UPSC Prelims 2010 · General Studies

Question 2: Science and Technology

If a potato is placed on a pure paper plate which is white and unprinted and put in a microwave oven, the potato heats up but the paper plate does not. This is because

aPotato is mainly made up of starch whereas paper is mainly made up of cellulose

bPotato transmits microwaves whereas paper reflects microwaves

cPotato contains water whereas paper does not contain water

dPotato is a fresh organic material whereas paper is a dead organic material

AnswerThe answer is (c).

A microwave oven sends out radiation that makes polar molecules, above all water, rotate and rub against one another, which produces heat. A potato is mostly water, so it heats up. Dry paper holds very little water, so there is almost nothing in it for the microwaves to act on and they pass through it. Remember: microwaves heat food through its water content (and to a lesser extent fats and sugars), not by heating the container directly.

Why each option is right or wrong

(a) The kind of carbohydrate does not matter; microwaves act on water molecules, not on starch or cellulose as such.
(b) Paper does not reflect microwaves (metals do); it mostly lets them through while the potato absorbs them.
(c) Right: the potato is full of water that absorbs microwave energy, while dry paper has almost none.
(d) Whether a material is fresh or dead organic matter makes no difference; water content does.

Explanation: written by UPSC Grade, not by UPSC.

Practise this paper timedPrevious question Next question

Source: UPSC, Civil Services (Preliminary) Examination 2010, General Studies, Series A. The paper on upsc.gov.in.