Chapter 10 Transfer of Thermal Energy

Question 1

When does the transfer of thermal energy take place?

Answer 1

Net flow of thermal energy occurs only when there is a difference in temperature. Thermal energy is always transferred from a region of higher temperature to a region of lower temperature.

Question 2

Define conduction.

Answer 2

Conduction is the transfer of thermal energy through a medium without any flow of the medium.

Question 3

Describe the process of transferring heat through a non-metal rod through conduction.

Answer 3

1) The particles at the hot end vibrate vigorously about their fixed positions.
2) They collide with neighbouring particles, making them vibrate more vigorously. (The kinetic energy of the vibrating particles at the hot end is transferred to the neighbouring particles.)
3) The neighbouring region of the rod becomes hot.
4) Eventually, the particles at the cooler end of the rod are also set into vigorous vibration.
5) The cooler end of the rod becomes hot.

Question 4

What is the additional mechanism of thermal transfer that takes place in metals? Describe the process of transferring heat through this process.

Answer 4

Free electron diffusion

1) The free electrons at the heated end absorb thermal energy, and hence gain kinetic energy.
2) The free electrons that gain kinetic energy move at greater speeds, and move to the cooler regions of the rod.
3) As these electrons move, they are free to move in the spaces between the particles before colliding with the atoms in the cooler parts of the rod, making them vibrate more vigorously.
4) Thermal energy is transferred via the motion of the free electrons. The cooler end of the rod becomes hot.

Question 5

Why are air and water poor conductors of heat?

Answer 5

The particles in liquids and gases are spaced further apart than those in solids. The collisions between the particles are therefore less frequent, which means the transfer of kinetic energy from the fast-moving particles (in the hot region) to neighbouring particles (in the cooler region) is slower.

Question 6

Define convection.

Answer 6

Convection is the transfer of thermal energy by means of convection currents in a fluid (liquid or gas), due to a difference in density.

Question 7

Describe the circulation process of a convection current in a liquid.

Answer 7

• When a liquid is heated, it expands
• Becomes less dense and rises
• The colder and denser liquid at the top sinks to replace it
• The colder liquid in turn gets heated up
• Difference in densities of water at different regions sets up a convection current

Question 8

Why does convection only take place in fluids (i.e. liquids and gases)?

Answer 8

This is because convection involves the bulk movement of the fluid that carries the thermal energy.

Question 9

Define radiation

Answer 9

Radiation is the transfer of thermal energy in the form of electromagnetic waves such as infrared radiation without the aid of a medium.

Question 10

What are the three factors that affect the rate of emission and absorption of infrared radiation?

Answer 10

1. Colour and texture of the surface
   - Dull and black surfaces > shiny and silver surfaces
2. Surface temperature
   - The higher the temperature of an object’s surface relative to the surrounding temperature, the higher the rate of emission of infrared radiation. (vice versa for absorption)
3. Surface area
   - The larger the surface area of the object, the higher the rate at which the object will emit/ absorb (CHOOSE ONE) infrared radiation.