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Theory:

In this exercise, an activity is explained to understand linear expansion.
 
Materials Required:
  1. Bulb
  2. Dry cell
  3. Candle
  4. Cycle spoke
  5. Coin (or broad-headed nail)
  6. Two wooden blocks.
Procedure:
  • Locate one end of the cycle spoke on a wooden block, and an electric wire is attached to it.
  • As shown in the figure, put a stone over the spoke to hold it firmly in place on the wooden block.
Linear expansion.jpg
Experiment on linear expanision
  • The spoke should be parallel to the ground.
  • Locate the second wooden block under the free end of the spoke.
  • Wind some electric wire around the coin (or nail) and locate it on the block.
  • You may place a stone over the coin to hold it in position.
  • Attach a bulb and dry cell to the free ends of the wires attached to the coin and the spoke and form the circuit as shown in the above figure.
  • When the tip of the free end of the spoke is in contact with the coin, the circuit is closed, and the bulb lights up.
  • Verify to ensure this. If the bulb does not glow, it indicates the circuit is not closed, so check your connections properly.
  • Now slide a page of your book between the coin and spoke, and then slide it out.
  • In this way, you would get a gap between the coin and spoke equal to the thickness of the sheet of paper.
Linear expansion:
When we heat a solid, if there is an increase in the body's length, this is known as Linear expansion. It is also called Longitudinal expansion.
 
The increase in the length is proportional to the rise in temperature, original length, and material type. The amount by which the unit length of a material increases when the temperature is raised by one degree is called the coefficient of linear expansion.
 
1. Does the bulb light up? If it does not, what could be the reason?
 
The bulb does not light up because the coin and the spoke do not touch each other. The circuit is not closed (open circuit).
  
2. Did the bulb light up after the spoke was heated using a candle for some time?
 
When the spoke gets heated, it will be expanded. Here, linear expansion takes place. So the spoke touched the coin. The spoke is heated, which causes the circuit to be completed (Closed-circuit), and the bulb is light up.
 
3. Why does the bulb go off sometime after the candle is taken away from the spoke?
The spoke does not get the heat once the candle is removed. The spoke is back to its original state. So the spoke does not touch the coin once again, and the bulb goes off.
 
4. What happens to the length of the spoke when it is heated or cooled?
The length of the spoke is expanded (increased) when the spoke is heated, and the length of the spoke is contracted (decreased) when the spoke is cooled.
When we heat the substance, it expands because the increase in the molecule's energy creates more vibrations and movements, usually creating more distance between themselves. Similarly, it contracts when cooled.
Reference:
TN Textbook: https://drive.google.com/file/d/1f27AArb7QaALnAURIxm6cLs0go6hWCDc/view - Page Number - 10.