Unit 5: Heat

1. Unit 5: Heat

2. What is heat? • What is temperature? • How are these related to each other?

3. Kinetic Molecular Theory • All matter consists of tiny particles • These particles are in constant motion • The amount of motion is proportional to temperature. Increased temperature means increased motion • ex water- solid when below zero, liquid when 0-100 degrees and steam above 100 • Solids, liquids and gases differ in the amount that their particles move and how much they interact

4. Kinetic Molecular theory

5. Heat • Is the thermal energy transferred from one object to another due to differences in temperatures. Measured in Joules (J) Qheat= Ethermal where Qheat = amount of heat; E= change in thermal energy

6. Temperature • Is a measure of the average kinetic energy of the molecules of a substance. Can be measured with a thermometer • Measurements of a thermometer: • Celsius (oC)- freezing point is 0oC and boiling point is 100 oC • Kelvin Scale (K)- sets absolute zero (temperature where no motion occurs) as 0 K (-273.15oC). K= oC +273 or oC= K-273

7. temperature conversions • Convert these temperature from Celsuis to Kelvin • 27oC = 300 K • 560 oC = ? K • -184 oC = ? K • -273 oC = ? K • Convert these temperatures from Kelvin to Celsius • 110K = -163 oC • 22 K = ? oC • 402 K= ? oC • 323 K = ? oC

8. THERMAL EXPANSION • When matter is heated, the particles move faster and move apart from each other. This causes the volume to increase. We call this expansion. • When matter is cooled, the opposite happens and we get contraction. 

9. Everyday examples • Air pressure • Tires have less pressure in the cold temperatures • Experiment: two balloons about the same volume- what will happen when one balloon is put outside in the cold

10. Everyday examples • Bridges and sidewalks are built in segments. They have spaces called expansion joints between them. The expansion joints allow the concrete and steel to expand without buckling and cracking. • The thumping sound you hear when you drive over a bridge in a car or bus is the sound of the tires going over the expansion joints.

12. Everyday examples • Thermometers- liquid in a thermometer will expand with increased temperatures making them rise • Global warming is causing ocean levels to rise (due to the expansion of the volume of water) threatening cities at sea level

13. Thermal Expansion • Substances vary in the amount of thermal expansion • Demo in small groups- ring and ball demo - compound bar

14. Linear Expansion- for a solid • Depends on the initial length, temperature change, and the type of materials it is made from • Change in length (Δ L) is directly proportional to the change of temperature (Δ T) and the original length (Lo) ΔL= α Lo * Δ T Where: L= length in meters (m); α = coefficient of linear expansion; Lo= original length (m); Δ T= change in temperature

15. Example • A lead rod is 2.5 m long at 20 oC. What is the change in length if it is heated to 100 oC and α = 27 x 10 -6 oC-1.

16. Solution • L =  Lo T = 27 x10-6oC-1 x 2.5 m x 80.0 oC = 0.0054 m

17. Volume Expansion- for gases • ΔV= βVo ΔT Where: ΔV- change in volume (m3 or L) β – coefficient of volume expansion Vo– original length (m3 or L) ΔT- change in temperature Note: β is different for different materials

18. Example • 1000 L of gasoline is in a tank at 20oC. What is the change in volume when the temperature drops to -20oC?( β = 950 x 10-6 oC)

19. solution • V= V0T = 950x10-6oC-1 X 1000L X -40oC = -38 L

20. Assignment • do expansion practice problems- check answers as you go • Expansion problems- do #1-8