# Energy transfer

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• Created by: Happydays
• Created on: 26-02-14 11:29

## Thermal energy

Thermal energy can be transfered by :-

conduction

convection

evaporation

and condensation.

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Thermal (infrared) radiation is the transfer of thermal energy (heat energy).

This type of energy moves by electromagnetic waves. It does not need particles to carry the energy.

All objects emit and absorb infrared radiation however we only notice this heat if an object is warmer that the surronding area.

eg. A light that is turned on emits energy as infrared radiation (although we don't want it to!). If you put your hand close to the bulb you can feel the heat.However, a while after the light has been turned off, the bulb no longer has more heat in it than the air around it,and it no longer feels hot.

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All objects emit and absorb thermal radiation. We don't always notice this because. But if we think about it we know this is true.

If we take ice cream out of the freezer it will absorb heat from the air in the room until it becomes the same heat as the air in the room.

Also if we put a mug of hot chocolate on the table beside our bed it will loose (emit) heat until it is the same temperature as the air in the room.

The hotter the object,the more energy it radiates.

The amount of thermal energy emited or absorbed depends on its surface,shape and dimensions.

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## Rate of heat transfer.1

An object will emit or absorb energy faster if there is a big difference in temperature between it and ist surroundings.

The rate of heat transfer can be slowed down by the use of insulation,which provides a barrier.

eg. We put another jumper on.

We put hot drinks in a thermos flask.

We put an insulating material in our house walls and roof to slow down heat loss.

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## Rate of heat transfer.2

Under similar conditions, different materials transfer heat at ifferent rates.

When at the same temperature ; dark and matt surfaces emit more radiation than light and shiny surfaces.

When at the same temperature ; darkand matt surfaces absorb more radiation than light and shiny surfaces. This is because shiny surfaces are good reflectors of infrared radiation.

We can design objects to reduce the rate of energy transfer.

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## An example

Black surfaces inside an oven emit infrared radiation better than shiny surfaces. This helps heat to transfer to the food inside the oven.

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