Demonstrating Conduction (Cambridge (CIE) IGCSE Physics): Revision Note

Experiments demonstrating thermal conductors

Good and bad thermal conductors

• Good thermal conductors are materials that transfer thermal energy quickly

  • For example: an aluminium pan or copper wire

• Bad thermal conductors (also called thermal insulators) are solids which do not transfer thermal energy well

  • For example: a wool blanket or layers of cardboard or paper

Comparing conduction in tiles and textiles

• This conduction experiment 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 it is at thermal equilibrium (the same temperature) with the floor

• Stand with bare feet, or use the palms of your hands instead

  • 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

  • In fact, they are at exactly the same temperature

Explanation

• Tiles and stone are good conductors of thermal energy

  • Where the foot touches the tiles, thermal energy is transferred away from the foot, making it feel cold

  • The foot has become colder since it lost thermal energy to the tiles

• Textiles such as rugs are thermal insulators, meaning they are bad conductors of thermal energy

  • Where the foot touches the rug, thermal energy is not transferred away from the foot

  • This foot feels relatively warmer than the one which has lost thermal energy to the tiles

  • This foot has lost very little thermal energy, so it stays close to its starting temperature

Comparing conduction in wood and metal

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

• 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

Explanation

• Metal is a good conductor of thermal energy

  • Where the paper touched the metal in this conduction experiment, thermal energy was transferred from the paper into the metal and along the length of the metal

  • This prevented the paper from getting hot

• Wood is a thermal insulator, meaning it is a bad conductor of thermal energy

  • Where the paper touched the wood, thermal energy 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 conduction experiment to demonstrate the relative conducting properties of different materials can be carried out using apparatus similar to that shown in the diagram below

• 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 thermal energy 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

• The better the conductor, the shorter the time taken for the wax to melt and the ball bearing to fall