# [Keywords] Transfer of Thermal Energy

### Important Facts That You Must Know First!

1. The rate of heat transfer determines whether an object is a good or poor conductor.
2. Air is a poor conductor of heat and many objects that reduce heat loss via conduction is because they contain/trap pockets of air. Some common examples are:
• fibreglass – used as insulator in the walls of houses.
• felt – sound or vibration damper, use in musical instrument, art and craft and even fashion.
• expanded polystyrene foam – ice box and in refrigerator.
• sawdust – used to cover ice to prevent it from melting too quickly.
• double-glazed windows trap a layer of air between glass panes – help to keep room cold/warm.
3. Water is a poor conductor of heat.
4. Convection currents occur only in fluids but not in solids. This is because convection involves bulk movement of the fluids which carry thermal energy with them due to a change in density.
5. Any objects always emit thermal energy and absorb energy radiated from the surroundings simultaneously. However, it can only be a net emitter or absorber at any one time. It is a net absorber if it is cooler than the surroundings, and a net emitter if it is warmer than the surroundings.
• Aluminium foil used to wrap food to cook in a barbeque. The dull side of the foil is on the outside as it absorbs heat from the barbeque fire faster. This is because dull surface is a good absorber of infrared radiation. The shiny side of the foil is on the inside in contact with the food to reduce heat loss due to radiation from the food. This is because shiny surface is a poor absorber of infrared radiation. NOTE: You need to know when you talk about the emitting or absorbing property based on the context of the question.
• Solar water heaters and cookers use the radiant heat from the sunlight to heat water and food respectively.

### Commonly Seen Question

• A kettle contains some water is put on a stove. After a while, the water in the kettle starts to boil.
• Describe how the heat is transferred from the stove to the water.
• When the kettle is on the stove, the bottom of the kettle gets hot. The water near the bottom of the kettle is then heated due to its contact with the kettle (this is conduction by the way).
• Describe how the heat is distributed in the water. Or how the convection current takes place in the water. Or how the water gets boiled.
• The water at the bottom gets heated, thus it expands, becomes less dense and rises. The cooler water at the top, being relatively denser, sinks to replaced the hot water that has left. The cold water get heated and the process repeats itself. This set up convection currents which circulate the heat throughout the entire water in the kettle.  NOTE: The underline part is to complete the explanation as to why the entire water gets boiled/heated.

• Explain why convection does not occur in solids.
• Molecules in solids do not move about freely (this is the property of molecules in fluids). They can only vibrates about their fixed positions. Since convection requires the bulk movement of molecules from one region to another region, it cannot take place in solids.

• Explain why the air conditioner would be less efficient if it were placed near the bottom of the wall. NOTE: I hope you know why it is placed at the top and know how to modify the explanation from water in kettle.
• If air conditioner is placed near the bottom of the wall, the cooler air that blown out will remain near the bottom of the floor. This is because cooler air, being relatively more dense (not denser) than the surrounding air, cannot float up. Convection currents will not form.  Since air is also a poor conductor of heat, the hotter air will take a long time to transfer the heat to the cooler air. NOTE: Efficient here refers to ability to cool the entire room.

• A solar cooker utilizes sunlight as its energy source as shown below. The solar cooker uses the curve mirror to focus sunlight onto a shiny, black pot.
• Explain the advantage of using a black pot for the solar cooker.
• Black surface is a good absorber of infrared radiation. The temperature inside the pot will increase faster compared to using a light-coloured or white pot and food will be cooked faster.
• Suggest and explain one method to cook food faster using the solar cooker.
• Replace the shiny, black pot with a dull, black pot. This is because a dull and black surface is a better absorber of infrared radiation than shiny and black surface.

• The image below shows a filament bulb standing upright in the air.
• Describe briefly two convection currents which are set up.
• One convection current that can be set up is inside the bulb. When the inert gas near the hot filament is heated, it expands, becomes less dense and rises. The cooler inert gas, being relatively denser, sink to replace the hot gas that has left. The cooler inert gas will get heated and the process repeats itself. This will set up convection currents. Due to the radiation and convection currents inside, the glass bulb is heated up as well. The air near the glass bulb gets heated, it expands, becomes less dense and rises. The cooler surrounding air, being relatively denser, sink to replace the hot air that has left. The cooler air will get heated and the process repeats itself. This will set up a second region of convection currents.