Cuesta College, San Luis Obispo, CA
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.
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