Chapter 10 - Transfer of Thermal Energy

1.

Describe, in molecular terms, how energy transfer occurs in solids.

  • When a substance (object) is heated at one end, the particles at the region gains thermal energy and vibrate faster.
  • These molecules collide with the less energetic neighbouring particles and transfer KE to them.
  • In this way, thermal energy is transferred along the entire substance (object) by molecular collisions from the hot end of the substance (object) to the cooler end.

2.

Why is metal a better conductor of heat than non-metals?

  • In metals, apart from transferring heat energy by the relatively slow process of molecular collision, another faster process of free electron diffusion takes place.
  • Metals contain free electrons that are mobile.
  • When heated, these free electrons gains KE and will diffuse to the cooler end at high speeds.
  • In the process, they will collide with atoms at the cooler end and transfer kinetic energy to them.

3. 

Explain why materials such as fiberglass, wool, polystyrene, fur are commonly used as heat insulators.

  • Fibreglass, wool, polystyrene and furs are poor conductors of heat because they are non-metals.
  • In addition, they contain trapped, still air, which is a poor thermal conductor as well.

4.

Describe, in terms of density changes, heat convection in fluids.

  • When a fluid is heated at the base, it will expand, become less dense and rise.
  • The cooler fluid at the top being denser, will sink.
  • This continuous, cyclic movement of the fluid known as convection current will eventually heat up the entire whole fluid evenly.

5. 

Explain why the heating element of a kettle should be positioned at the base.

  • Heated liquid rises and cooler liquid will sink.
  • Placing the heating element at the base allows convection current to set up, eventually heating up the entire liquid.

6.

In some water heating system, the heating element is placed in the middle instead of the base. State one advantage of this arrangement.

  • Less energy will be used as only the upper half of the water in the container will be heated.

7. 

Two cars, one white, one black are parked in the sun. After some time, one is much hotter to touch than the other. Which car is it? Explain your answer.

  • The black car feels hotter.
  • This is because black surfaces are better absorbers of infrared radiation than white surfaces.

8.

Two kettle, one silver, one white, contains water at 90 oC. After some time, one is cooler than the other. Which kettle is it? Explain your answer.

  • The black kettle feels cooler.
  • This is because black surfaces are better emitters of infrared radiation than silvered surfaces.

9.

Explain why heat loss from a cooking pan is reduced by fitting it with a lid.

  • The lid prevents hot air from rising, hence reducing heat loss due to convection.
  • It also reduces heat lost by evaporation (if a liquid is present in the pot).

10. 

*Explain why the filament reaches a constant temperature, even though heat is produced continually as current flows through the bulb.

  • As electricity passes through the filament, thermal energy is produced and the temperature of the filament increases.
  • At the same time, thermal energy is lost to the surrounding air by radiation as the temperature of the filament is higher than the surrounding.
  • The filament reaches a final constant temperature when the rate of increase of thermal energy by the filament is equal to the rate of thermal energy loss by the filament to the surrounding.
  • It is said that the filament have reached thermal equilibrium.