Demonstrating Conduction (Cambridge O Level Physics)

• Good thermal conductors are solids which easily transfer heat
  • For example; a metal pan or a ceramic tea cup
• Bad thermal conductors (also called insulators) are solids which do not transfer heat well
  • For example; a woolen blanket or layers of cardboard or paper

Comparing Conduction in Tiles and Textiles

This demonstration shows why homes use rugs and carpets

• Find a tiled or stone area of floor 
  • In the same room leave a rug or bath towel (not a thin cloth, it must be thick)
  • The textile must stay in place on the floor for several hours to ensure they are at thermal equilibrium (the same temperature)

Standing on a Tile and a Rug

One person with a foot on each allows a qualitative comparison of temperature between the two

• Stand with bare feet (hands can be used)
  • Place one foot on the tiles or stone area, and the other on the textile (towel or rug)
  • Observe the apparent temperature of the two materials as felt through the feet
  • It will feel as though the tiles are cold while the rug is warm, however, they are at exactly the same temperature

Transfer of Energy through Tile and Rug

Energy is transferred by heating from the warm body to the cooler tiles by conduction

Explanation

• Tiles and stone are good conductors of heat
  • Where the foot touches the tiles, heat is transferred away from the foot, making it feel cold
  • The foot has become colder since it lost heat to the tiles
• Textiles such as rugs are good insulators, meaning they are poor conductors of heat
  • Where the foot touches the rug, heat is not transferred away from the foot
  • This foot feels relatively warmer than the one which has lost heat to the tiles
  • The foot has stayed at its starting temperature

Comparing Conduction in Wood and Paper

• A cylindrical rod made of half wood and half metal is wrapped tightly in paper

Testing Conduction in Wood and Metal

The paper is heated. The two different materials conduct energy away from the paper's thermal store at different rates

• Using a gentle flame, and holding the rod clear of the top of the flame, gently heat the paper at the join of the wood and metal
  • Turn the rod so that the paper is well-heated all around the circumference of the rod
  • Stop when the paper is clearly discoloured

• Remove the rod from the flame, gently unwrap the paper and observe the burn pattern
  • A distinct pattern is seen;
    • Where the paper touched the metal surface it is undamaged
    • Where the paper touched the wood surface it is charred

Pattern of Charring on the Paper after the Experiment

The side of the paper of the wood shows signs of being burned, so there was more energy in its thermal store on this side

Explanation

• Metal is a good conductor of heat
  • Where the paper touched the metal, heat was transferred from the paper into the metal and along the length of the metal
  • This prevented the paper getting hot
• Wood is a good insulator, meaning it is a poor conductor of heat
  • Where the paper touched the wood, heat was not transferred from the paper
  • This meant that the paper did get hot enough to start to burn

Demonstrating Different Rates of Thermal Conduction in Metals

• A simple experiment to demonstrate the relative conducting properties of different materials can be carried out using apparatus similar to that shown in the diagram below

Apparatus for Conduction Ring Experiment

The above apparatus consists of 4 different metal strips of equal width and length arrange around an insulated circle

• Ball bearings can be stuck to each of the strips and equal distance from the centre, using a small amount of wax
• The strips should then be turned upside down and the centre heated gently using a candle, so that each of the strips is heated at the point where they meet
• When the heat is conducted along to the ball bearing, the wax will melt and the ball bearing will drop
• By timing how long this takes for each of the strips, their relative thermal conductivities can be determined