Thermal Equilibrium Conduction Convection Radiation

1. Heat Transfer Thermal Equilibrium Conduction Convection Radiation

2. Thermal Equilibrium • Heat is defined as a transfer of thermal energy because of a difference in temperature. • Thermal energy always transfers from areas of higher temperature to areas oflower temperature. • Energy transfer stops when objects are at the same temperature. • When objects are at the same temperature as their surroundings, they are in thermal equilibrium.

3. Thermal Energy Transfer If two objects of different temperatures are in a room at 20° C (comfortable room temperature), what will happen? B A 20° C 20° C 20° C 70° C 5° C Thermal energy will transfer from container A to the room. Thermal energy will transfer from the room to container B. Why? Because they are all at differenttemperatures.

4. Thermal Energy Transfer • Thermal energy can only be transferred in three possible ways. • Three processes: Conduction Convection Radiation

5. Conduction Conduction: - transfer of thermal energy when molecules collide - transfer of energy through an object or between objects in contact Examples: - at home: conduction from a stove top to the copper bottom of a pan - on Earth: conduction from the ground to the air just above the ground

6. Conduction Why are the bottoms of some pots and pans coated with copper? Why are pot handles long and sometimes made of wood or plastic? Why is the bottom of the troposphere (near ground) so much warmer than higher up in the troposphere?

7. Convection Convection: - transfer of thermal energy by a moving fluid - moving fluids can include liquids and gases, because they can flow - occurs because of differences in density Examples: - at home: boiling water churning in a pot; stirring a hot drink or while preparing food - on Earth: warm air rising and cool air sinking because of their different densities

8. Convection Look familiar? Warm water in a pot rises because it is less dense, and cooler water sinks because it is more dense, and so the water circulates. Warm air and cool air in the atmosphere circulate in much the same way, generating sea and land breezes as well as the surface winds.

9. Radiation Radiation: - transfer of thermal energy across space - any object at a higher temperature than its surroundings will radiateheat - radiation can be absorbed or reflected Examples: - at home: heat radiating from different sources: fireplaces, radiators, stoves, light bulbs - on Earth: radiation from the Sun warms the surface of the Earth (differently in different places) Question: Why is radiation the only process that can transfer thermal energy from the sun to the Earth?

10. Radiation Why wear light colored clothes in summer? Why does snow melt quicker on a dark roof? Why does the sand warm up more quickly than the water? Why carry a space blanket when you go camping or hiking?

11. What does this all mean? Solar radiation warms the Earth’s surface, and the ground absorbs more of the sun’s heat. Conduction from the ground warms the air close to the ground. Warm air is less dense and rises (Archimedes Principle). Warm air rising and cool air sinking cause convection. We have a sea breeze!