NEET · 21 September 2026
Black-body radiation: compare emitted power
Corrected practice variant — no response statistics exist for this version.
A spherical ideal black body of radius r at absolute temperature T emits 450 W. A second spherical ideal black body has radius r/2 and absolute temperature 2T. What power does the second body emit?
A. 225 W
B. 450 W
C. 1000 W
D. 1800 W
Choose A–D and record your reasoning before revealing the review.
Reveal corrected-practice answer
D — 1800 W. P′/P = (r′/r)²(T′/T)⁴ = (1/2)² × 2⁴ = 4. This is emitted power, not net exchange with an unspecified environment. The original fixed 12 cm and 500 K values were removed to make the supplied 450 W premise internally consistent.
OpenStax, emitted radiation and Stefan–Boltzmann law
This is a separate correction; it does not claim that the original app question or explanation has been updated.
This opens general daily practice, not a direct link to this corrected question.
Historical original — inconsistent premises; not results for the new practice
Historical item: itm_4bbc8aaeb73745f7; release 2026.09.16-0354; scored form 4434be5cbf7e0f6a. Numeric item revision is unknown. One additional unknown-binding skip is excluded from these statistics.
A spherical black body with a radius of 12 cm radiates 450 watt power at 500 K . If the radius were halved and the temperature doubled, the power radiated in watt would be:
A. 225
B. 450
C. 1000
D. 1800
Choose A–D and record your reasoning before revealing the review.
Historical original review and statistics
Stored key: D — 1800 W, conditional on using the supplied initial power of 450 W. For emitted power, P = εσ(4πr²)T⁴. With unchanged emissivity, P′/P = (1/2)² × 2⁴ = 4, so the intended ratio calculation gives 4 × 450 = 1800 W.
The original premises are inconsistent for an ideal black body. With ε=1, r=0.12 m and T=500 K, the initial emitted power is about 641.3 W, not 450 W. At r=0.06 m and T=1000 K it would be about 2565.2 W, which is not an offered option. Calling 450 W net heat loss does not repair the ratio without specifying surrounding temperature. Thus D is the intended answer under the supplied-power ratio, not a resolution of all the literal data. We do not claim that seven students failed a physically consistent question.
Explanation correction: using only the radius factor gives 112.5 W; using T² instead of T⁴ with the area factor gives 450 W. Neither establishes the stored explanation's proposed route to A=225 W. Choice counts do not establish learners' calculations. A separate corrected practice version removes the incompatible fixed radius and temperature; its response statistics are unknown.
Observed original-item records, 20 September daily set, answered on 20 September IST: 3 correct, 7 server-marked wrong / 10 answered (70.00% wrong), plus 4 explicit skips. A=4, B=3, C=0, D=3. Leading wrong option A: 4/10 answered (40.00%) or 4/7 wrong (57.14%). These denominators differ. Counts do not show why an option was chosen.
First responses per account to the matched question/key within the named window; partial sessions included, staff/test closure and unknown bindings excluded. Unreached items are not skips. Accounts are not verified unique students. Response language is unknown; no EN/HI comparison is supported. Statistics belong to the original served item, not this revised explanation. Snapshot: 21 September 2026, 09:43:44.889 IST; latest event 09:15:34.770 IST. Late arrivals can change counts.
OpenStax, emitted radiation and Stefan–Boltzmann law
Which premise or relationship did you use? Share a reasoned challenge if you disagree.
This is a separate correction; it does not claim that the original app question or explanation has been updated.