Heat – Thermal Energy

1. Heat – Thermal Energy ISCI 2002

2. What is Heat? • Place your finger on the handle of a ‘hot’ pan. Ouch! • Heat is energy that is transferred from one ‘system’ to another (two systems have different temperatures). • Heattransferred to atoms and molecules – increases the KE of each

3. What is Heat? • Heat – another form of energy • Energy transmitted by electromagneticwaves • Sun • Infrared radiation • Energy conversions • Units of heat: • Calorie (cal) • Joule (J)

4. Laws of Thermodynamics • Thermodynamics – “movement of heat” • Thermal energy transfers as heat – no net loss or gain • 1st Law of Thermodynamics • When heat flows into or out of a system, the gain or loss of thermal energy equals the amount of heat transferred. • Heat spontaneously flows from higher to lower substances, never from lower to higher temperature substances • 2nd Law of Thermodynamics

5. Energy Conversion

6. KE and Temperature • KE – Temperature Relationship • Temperature is the ‘average’ KE of all atoms and molecules in a system. • Scales • Fahrenheit • Celsius • Absolute Zero

7. Heat, Temperature, Mass and the Specific Heat of a Substance • Apply the exact same amount of heat to two exact volumes of water. • T ~ Q (heat) Both beakers contain 1.0 kg of water Same amount of heat applied Rise in temperature will be exactly the same.

8. Heat, Temperature, Mass and the Specific Heat of a Substance • Next, you apply same amount of heat but double the volume of water in one beaker. • T ~ 1/m • Temperature of the 2.0 kg beaker will be ½ less What if you apply the same amount of heat, but double the volume in one beaker? Left – 1.0 kg Right – 2.0 kg

9. Heat, Temperature, Mass and the Specific Heat of a Substance • Finally, add the same volume of liquid to each beaker, but instead of water add ethyl alcohol to one of the beakers. • T ~ 1/c • C = specific heat of a substance Volumes are 1.0 kg, but on the right is ethyl alcohol Ethyl alcohol’s temperature will rise twice as fast as the temperature of the water.

10. Putting it All Together • Q = mc T • Heatisproportional to the mass of an object times the specific heat of a substance times the change in temperature • Specificheat – quantity of energy required to raise the temperature of 1 g of a substance 1 C

11. Phase Changes of Water and Heat A – B : Ice temperature rising to 0 B – C : Ice temp remains at 0 even though heat constantly added; needed to change solid to liquid. Amount of heat need is called heat of fusion C – D : temperature rises from 0 to 100 C no phase changes occur; only raising temp of water D – E : At 100 C phase change occurs; amount of heat needed to do this is called heat of vaporization

12. Thermal Expansion • As heat is transferred through a substance • Molecules move faster; move far apart • Liquids expand more than solids • Engineering Applications • Bridges, Concrete, etc. • Water expansion • Expands except in 0-4 C range • Occupies greater volume in this range; ice less dense than water • Freezing increases water’s volume (decreases density)

13. Heat Transfer • Conduction • Convection • Radiation

14. Radiation • RadiantEnergy – Electromagnetic Waves • Any substance above absolute zero emit radiant energy • f is proportional to T • Humans emit ‘infrared’ radiation • If a substance is real hot – emits light – 500 C (long waves – red light); 1200 C (white hot) • Sun – emits short wave radiation; Earth emits longer wave radiation (terrestrialradiation) – cool surface

15. Electromagnetic Spectrum

16. Greenhouse Effect • How a real greenhouse works • Glass and visible light • Traps ‘infrared’ sunlight • Earth’s Greenhouse Effect • Carbon dioxide acts like glass • Greenhouse gases • Allowed for life to thrive • Venus vs. Earth

17. Absorption of Radiation • Good emitters are good absorbers of radiation • A black object filled with hot water will cool faster. Why?