Radiation Definition: Radiation is the continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium This means that energy is transported through space by electromagnetic waves.
Radiation involves waves to transfer thermal energy. It is possible even in a vacuum. That is how the heat from the Sun travels to the Earth. The Sun emits electromagnetic waves. These waves makes us feel warm. Thermal energy from infrared waves is called radiant heat.
The hotter the object, the greater the amount of radiant heat emitted.
All sources of heat radiate heat to a certain extent. Even human beings radiate heat to a certain extent.
Some factors affect the rate of infrared radiation, like: 1) Colour and Texture of the surface. Black and dull surfaces absorb infrared radiation better than whte and shiny surfaces.
2) Surface Temperature. The higher the temperature of the surface of the object relative to the surrounding temperature, the higher the rate of infrared radiation.
This means that there is a direct proportion between the surrounding temperature and the temperature of the object's surface.
3) Surface Area. The larger the object is, the faster it will emit infrared radiation.
This means that the size of the surface area of the object is directly proportional to the rate of infrared radiation emission.
Convection Definition: Convection is the transfer of thermal energy by means of currents in fluids (liquids or gases) This means that the transfer of heat is actually by movement of the warmed matter, which is either liquid or gas.
Convection occurs only in liquids and gases and not in solids because convection requires the bulk movement of the fluids which carry thermal energy. Solids transfer thermal energy from one particle to another through vibration without any bulk movement of the particles.
How does convection work?
In water:
When water is heated, it expands, making it less dense than the water that is not heated, therefore it rises, as shown in the graphic above as the red arrow. When the heated water rises, the cooler regions of the water will be denser than the heated water, hence sinking to the bottom, as shown in the graphic as the blue arrows. The cooler water will then reach the heat source and start heating up, hence being the less dense one, whereas the heated water will start cooling at the top, becoming more dense and sinking to the bottom. It will then repeat itself, forming a convection current.
In air:
The air above the heat source gets heated up and becomes less dense than the air around it, hence rising. The air at the top, being denser than the air that has rised, sinks to the bottom and reaches the heat source. It gains heat and rises, while the air at the top reaches the cool area with ice and loses heat, hence becoming denser and sink to the bottom, forming a convection current.
CONVECTION IN DAILY LIFE
Whenever we are at the seaside during daytime, there will usually be a sea breeze. This breeze is a result of convection. There is also a land breeze, that is, the wind from land will be movig towards the sea. This happens at night.
Because water has a higher heat capacity than land, the temperature of the sea does not really much. However, temperatures on land could change alot between day and night. For example, during the day, temperature on land increased by more than 10 Degrees Celsius, but the temperature in the sea might have only increased by about 1 Degree Celsius. Likewise for night time, if the temperature on land dropped by over 10 Degrees Celsius, temperature in the sea might have dropped by only few Degrees Celsius.
Therefore, during daytime, the air above the land will be much warmer than the air above the water, hence rising while the cool air will move towards the land, causing the sea breeze. During night time, the air above the sea will be warmer than the air above the land so the air above the sea will rise and the air above the land will move towards the sea, causing land breeze.
Convection can also occur in houses during winter. The air in the house will be cold, but once it is near the heater the air heats up and become less dense than the surrounding air, causing it to rise. Once it rises it loses heat to the surrounding air and become denser so it sinks back down. It will then move nearer to the heater and rise again. This forms a convection current in the house during winter. It is illustrated in the picture on the left.
Conduction Definition: Conduction is the process of thermal energy transfer without any flow of the material medium This means that it is the transfer of energy through matter from particle to particle.
Conduction requires an object in between to transfer the thermal energy. No movement is required in conduction as the object stays in its place during conduction. The object is either called a 'good conductor of heat' or a 'poor conductor of heat', otherwise known as 'insulator', depending on the rate it conducts heat at. Usually, metals are good conductors of heat whereas non-metals are insulators of heat.
All metals and non-metals, which are known as solids, are made up of tiny particles called atoms and molecules. The only difference is that metals have many free electrons while non-metals don't. These free electrons speed up the rate of conducting heat as they move randomly between the atoms and molecules. This will be further explained in the next paragraph.
The process of conducting heat can be illustrated with two rods, one metal and one non-metal. When you heat up the rods at one end, the particles will start vibrating more rapidly, knocking into their neighbouring particles, transferring the heat over. The neighbouring particles will start vibrating too and knock into their neighbouring particles, and it will carry the heat all the way throughout the rod. However, in metals, they have free electrons that do not seem to be attracted to any nucleus, hence are moving around very fast and by themselves. When the particles start to vibrate faster, these electrons will speed up and diffuse into the cooler parts of the metal and transfer their kinetic and heat energy to them. In this way, the electrons are helping to increase the rate of which heat spreads throughout the metal, hence metals are good conductors of heat.
Conduction in liquids and gases is of the same concept as conduction in solids. However, the particles in liquids and gases are quite far apart and hence, the chances of the particles knocking into each other is quite low. Since the particles do not really touch each other, chances of passing heat is low. Therefore, liquids and gases are poor conductors of heat as compared to solids.
The experiment below illustrates that water is a poor conductor of heat.
The water at the top nearest to the flame is boiling, but the ice cube wrapped in metal gauze at the bottom of the test tube has barely melted. This further illustrates the point that water is a poor conductor of heat.
Cooking pots are made of metal and act as the medium in cooking because without the pot, the food is more likely to catch fire. Also, the pot must be a good conductor of heat as it is acting as the medium for conduction of heat. Another reason it must be a conductor of heat is because most conductors of heat are metal, and metal will not melt or burn so easily like wood or plastic at cooking temperature.
Thermal energy is transferred by three processes: Conduction, Convection and Radiation. These three processes are found in our everyday life, like cooking, and will be explained in greater detail in the next three posts.
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