UPSC Prelims 2010 Q2: Science & Technology – Physics
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
- (a) Potato is mainly made up of starch whereas paper is mainly made up of cellulose
- (b) Potato transmits microwaves whereas paper reflects microwaves
- (c) Potato contains water whereas paper does not contain water
- (d) Potato is a fresh organic material whereas paper is a dead organic material
Answer: (c) Potato contains water whereas paper does not contain water
Explanation
Why a potato heats up in a microwave but paper doesn't
Microwave ovens heat food by exciting polar water molecules, not by heating all materials equally.
| Option | Verdict | Reason |
|---|---|---|
| (a) Potato is starch, paper is cellulose | Incorrect | The starch vs cellulose distinction is not the operative mechanism for microwave heating |
| (b) Potato transmits microwaves, paper reflects them | Incorrect | This reverses the real mechanism — paper is largely transparent to microwaves (doesn't absorb them), it doesn't 'reflect' them like metal does |
| (c) Potato contains water, paper does not | Correct | Microwaves specifically excite water molecules, causing them to vibrate and generate heat; potato's high water content heats up, while dry paper has negligible water to absorb microwave energy |
| (d) Potato is fresh organic, paper is dead organic | Incorrect | The freshness/deadness distinction is irrelevant; the real factor is water content |
Why microwaves heat selectively:
| Point | Detail |
|---|---|
| Mechanism | Microwaves make polar water molecules vibrate, generating heat |
| Potato | High water content → heats up |
| Paper | Negligible water → passes through largely unaffected |
Answer: (c) Potato contains water whereas paper does not contain water.
Why each option is right or wrong
- (a) Incorrect. The starch vs cellulose chemical composition is not what determines microwave heating.
- (b) Incorrect. Paper does not 'reflect' microwaves like metal; it is largely transparent to them due to lacking water.
- (c) CORRECT. Microwaves heat materials by exciting water molecules; potato's high water content heats up while dry paper does not.
- (d) Incorrect. The fresh vs dead organic material distinction is not the relevant physical mechanism here.
Key point
Microwaves heat food by exciting water molecules. Potatoes contain water and heat up; dry paper has negligible water and stays largely unaffected.
Remember
Microwave heating works by exciting water molecules — potato (water-rich) heats up, dry paper (water-poor) does not.
Common mistakes
- Attributing microwave heating to chemical composition (starch/cellulose) instead of water content.
- Confusing paper's microwave 'transparency' with metal's microwave 'reflection'.
- Overlooking that water content, not organic freshness, is the real determining factor.
Exam tip
Microwaves heat water, not 'organic matter' generally. Any material lacking water stays cool in a microwave.
Sources
- NCERT Class XII Physics — Electromagnetic Waves (microwaves)
- Encyclopaedia Britannica — microwave oven