Applications of Thermal Energy Transfer

Common Applications of Conduction:

If heat has to be transferred quickly through a substance, good conductors are used (metals). If heat is unwanted we use insulators to prevent themal energy from transferring quickly or minimise the loss of thermal energy.

- Uses of Good Conductors of Heat (Metals)
1) Cooking utensils
- Made of aluminium of stainless steel
- Direct heating is involved
- Requires the heat energy to cook the food

2) Soldering iron rods


















- Made of iron, but tip is made of copper - copper is a much better conductor of heat

3) Heat exchangers















(Click picture to Enlarge)

The hot dirty water goes in from the tube inlet, passes the water to the cold clean water, which enters in the shell inlet and exits from the shell outlet. The hot dirty water then exits from the tube outlet.

- Uses of Bad Conductors (Insulators) of Heat (Non-Metals)
1) Handles of appliances and utensils
- Handles of cooking kettles, cooking pot, pans, irons and soldering iron rods are made of insulators of heat like wood or plastic.
- Prevents hands from getting scalded when picking it up

2) Table mats
- Usually made of cork
- Prevents table-top from getting damaged by the intensed heat from the hot kitchenware

3) Sawdust
- Used to cover ice blocks - good insulator

4) Wooden/Plastic ladles
- Good for stirring or scooping hot soups/scooping cooked rice that has just been cooked

5) Woollen clothes
- Traps heat
- Keeps people warm on cold days/winter

6) Fibreglass, felt and expanded polystyrene foam
- Traps large amounts of air
- Used as insulators in the walls of houses, ice boxes, refrigerators, etc.
- Double-glazed windows have air trapped between two panes of glass - reduces thermal energy transfer through windows


Common Applications of Convection:
1) Electric kettles
- Heating coil is always placed at the bottom of the kettle - aids transfer of thermal energy in water (convection)
- Water around the heating coil is being heated up and rises to the top as it becomes less dense
- The cooler water at the top will then sink as it is denser than the warm water
- It comes into contact with the heating coil and gets heated up, hence rising
- Hence a convection current is formed

2) Household hot water systems




























3) Air conditioners
- Always installed near to the ceiling of a room to facilitate convection
- Rotary fan inside an air conditioner releases cool dry air into the room (dense, hence sinks)
- Warm air below (less dense) rises and is being cooled as it is near the air conditioner.
- Hence air is recirculated and the temperature of air will eventually fall to the desired value

4) Refrigerators
- Similiar to air conditioners
- Freezing unit is placed at the top to cool the air and facilitate convection
- Convection currents help to cool the food items inside

Common Applications of Radiation:

1) Teapots

- Shiny surfaces are bad emitters of radiation - keep tea warm for a longer time than black teapots

- Shiny surfaces are bad absorbers of radiation - keep cold liquids cool for a longer time than black containers

2) Greenhouses

- A greenhouse is used in cold climates to help plants grow better

- Traps heat in

- During day, infrared radiation from the Sun enters the greenhouse by passing through the glass roof

- Contents in the greenhouse get warm - emit infrared radiation

- Infrared radiation emitted cannot pass through glass roof - trapped in greenhouse

- Over time, the amount of infrared radiation in the greenhouse gets trapped - temperature in the greenhouse increases

3) Vacuum flasks (Thermos flask)

- Designed to keep liquids hot by minimising heat loss in four possible ways: conduction, convection, radiation and evaporation

- Stopper is usually made of plastic, which is an insulator of heat

- Conduction through the trapped air above the liquid is minimal because air is an insulator of heat

- Conduction and convection through the sides of the flask are prevented by vacuums between the double-glass walls of the flask

- To minimise heat loss through radiation - walls of the glass are silvered - reflects radiant heat back into the hot liquid

- Convection and evaporation can only occur when the plastic stopper is removed

- Radiation is harder to stop as it occurs through vacuums

Transfer of Thermal Energy - Radiation

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.

Transfer of Thermal Energy - Convection

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.

Transfer of Thermal Energy - Conduction

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.

Transfer of Thermal Energy

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.