Thermal Processes

An experiment is conducted to find out which metal is the best thermal conductor. Drawing pins are placed on melted wax on the metal rods and the wax is left to harden. The rods are inserted into a hot water bath to the same depth. Some of the pins fall off after some time.

Which metal is the best thermal conductor?

• metal A

• metal B

• metal C

• metal D

A match will not ignite if it is held 10 cm from one side of the flame. It will ignite if it is held 10 cm above a flame. This is due to energy transferred by ____________.

• conduction

• convection

• diffusion

• radiation

Energy is transferred from the sun to Earth through a vacuum.

How is energy transferred from the sun to Earth?

• by conduction only

• by convection only

• by radiation only

• by convection and radiation

The outer surface of an electric kettle is always kept polished and shiny. What is the purpose of this?

• to reduce energy transfer by conduction

• to reduce energy transfer by convection

• to reduce energy transfer by radiation

• to reflect any radiation incident on its outer surface

Which of the following occur in a metal to transfer energy by conduction?

• A

• B

• C

• D

The following shows the cross-section of a plastic container that a manufacturing company has created.

The company claims that the container can keep food warm for a duration that surpasses other containers. It offers the following explanations to justify their claims.

1         The plastic cover will reduce energy transfer by conduction as plastic is a thermal insulator.

2         The black inner walls will transfer energy by heating to the food. This keeps the food warm since black surfaces are good absorbers of thermal radiation.

3         The vacuum between the interior and exterior walls of the container will prevent energy transfer by heating to the surroundings through conduction, convection and radiation.

Which of the claims explain(s) why the plastic container keeps food warm for a long period of time?

• 1 only

• 1 and 2 only

• 2 and 3 only

• 1, 2 and 3

Stainless steel has a lower thermal conductivity than aluminium. A consultant for a cookware manufacturer wishes to make a pan that will have these two features.

·        Transfers energy from a flame to the entire pan as quickly as possible

·        Have a cooking surface that stays as hot as possible when heated

 

Which pan would be recommended based solely on its thermal properties?

• aluminium pan with a black outer surface and a shiny inner surface

• aluminium pan with a shiny outer surface and a black inner surface

• stainless steel pan with a black outer surface and a shiny inner surface

• stainless steel pan with a shiny outer surface and a black inner surface

The following shows four spherical objects of the same colour and same type of surface. Which object emits infrared radiation at the highest rate?

• A

• B

• C

• D

Containers A and B are filled with equal volumes of hot water at the same temperature. After some time, the temperature of water in container A has a much higher temperature than container B.

What are the possible reasons?

1   Container A is painted white and container B is painted black.

2   Container A is covered with a lid and container B is not covered.

3   Container A is made of plastic and container B is made of aluminium.

• 1 and 2 only

• 1 and 3 only

• 2 and 3 only

• 1, 2 and 3

An experiment was carried out using four bars. The bars are of the same size, and the end of each bar is placed in water at 90 °C.

The time taken for the temperature at the other end of each bar to rise by 2 °C is recorded.

A metal tank is to be made with the least energy transfer from its contents to the surroundings by heating. Which material should be used for the tank and its insulation?

• Tank - aluminium

  Insulation - fibreglass

• Tank - aluminium

  Insulation - polystyrene

• Tank - copper

  Insulation - fibreglass

• Tank - copper

  Insulation - polystyrene