HEAT AND ENERGY

TRANSFER

Images from Microsoft Office Clipart

ENERGY• Energy is the ability to do work

• Massless and does not take up space

• It can be measured only by its effects on matter

• Comes in many forms such as energy of motion, stored (potential) energy, chemical energy, or energy associated with nuclear forces

NASA Image from http://www.nasa.gov/audience/forstudents/k-4/dictionary/Energy.html

HEAT• Heat is transferred energy that arises

from the random motion of molecules

• The ability to transfer and store heat is related to the structure of a substance

• Heat transfer involves at least one substance losing energy and another gaining energy

NASA Image from http://science.hq.nasa.gov/kids/imagers/ems/infrared.html

Transfer of Energy through Heat• Conduction (solids)

• Convection (fluids – gases and liquids)

• Radiation (light, heat, radio waves - can pass through a vacuum)

NASA images from http://spaceplace.nasa.gov/review/beat-the-heat/

Conduction

• Transfer of heat within and between substances that are in direct contact with each other

• The better the conductor, the more rapid the heat transfer

Images from Microsoft Office Clipart

Convection

• Convection is the transfer of heat by the bulk motion of the substance containing the heat.

• Natural convection happenswhen warm (less dense) substances rise– Natural convection plays a roll in

plate tectonics, global wind and ocean currents.

• Forced convection is a result of moving the substance intentionally, such as with a fan– heating systems and computer

cooling fans are examples of forced convection

Image from NASA: http://www.virtualskies.arc.nasa.gov/weather/3.htmlImage from EPA: http://www.energystar.gov/index.cfm?c=business.EPA_BUM_CH8_AirDistSystems

Electromagnetic Energy - Radiation

Wikimedia Commons from NASA

KEY TERMS• Energy – ability to do work• Heat – Heat is transferred energy that arises

from the random motion of molecules• Temperature – a relative term reflecting how

vigorously the atoms of a substance are moving and colliding.

UNITS OF HEAT• Calorie– The amount of heat required to raise the temperature

of one gram of water 1oC. • BTU (British Thermal Unit)– The amount of energy needed to heat one pound of

water one degree Fahrenheit• Joule– The international unit of energy, not defined by a

temperature change– 4.184 Joules in one calorie– 1,054.35 Joules in one BTU

Cooling a Hot Glass of Liquid

Thermos

Heat Capacity• A temperature difference causes heat to

transfer from one place to another• Upon gaining or losing energy, an object will

increase or decrease its temperature• The heat capacity is a constant that tells how

much heat needs to be added per unit temperature rise

Example of Heat Capacity

• 1 kg or iron has a heat capacity of 450 J/K• 1 kg of water has a heat capacity of 4181 J/K

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• In the winter homes lose heat to the outside through conduction, convection, radiation, and infiltration

• These losses can be reduced by good home design, but there is always some loss of heat

• To keep the inside of a home warm the lost heat needs to be replaced

Heating Losses in a Home

Image from energy.gov: http://energy.gov/public-services/homes

Temperature Swings in a Home

• Homes lose and gain energy, resulting in temperature changes

• Temperature changes can be lessened if a home contains materials with a high heat capacity

• Solar homes gain energyduring the day. Very highheat capacity allow theenergy to be stored inthe house without overheating