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Transfer of Energy

Heat Transfer • Heat Energy is transferred from hotter object to colder object • Examples? – Cooking pasta – Making ice – Making popcorn – Roasting marshmallows

Conduction • Stir your hot soup with a metal spoon • Pretty soon you need a pot holder because the end of the spoon you are holding gets hot • This is heat transfer by conduction • Energy travels up the spoon from the end in the hot soup to the end in your hand by direct contact!

Conduction • Transfer E as heat between particles in direct contact • Example – Roasting marshmallow with wire – Air molecules collide with wire • Transfer energy to wire • Particles in wire move faster, have more E • E then transfer to hand, detect warmth

Conduction • For example, a spoon in a cup of hot soup becomes warmer because the heat from the soup is conducted along the spoon. • Fun fact: Have you ever noticed that metals tend to feel cold? Believe it or not, they are not colder! They only feel colder because they conduct heat away from your hand. You perceive the heat that is leaving your hand as cold.

Conduction

Conduction Example • Put your hand on the desk. – How does it feel? – Is it colder or is the heat transferring by CONDUCTION? • Popcorn in a pot – The heat is transferred by direct contact from the pan, to the oil, to the kernels of popcorn.

Convection • Energy transfer through movement of fluids at different temperatures due to density • (fluids - liquids and gases) • Examples – Camp fire • See soot, embers rise & swirl • Rise upwards – warm air expands – Why?

Convection • Convection current – flow of fluid due to expansion caused by heat followed by cooling and contraction • Examples – Wind current – Boil water in pot

Convection • You can hold your fingers beside the candle flame without harm, but not above the flame. Why?

Convection Examples • Water…boiling – What is happening? • Hot air popcorn – What is happening? – The hot air transfers the heat to the cooler kernels, and when enough hot air heats the kernels they pop.

Convection • Explains why breezes come from the ocean in the day and from the land at night

Radiation • Transfer of E by electromagnetic waves • No movement of matter – will work in vacuum or outer space • No contact or movement of fluid • Infrared, visible light, UV rays • Example – Fire emits radiation (infrared), skin absorbs E, increase in temperature

Radiation • Radiation can be absorbed and emitted • Color – Summer day: wear light or dark clothes? – Winter day: wear light or dark clothes? – Why?

Radiation • Interior of a car on a sunny day • Sunlight comes in as visible light • Seats and interior are much cooler so they radiate in the infrared instead of visible • Glass in the windows blocks infrared so energy can’t get out • Car interior heats up!

Radiation • Radiation: Electromagnetic waves that directly transport ENERGY through space • Sunlight is a form of radiation that travels through space to our planet without the aid of fluids or solids. The energy travels through nothingness! Just think of it! The sun transfers heat through 149 600 000 km of space. • Because there are no solids (like a huge spoon) touching the sun and our planet, conduction is not responsible for bringing heat to Earth. • Since there are no fluids (like air and water) in space, convection is not responsible for transferring the heat. • Radiation brings heat to our planet.

What type of heat transfer is involved? • Heating a room with a fireplace • Egg cooking in a frying pan • Roof of a house becoming hot

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