Heat Transference 24/04.

Question 1

What is heat and what is it continually used in the home for?

Answer 1

Heat is a form of energy and it is continually used in the home to:

• Cook food.
• Heat rooms.
• Heat water.

Question 2

What does heat do in the body?

Answer 2

In the body, some of the energy released from food during digestion is converted into heat to maintain body temperature.

Question 3

When is heat produced?

Answer 3

Heat is produced when the molecules in a substance vibrate and move rapidly. The faster the movement, the more heat energy is produced.

Question 4

The amount of heat energy produced is measured on what?

Answer 4

The amount of heat energy produced is measured on a temperature scale, using a thermometer, which indicates the degree of heat.

Question 5

What scale is used to measure temperatures?

Answer 5

Temperatures are normally measured on the Celsius scale, although, the Fahrenheit scale is also sometimes used.

Question 6

What is the underlying principle in the cooking of food?

Answer 6

Heat must pass from the cooker to the food at a suitable rate.

Question 7

How can heat energy be transferred?

Answer 7

Heat energy can be transferred from one point to another.

Question 8

How does heat flow and is this process reversible or irreversible?

Answer 8

Heat flows from a higher temperature to a lower temperature until a constant temperature is reached (this process is reversible).

Question 9

Heat can be transferred by three means. Name them?

Answer 9

Heat can be transferred by conduction, convection and radiation.

Question 10

When does conduction happen?

Answer 10

Conduction happens through solids by contact molecules vibrating rapidly with neighboring molecules and generating heat.

Question 11

What is conduction?

Answer 11

Conduction is the transfer of heat through a solid.

Question 12

What are examples of conduction?

Answer 12

• metal spoon in hot liquid.
• frying bacon in a pan.
• cake in a cake tin.

Question 13

When a metal spoon is placed in boiling water [it gets hot] heat energy is transferred along the whole length of the spoon. Why?

Answer 13

This is because molecules at the end of the spoon start to vibrate quickly as heat energy from the water is transferred to them.

Question 14

When a metal spoon is placed in boiling water it gets hot]. What is this method of heat transference?

Answer 14

Conduction.

Question 15

What are the advantages of conduction?

Answer 15

• quick e.g. frying.
• boiling does not need constant attention.
• several dishes can be baked at once as heat from all oven shelves passes to baking tins.

Question 16

When does convection happen?

Answer 16

Convection happens through liquids and gases.

Question 17

What is convection?

Answer 17

Convection is the transfer of heat through liquids and gases.

Question 18

What are examples of convection?

Answer 18

• baking and roasting- movement of air molecules.

- boiling and steaming - movement of liquid molecules.

Question 19

In what way does a convection current set up until a constant temperature is reached?

Answer 19

As a liquid is heated it expands and rises, cooler liquid moves to take its place, it is then heated in turn and then rises and in this way, convection currents are set up until a constant temperature is reached.

Question 20

Advantages of convection?

Answer 20

• may cook several dishes at once - tiered steamer.
• does not need constant attention.
• no added fat if steam or water is used - more healthy.

Question 21

Disadvantages of convection?

Answer 21

• some methods take a long time - steaming.
• some water-soluble nutrients are lost.
• boiled and steamed dishes lack color and texture.

Question 22

How is heat energy is conduction and convection similar?

Answer 22

In conduction and convection heat energy is transferred from one place to another through a medium.

Question 23

Since radiation does not travel through a medium, how does it travel?

Answer 23

They travel through space or vacuum, the rays from the heat source travel in straight lines and fall onto the food in its path.

Question 24

What are examples of radiation?

Answer 24

• grilling - partly depends on radiation including roasting.
• infra-red grills - have electrically heated wire elements.
• microwave oven - food cooked by microwaves.

Question 25

Advantages of radiation?

Answer 25

• quick method like grilling.

- attractive brown crisp surface.

Question 26

Disadvantages of radiation?

Answer 26

• needs careful attention as it is easy to overcook.
• food needs to be turned and basted.
• only suitable for thin pieces of food.

Question 27

Most cooking methods use a combination of all 3 heat transference methods e.g. a sponge cake.
Explain how a sponge cake uses all three methods.

Answer 27

A sponge cake is cooked by:

• heat reflecting from the oven walls (radiation).
• heat circulating in the air within the oven (convection)
• heat transferred from the cake tin to the cake mixture (conduction).

Question 28

What can be a prevention of radiation and why?

Answer 28

Prevention of radiation is a vacuum flask because no heat is conducted across the vacuum and heat rays are reflected off the silvered interior into the liquid.