Lower Secondary Science - Transfer of Heat Energy and its Effects

Here are some main points to take note for students who are preparing for the upcoming WA:

The SI unit of temperature is kelvin (K).

Describe some effects and applications of expansion and contraction in everyday life

• Application

  • Riveting - heat up a rivet and insert it through the hole of 2 metal plates. When the rivet cools, it will contract and hold the plates tightly together

  • Fitting a wheel into a metal tyre - heat up metal tyre first so that it can expand. Fit wheel inside. When tyre cools down and contracts, it will fit nicely around the the wheel

  • Bimetallic - used for thermostats where temp can be regulated automatically. Common metals - iron and brass. Brass expands and contracts greater than iron. The two metals are connected together, forming a bimetallic strip. This will cause the strip to bend, thus disconnecting the circuit when needed.

    • Used for fire alarms, ovens and electric iron
      

• Problem

  • Railway tracks built continuously will buckle during a sunny day as they gain from the sun and expands. Due to the lack of room for expansion, they will bend sideways. Solution --> ensure small gaps are present between sections of tracks

  • Concrete road surfaces --> similar to railway tracks

  • Pipes - similar to above except that pipes may leak when they don't have room to expand or contract. The solution would be to create curved joints to allow the pipes room for change in volume

  • Bridges - when both ends of a bridge is fixed, the bridge can be damaged easily due to contraction and expansions that results in buckling (i.e. bending). Solution --> movable joint at one end will allow room for the bridge and expand and contract.

Transfer of heat energy through conduction, convection, and radiation

1. Conduction - transfer of heat from higher temperature to lower temperature through a medium without a movement of the particles

   1. 2 processes of conduction

      1. Vibration of particles --> transference of vibration between particles as particles near the heat source gain energy and increase in vibration. These particles collide with neighboring particles, transferring the vibration to them. This process is slow.

      2. Free electron diffusion --> as metals contain delocalised electrons, these electrons can transfer heat energy much faster by traveling freely from the heat source to the heat sink. This process is fast and it's the reason why metals are good conductors of heat.

   2. In general, solids are the best conductors while gases are the poorest. Why? It's due to distance between particles.

2. Convection - transfer of heat from higher temperature to lower temperature through a medium with a movement of the particles

   1. It can only take place in liquid and gases. Why? Think of kinetic model

   2. Convection current has to do with density (important keyword). --> region particles gain heat and expands, which then decreases the density. Less dense fluid will rise, while denser fluid sinks. This sets up a convection current.

3. Radiation - transfer of heat from higher temperature to lower temperature without movement.

   1. Factors - surface area, color, texture, temperature

   2. Greater surface are = greater rate of radiation

   3. Dark colors are good radiators or good absorbers and emitters of heat. Bright colors are the opposite

   4. Dull surfaces are good radiators or good absorbers and emitters of heat. Smooth surfaces are the opposite

   5. Higher the temperature = greater rate of radiation

Case Study - Vacuum Flask - how does it keep the beverage warm?

1. Lid --> reduces the air escaping from the inside of the flask --> thus, reducing the set up of convection current

2. Vacuum Layer between Walls --> Prevents heat transfer through conduction and convection as they require a medium to travel

3. Shiny Silver Wall --> Reflects radiant heat back into the beverage, keeping it warm for a longer time

4. Trapped air at top of beverage --> air is a poor conductor of heat. This reduces the heat loss via conduction

While these are the main points, further details can be found in your notes. Make sure you read up!

This topic can be easily mastered so long you are able to remember the keywords and proper answering technique. Thus, mind maps or cheat sheets are important.