Question

Assertion: Bimetallic strips are used in fire alarms.
Reason: Wires on electric poles are mounted tightly, in winter.
A. Both assertion and Reason are correct and Reason is the correct explanation for assertion
B. Both assertion and Reason are correct but Reason is not the correct explanation of Reason
C. Assertion is correct but Reason is incorrect
D. Assertion is incorrect but Reason is correct

Answer 1

The typical property of any solid is to expand when heated. The amount of expansion depends on various factors like initial length and temperature difference we applied to it and some inbuilt properties of that solid too. Based on this information we can solve the given question.

Complete step by step answer:
When heat is given matter expands and when heat is taken out it contracts. It might be any solid or liquid or gas. Now heat can be given by increasing the temperature and it can be taken by decreasing the temperature. For the same amount of temperature difference created the amount of heat consumed depends on the material. Obviously when heat consumed is different then the expansion also will be different. This clearly means there is some materialistic property which decides the amount of expansion of the material irrespective of the temperature difference because the same temperature difference is creating different amounts of expansion in different materials.
Now let us assume the temperature difference created is $\Delta T$ and the initial length of the solid is $l$ and the coefficient of thermal expansion is $\alpha$so when the temperature difference is created the amount of expansion a solid undergoes is $\Delta l = l\alpha \Delta T$
Where $\Delta l$ is the change in length.
From the formula it is clearly evident that the expansion is directly proportional to the initial length and temperature change and the coefficient of thermal expansion $\alpha$.
It is to be noted that the $\alpha$ is the materialistic property and constant for a material and doesn’t vary with the temperature.
So even though temperature is changed $\alpha$ remains the same.
In bimetallic strips as the name suggests generally two metals are used. They are used in fire alarms because in case of fire the metal which is having big alpha expands more while with smaller alpha expands less and due to overall increase in the length of strip the open circuit will become closed and due to current conduction the fire alarm rings. So the assertion is correct.
When temperature is reduced, contract. So wires on electric poles are mounted loosely. Hence the reason is wrong.
So assertion is correct while the reason is incorrect.

Hence option C will be the answer.

Note:
There are some materials which contract on heating like grey cast iron, water at 4 degree Celsius, our hair and Indian rubber. In winter if wires are mounted tightly then due to contraction the wires may break. Between adjacent railway tracks, a gap is maintained because in summer due to temperature increase the tracks may expand and overlap on each other.