How is Heat Transferred?

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Introduction

There are three methods of heat transfer: radiation, convection and conduction. Heat and temperature are related but not the same. Temperature can be measured using a thermometer but we cannot measure the amount of heat energy it contains using one.


Temperature can be measured in Degrees Celsius (°C).

Heat is a form of energy and is measured in joules (j). Another name for heat energy is Thermal energy. The amount of heat energy in an object depends on its temperature, mass and material.

Radiation

There is nothing between the Sun and the Earth. Therefore, energy cannot be transferred using conduction or convection. Thermal energy is then transferred by radiation or infrared radiation. It is similar to light: it does not need anything to travel through. Radiation can also go through glass and other transparent substances.

Hot substances emit infrared radiation which can be absorbed or reflected. All shiny materials reflect the most infrared radiation.


For example, to keep liquids warm or cold, the inside of a thermos is a shiny grey colour. The radiation is reflected and the liquid is kept at the desired temperature.



Thermal imagers also use radiation. They convert infrared radiation into temperature maps. This can be used to see things during the night, film hot parts of the Earth from space or show the temperatures in the human body.

Thermal imagers can also forecast the weather by taking pictures of clouds.

Conduction

Conductors let energy flow through them. When heat travels through a solid, this is called convection. Some thermal conductors are better than others. Insulators are poor heat conductors.

Metals are good thermal conductors and wood and platic are good thermal insulators.

When a solid is heated, the particles start to vibrate as it gains more thermal energy. The particles bump into each other and pass the energy onwards. Conduction happens best in solids because the particles are so close together meaning that the energy is passed on quickly.


When the particles in a solid start to vibrate, they start to gain more space and this causes the solid to expand. When the particles lose the thermal energy, the particles come back into place and the solid returns to its origianl form.


When a solid expands, its mass stays the same altough it takes up more space decreasing its density.

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Convection

Liquids and gases can both be called fluids. Thermal energy is not very effective when it passses through fluids by conduction, although it can by convection.


When a fluid is heated, the particles move around and it becomes less dense. if only one part of the fluid is being heated, that part starts to rise becuase it is less dense than the other part. Coolder fluid comes in and takes its place. This is how a convection current is formed.

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Convection currents can play a huge role in the weather on Earth. Some places are hotter that others and are warmed more by the Sun. Warm places on Earth heat air above them and a convection currents forms. In our everyday life, a convection current is called wind.



Convection currents also take place when a substance is colder than the air around it as showed in this diagram:

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Some birds like the Ruppell's Vulture use convection currents to stay high in the sky. The highest altitude for a bird ever recorded was at 11300 meters when it hit an airliner.


Changing State

The three states of matter can be changed from one to the other by heating or cooling them. If you heat ice, a solid, it will melt into water when it reaches its melting point. The water can then be evaporated in water vapor, a gas.


It is possible to change a gas back into a liquid. This process is called condesation. When cooling a liquid, it will change into a soid at its freezing point. The freezing point and melting point of a substance are always the same.

When you change a solid to a liquid, the thermal energy is used to break the bonds between the particles in the solid. When that has been done, the energy is no longer needer and the heat is tranferred to its surroundings.


When the human body is hot, it sweats to help you cool down. The skin produces the sweat and it absorbs the heat as it evaporates. This is why many sports clothes 'wick' sweat. This means it spreads the sweat so it can evaporate quicker.

Protective Clothing

Many different people in the world need specific protective clothing. Someone who works in an environment at a very high temeperature need clothing to protect him from the heat. Protective clothing are tested on manekins to see if they are effective and could potentially keep someone alive.
The safety suits are made to keep the passengers safe if they have to ditched in the sea in an emergency. If they wear the correct clothing underneath, someone ould survive three hours in the water.