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
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