts the behavior of gases very well, except under condi;ons of high pressures or low temperatures.