Physics Unit 5: Heat and Temperature

1. Physics Unit 5: Heat and Temperature

2. Temperature What does temperature indicate? Measurement of the average Kinetic Energy of the molecules NOT A MEASURE OF HEAT

3. Measuring Temperature • We use a Thermometer to measure temperature • Thermometers rely on expansion • Most objects expand when their temperature increases • Alcohol or Mercury Thermometers use expansion and contraction to measure temperature.

4. The warmer a liquid is, the more the particles move and thus they expand (take up more space) and rise up through the tube • Liquid thermometers can only be used between certain temperatures, why? • Because the liquid will freeze at low temps • Because the liquid will boil at high temps • So, some thermometers use a solid metal to measure temperature through expansion • Like a metal coil used in refrigerators • Digital thermometers use electrical current to measure temperature.

5. Temperature Scales • There are three temperature scales 1) Fahrenheit Scale: water boils at 212 oF and freezes at 32 oF, normal body temp is 98.6 oF. 2) Celsius Scale: like the metric system, it is bases on powers of 10. - water boils at 100 oC and freezes at 0 oC. 3) Kelvin Scale (Science): water boils at 373 K and freezes at 273 K. (no degree symbol b/c it does not vary or change based off of location) - absolute zero (0 K): motion stops…the lowest possible temperature you can go. (-273 oC)

6. Since the Celsius and Fahrenheit Scales can go below their zero mark, they have negative values. • This is why they are reported in degrees • Since KELVIN starts at zero, and cannot go below that absolute zero, then you cannot have negative Kelvin • This is why Kelvin is not reported in degrees • Kelvin is always positive

7. Converting Temperature • Celsius to Kelvin: tK = toC+ 273 Ex: 23 oC to K • Kelvin to Celsius: toC= tK - 273 Ex: 338 K to oC • Celsius to Fahrenheit: t oF = 1.8 (t oC) + 32 Ex: 33.5 oC to oF • Fahrenheit to Celsius: t oC = (t oF – 32) Ex: 147 oF to oC 1.8

8. Temperature and Energy Transfer • When you hold an ice cube, the ice melts because of the energy transfer. • The particles of your hand are moving faster than those of the ice cube • The particles of your hand collide with the particles of the ice cube and cause the particles of the ice cube to move faster • When the ice cube particles move faster (higher kinetic energy) from this collision, the temperature of the ice cube rises and thus it melts.

9. The energy transfer between particles of two objects due to a temperature difference between the two objects is called heat. • The transfer of heat ALWAYS goes from higher temperature (faster moving) to lower temperature (slower moving) • This happens until equilibrium is reached! • A seat may feel cool at first, but your body will warm it up until they reach the same temperature (equal temp equilibrium) • Heat gained = heat lost • Law of conservation of energy

10. Methods of Energy Transfer • Conduction: involves objects in direct contact • Fast moving molecule will collide with slow moving molecule, transferring energy • Works for all 3 phases of matter • But gases are very poor conductors because the particles are so far apart • Liquids are okay • Solids are best insulators, but vary by substance • Heat Conductor: Substance that moves heat more effectively (like metal skillets used to cook) • Insulator: Substance that will not conduct heat well (like fiber glass insulation, wood on skillet handle)

11. 2. Convection: movement of warm fluids • The fluid (air) transfers the heat • Molecules move in currents • Only in a fluid: (Liquid or Gas) • Heated portion speeds up and becomes less dense, creating a current of heat • Air currents are a result of convection

12. 3. Radiation: does not require direct contact • Heat is transferred through space • Does not involve the movement of matter • Travels as waves • Electromagnetic radiation (Gamma, UV, Visible, X-rays, Microwaves, Radio, Infrared,)

13. Measuring Heat • calories (cal) or joules (j) are used to measure heat • calorie: Amount of heat needed to raise the temperature of one gram of liquid one degree Celsius • Since heat is related to kinetic energy, the unit of joules are often used • 1 cal = 4.2 joules

14. What Are Food Calories? • Calories are really kilocalories • 2000 Cal diet is really 2,000,000 calories • We use capital C (kilo) because it is easier

15. Specific Heat (s) : Specific Heat (s) : amount of energy required to change the temperature of one gram of a substance 1 oC • How well a substance conducts heat • Varies from one substance to another • Heat always travels from high concentration to low concentration!! • Heat lost = Heat gained

16. Water has a specific heat = 1 cal/goC or 4.184 J/goC • Water has the second highest specific heat capacity of all known substances. So it requires high amounts of heat energy to raise water temperature. • water also has a high energy/heat requirement for evaporation • SIRON = 0.449 J/goC • Which would heat up faster, 5.00 grams of iron or 5.00 grams of water? •   Which would cool down faster, 5.00 grams of iron or 5.00 grams of water? •  Which is a better thermal conductor? •  Which is a better insulator?

17. Q = s x m xDT Q = energy (heat) required (J) or (cal) s = specific heat capacity (J/goC) or (cal/goC) m = mass of the sample in grams DT = change in temperature in oC • A 2.8 g sample of a pure metal requires 10.1 J of energy to change its temperature from 21 oC to 36 oC. What is the specific heat of the metal? s = Q = 10.1 J = 0.24 J/goC m xDT (2.8 g x 15oC)

18. Applications • Heating Systems: • Work can increase ave kinetic energy, like when lighting a fire by using the friction of two sticks • Our bodies act like a heating system to regulate our body temp to stay at 37 oC or 98.6 oF, we burn stored calories and nutrients to provide the energy we need to raise our temp in the cold. • The sun can be used to heat a system • by cold-blooded animals to help maintain their temp

19. Cooling systems: • Liquids can be evaporated or condensed to allow for transfer of energy in either direction so as to cool or heat a system. • Evaporation causes a cooling effect because gases are farther apart and thus cannot transfer energy as well through physical contact