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Question

Specific heat of water is 4.2 J/gC . If light of frequency 3 x 109 Hz is used to heat 400 gm of water from 20C to 40C the number of photons needed will be

A
1.69×1029
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B
1.68×1028
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C
2.80×104
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D
2.80×105
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Solution

The correct option is B 1.68×1028
C=4.2
f=3×109Hz
m=400gm
energy required
E=mc(ΔT)=400×4.2×20=33600J
each photon energy =hv=6.623×1034×3×109=19.9×1025J
no.ofphoton=3360019.9×1025=1.68×1028
Hence,
option (B) is correct answer.

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