Transmission of Heat

Objectives 1. Define conduction, insulators, convection and

radiation 2. Explain sea breezes 3. Relate good conductors, absorbers, reflectors

and emitters 4. Define Newton’s Law of Cooling. 5. Explain the Greenhouse Effect and a thermos

bottle

Heat Transfer • Heat

– thermal energy that flows from a warmer material to a cooler material

• Like work, heat is... – measured in joules (J) – a transfer of energy

Three Ways to Transfer Heat

Heat Transfer • Heat is always transferred

from hot to cold. • Insulators slow the

transfer of heat due to air pockets.

• Conductors easily allow the transfer of heat, like metals.

Conduction

• Heat is transferred due to objects touching each other or through collisions.

• Occurs best in solids. • Heat continues to be

transferred until both objects reach the same temperature, called a thermal equilibrium.

Conduction

• Conductors – Transfer energy with little loss – All metals

• Insulators – Do not transfer energy well – Styrofoam, wood

Convection • Transfer of heat through a liquid or

gas through moving currents, called convection currents. (transfer by mass)

• The cause of wind and weather. • Sea Breezes

– During the day, air above land is hot and rises

– Wind blows towards the land – At night, air above land cools

and sinks – Wind blows away from the land

Convection Currents

Convection Currents

Convection and Conduction at Work

Radiation • Transfer of heat through

electromagnetic radiation (light from stars or light bulbs).

• Transferred in all directions.

• No contact required! • Dark or dull objects absorb

more than light or shiny objects do.

Radiation • Heat transfer by electromagnetic waves • Allows for heat transfer through vacuum

(space) • No contact required! • Shorter wavelength is higher energy

– Infrared – heat – Visible light - more energy – Ultraviolet – more energy

Absorption, Reflection and Emission

• Objects that are good absorbers get hotter • Good absorbers are poor reflectors • Good reflectors are poor absorbers • Good absorbers are good emitters • Examples

• Light colored clothes reflect energy and stay cooler • Coffee cools faster without cream in it

Newton’s Law of Cooling

• The rate of loss of heat from an object is proportional to the temperature difference between the object and its surroundings. – Rate of cooling ~ ∆ T – Hot apple pie is cooled faster in a refrigerator.

The Greenhouse Effect

• Natural process • Sun emits all frequencies of radiation

ultraviolet (UV) – visible light – infrared (heat) • Ozone absorbs UV wavelengths • CO2 & CH4 reflect infrared • Visible light passes through • As objects absorb light they get warm and emit infrared • Infrared cannot go through the atmosphere so it gets trapped

Thermos Bottle

• Double glass-walled bottle with a vacuum

between the layers • Vacuum prevents heat loss by conduction and

convection • Inside of glass layers is lined with silver • Silver lining prevents heat loss by radiation