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Linear and Cubical Expansion
A solid has a definite shape, so when a solid is heated, it expands in all directions i.e., in length, area and volume, all increase on heating.
The expansion in length is called linear expansion and the expansion in volume is called cubical expansion.
Why is the iron rim of a bullock cart wheel heated before it is fitted onto the wheel? Why is a small gap left between two lengths of railway lines?
We can perform an interesting experiment to find out an answer to these questions. All we need to do is to heat a cycle spoke.
Take a bulb, dry cell, candle, cycle spoke, coin (or broad - headed nail) and two wooden blocks.
Place one end of the cycle spoke on a wooden block and connect an electric wire to it. Put a stone over the spoke to hold it firmly in place on the wooden block, as shown in Figure . The spoke should be parallel to the ground. Place the second wooden block under the free end of the spoke. Wrap some electric wire around the coin (or nail) and place it on the block. You may put a stone over the coin to hold it in place.
Connect a bulb and dry cell to the free ends of the wires connected to the coin and the spoke and make the circuit shown in the figure.
When the tip of the free end of the spoke touches the coin, the circuit is completed and the bulb lights up. Check to ensure this. If the bulb does not light up, it means the circuit is not complete, so check your connections properly. (Note: We will learn about electric circuit elaborately in electricity lesson.) Now slide a page of your book between the coin and spoke and then slide it out. That way you would get a gap between the coin and spoke equal to the thickness of the sheet of paper.
* Does the bulb light up? If it does not, what could be the reason?
The bulb does not light up. There is a gap between the coin and the spoke. So the electrical circuit is not complete. Therefore the bulb does not light up.
You saw that the bulb does not light up when the spoke does not touch the coin. Now light the candle and heat the spoke with it.
• Did the bulb light up after the spoke was heated for some time?
Yes, the bulb lit up.
• If it did, then explain how the spoke touched the coin after it was heated.
When the spoke was heated it expanded in length and it
touched the coin.
• Why does the bulb go off some time after the candle is taken away from the spoke?
When the candle is taken away the spoke becomes cool. It contracts. There is a gap between the spoke and the coin. As the circuit breaks the bulb goes off.
• What happens to the length of the spoke when it is heated or cooled?
When heated the spoke undergoes linear expansion. When
cooled the spoke contracts in length.
Take a metal ring and metal ball of such size that the ball just passes through the ring.
• Heat the ball and check whether it passes through the ring.
__ Passed through
✔ Not passed through
• Now let the ball cool down, and check whether it passes through the ring.
✔ Passed through
__ Not passed through
Solids expand due to heat and come back to the original state if heat is removed.
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