Physics quiz question: step-down transformer output current

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:

ε₂/ε₁ = 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