# Thermal Transfer

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• Created by: DiHydro
• Created on: 16-05-16 16:36

## Thermal Transfer and Conduction

• Thermal Energy - Heat - A form of energy, measured in joules.
• Temperature

A way of describing how much heat energy something has, usually measured in degrees Celsius (°C). When heat energy is transferred to an object, the temperature increase depends upon:

• The substance the object is made from.
• The mass of the object.
• The amount of energy transferred to the object.

This means that the more heat energy transferred to an object, the greater its temperature increase.

• Heat Transfer

Heat energy always flows from a hot object to a cool object. The bigger the difference in temperature, the faster the heat is transferred

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## Conduction

Conductors

When heat energy travels through something it is called conduction. Some materials are better thermal conductors than others.

• Conductors - Something that lets energy pass along it.
• Insulators - Something that does not let energy pass along it.

Thermal Conduction

• Hot particles vibrate a lot
• Cold particles vibrate a little

When a solid is heated, the particles in it gain energy and vibrate more. The particles bump into each other and pass the energy on. The solid gets hotter. This continues until all particles are vibrating with lots of energy.

Expansion

Many substances expand when heated. When heated, the particles in a substance vibrate more. This means they need more space, so the substance expands.

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## Convection

1. Water is heated and particles gain thermal energy, they spread out from each other.

2. This creates an 'area' of less dense water and this 'area' of water rises.

3. Less dense water hits the top of the tube and (having no further up to go) travels along the tube.

4. As the 'area' of heated water travels, thermal energy is transferred to cooler regions that it travels through.

5. As the water loses thermal energy, particles come closer together and it becomes more dense.

As it is more dense, the 'area' of water falls back down.

6. The cooler, more dense water is drawn back to the area that is being heated. This occurs due to a slight pressure difference caused by new 'areas' of water becoming less dense and rising.

* This establishes a convection current that continues until the whole volume is at the same temperature.

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No particles are involved in radiation, it travels as a wave of energy. This means it shares properties with light:

• They can travel through vacuum
• Move at 3x10^8 m/s
• Can be reflected
• Do not pass through opaque materials

All objects radiate thermal energy through infrared radiation. The hotter the object is, the more IR radiation it radiates. When radiation hits something, it can be  absorbed or reflected. Light coloured and/or shiny objects reflect more infrared radiation. Dull and/or dark objects absorb & emit more radiation.

Thermal energy transfers will always occur if there is a difference in thermal energy. They cannot be stopped but can be reduced. Reducing thermal energy transfer is known as insulating.

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