Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).

Assertion (A): Thermal conductivity of a single grain is always greater than that of bulk grain.

Reason (R): Thermal conductivity of air is comparatively less than that of food grain.

In light of the above statements, choose the correct answer from the options given below.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

Think about what fills the spaces between individual kernels when grain is stored in bulk.

Answer 1

The Correct Option is A

Step 1: A single grain kernel is a solid, more or less continuous body, so heat conducts through it fairly efficiently, giving it a certain thermal conductivity value.

Step 2: A bulk mass of grain is not a solid continuum, it is made of many individual kernels with air filled gaps between them. Heat travelling through bulk grain has to pass through both the solid kernels and these air pockets.

Step 3: Air is a poor conductor of heat compared to the solid material of a food grain, so wherever the heat path runs through an air gap, it slows down a lot.

Step 4: Because a bulk grain mass has these poorly conducting air pockets mixed in throughout, its overall effective thermal conductivity ends up lower than that of a single solid kernel with no internal air gaps. This confirms Assertion (A) is true.

Step 5: Reason (R) states exactly why this happens, air conducts heat much worse than the grain material itself, which is precisely what drags down the bulk value. So (R) is true and it correctly explains (A).

Step 6: Both statements are true and (R) is the correct explanation of (A), so option 1 is correct.