20161118

Physics quiz question: Samsung Galaxy S7 pressure difference underwater

Physics 205A Quiz 5, fall semester 2016
Cuesta College, San Luis Obispo, CA

"Samsung Galaxy S7 Edge review"
Gareth Beavis
techradar.com/reviews/phones/mobile-phones/samsung-galaxy-s7-edge-1315189/review/11

A Samsung Galaxy S7 smartphone is claimed to be "water-resistant in up to 1.5 m of water for up to 30 minutes."[*] (ρwater = 1.0×103 kg/m3; Patm = 101.3 kPa.) The difference in pressure between atmospheric pressure and 1.5 m underwater is:
(A) 1.5×103 Pa.
(B) 1.5×104 Pa.
(C) 8.7×104 Pa.
(D) 1.16×105 Pa.

[*] samsung.com/us/support/answer/ANS00047867/.

Correct answer (highlight to unhide): (B)

For static fluids, the energy density relation between pressure difference ∆P and difference in depth ∆y is given by:

0 = ∆P + ρ·g·∆y.

The difference in pressure ∆P between atmospheric pressure (at the surface of the water) and 1.5 m below the surface:

P = –ρ·g·∆y,

P = –ρ·g·(y(–1.5)y(0)),

P = –(1.0×103 kg/m3)·(9.80 m/s2·((–1.5 m) – (0)),

P = +14,700 Pa,

such that the difference in pressure between atmospheric pressure and 1.5 m underwater (to two significant figures) is 1.5×104 Pa.

(Response (A) is ρ·∆y; response (D) is the absolute pressure 1.5 m underwater; while response (C) is the pressure difference subtracted from atmospheric pressure.

Sections 70854, 70855, 73320
Exam code: quiz05b0oM
(A) : 18 students
(B) : 19 students
(C) : 12 students
(D) : 6 students

Success level: 35%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.36

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