20160613

Physics quiz question: step-down transformer output voltage

Physics 205B Quiz 6, spring semester 2016
Cuesta College, San Luis Obispo, CA

A step-up transformer with 20 times more coils in the secondary than in the primary coil is used with an input voltage of 120 V, and an input current of 0.50 A in the primary coil. The current in the secondary coil is:
(A) 2.5×10–2 A.
(B) 6.0 A.
(C) 10 A.
(D) 2.4×103 A.

Correct answer (highlight to unhide): (A)

The transformer equations are:

ε21 = N2/N1 = I1/I2,

where the input (alternating root-mean-square) voltage and current in the primary coil are ε1 = 120 V and I1 = 0.50 A, respectively. Since the secondary coil has 20 times more turns than the primary, then the output voltage ε2 will be stepped up by a factor of 20, while the output current I2 will be stepped down by a factor of 20:

I2 = I1·(N1/N2),

I2 = (0.50 A)·(1/20) = 2.5×10–2 A.

Response (B) is ε1·(N1/N2); response (C) is I1·(N2/N1); response (D) is ε1·(N2/N1).

Sections 30882, 30883
Exam code: quiz06eL3k
(A) : 24 students
(B) : 3 students
(C) : 10 students
(D) : 2 students

Success level: 62%
Discrimination index (Aubrecht & Aubrecht, 1983): 1.00

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