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St. Anthony's Canossian Secondary SchoolTransfer of Thermal Energy

Name: Class: Sec Date:

How to answer question on heat transfer?

When answering questions on heat transfer, you should include the following information:

What is the process of heat transfer?

If the material of the object is mentioned in the question, the process is likely to be conduction.

If the surface texture, colour is mentioned, it is likely to be radiation.

If it is about the position of the heating or cooling unit, then the process is convection

Is heat entering or leaving the object?

Heat will enter an object if it is cooler than the surrounding.

Heat will leave an object if it is hotter than the surrounding.

Should the rate of heat transfer be fast or slow?

If you want to keep the temperature of the object constant, the rate of heat transfer should be slow.

If you want the object to cool down or heat up fast, then the rate of heat transfer should be fast.

What kind of material should be used?

If we want to slow down (speed up) conduction, we should use good insulator (good conductor).

If we want to slow down (speed up) radiation, we should use poor emitter/absorber (good

emitter/absorber).

Sampl e Template:

xxx is a ( good / poor ) ( conductor / emitter / absorber ).

Hence the rate of heat transfer ( into / out ) of the object by [method of heat transfer] will be

( faster / slower ). This will enable .

Note: When answering questions on radiation, DO NOT mention that a surface is a good/poor emitter and

absorber. You must decide whether the surface is acting as an absorber or emitter. This can be done by

comparing the temperature of the object with the surrounding. If it is hotter than the surrounding, it will emitheat. If it is cooler than the surrounding, it will absorb heat.

Good conductors Metals

Poor conductor (Good Insulator) Wood, plastic, rubber, wool, Styrofoam, liquid, gas, vacuum

Good emitter / Good absorber Black, dull, rough surface

Poor emitter / Poor absorber Shiny, white, smooth surface

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Example:

1. Why do ices kept in Styrofoam box take a long time to melt?

Styrofoam is a good insulator of heat.

Hence, the rate of heat transfer into the box by conduction will be slower.

This will enable the ice to stay cold for a longer period of time.

2. Would you use a black or white cup if you want your cup of hot coffee to remain hot for a longer periodof time?

I will use a white cup because white surface is a poorer emitter of heat then black surface.

Hence, the rate of heat transfer out of the cup by radiation will be slower.

This will enable the coffee to stay hot for a longer period of time.