Chapter 21 – temperature, heat & expansion

1. Chapter 21 – temperature, heat & expansion

2. 21.1 - temperature • How hot or cold something is compared to a standard • A measure of atomic movement – the random kinetic energy of particles • The first two scales are based on water: • Celsius: boiling = 100 °C, Freezing = 0 °C • Fahrenheit: 212 °F, Freezing = 32 °F • The third based on lowest possible temp. – absolute zero = -273 °C • The “kelvin” (K), no kinetic energy left to give, no negative values

3. Temperature & Kinetic Energy • A measure of random average KE • Therefore: hotter = faster particle motion • Temperature ≠ total KE • Higher temp is not necessarily the same as greater energy

4. 21.2 - heat • Energy in transit = heat • Objects do not have heat, they have energy – internal energy • Heat flows when there is contact (thermal) between substances • Always flow from higher temperature to lower temp – nothing to do with total energy

5. 21.3 – thermal equilibrium • No heat flow when objects are in contact are in thermal equilibrium • Thermometer register their own temp, but they are in equilibrium with something else • It must have a negligible effect on substance being measured

6. 21.4 – internal energy • The total energy within a substance • Includes: rotation, atomic jiggling, PE due to forces • Changes when substance takes in or gives off heat – a measure of temperature

7. 21.5 – measurement of heat • Heat transfer is determined by the temperature change of a known amount of a substance • Need to know: mass & type of matter • calorie = heat required to raise temp of 1 g by 1°C • 1 kilocalorie (Calorie) = 1000 cal, the standard “food calorie”, determined by combusting food • SI unit: joule (J) = 1 kg of water by 1°C

8. 21.6 – specific heat capacity • Recall: how substances react to heat depends on the substance (& mass) • Because of chemical composition • Each has a specific change in temperature • Specific heat capacity (c) = heat req. to raise 1 kg by 1 °C, unit: J/kg °C • “thermal inertia”

9. Temperature is a measure of translational KE Some materials, most of the heat goes into this They are “easy to heat & cool” low specific heat, ie. lead, tin Others, much heat goes into rotation, bonding, & molecular vibrations (water) Hard to heat & cool Water has very high specific heat

10. 21.7 – the high specific heat of water • Water has very high “thermal inertia” • Used in cooling systems because it absorbs a lot of heat with only a small change in temperature • Effects climates around the world • Europe warmer (than should be) because of Gulf Stream • Large bodies of water moderate temperature

11. 21.8 – thermal expansion • As matter is heated, there is more translational KE, and the molecules get further apart • Most matter expands when heated & vice versa • Gases > liquids > solids Expansion joints • Construction must take this into account • Joints are included • Materials must expand at same rate

12. Bimetallic Strips • Two strips of metal with different expansion rates, bonded together • Heating (cooling) causes bending

13. Thermostats • Application of bimetallic strip • Changes in temp. cause heater (or AC) to turn on • Used in ovens, toaster, etc. Glass • Inside/outside of glass may expand differently causing breaking • Special glass is often used

14. 21.9 – Expansion of water • Water is most dense at 4°C, a liquid not solid • Recall: ice floats, less dense than water

15. Due to open structure of ice crystals & angular structure of molecule

16. When temp of water at surface reaches 4°C, it is most dense and sinks • Bodies of water will not freeze until it all is 4°C • Causes ponds and lakes to freeze from “top down” • Layer of ice forms at surface and gradually gets thicker • Forms an “insulating blanket”