HW10solutions

# HW10solutions - Physics 213 Q30.2 Homework 10 Solutions...

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Physics 213 Homework 10 Solutions Spring 2009 Q30.2 [Transformer] Transformers work because of Faraday's Law of magnetic induction — a changing magnetic flux produces an induced emf. Since the areas and orientations of the transformer's coils are fixed, a changing magnetic field is required to produce a changing magnetic flux, which requires a changing electric current. The induced emf 2 = -N 2 (d B /dt) in the secondary coil is nonzero as long as the electric current i 1 in the primary coil is changing. A DC (constant) current won't produce a changing magnetic flux, so a transformer won't work with DC. An AC current is needed since it is always changing. 30.9 [Inductor] (a) 3 (0.260 H)(0.0180 A/s) 4.68 10 V L di dt (b) From Figure 30.6 on page 1036 of the textbook, we see that the rule is V cd =Ldi/dt , where c is the upstream point and d is the downstream point as defined by the current. In our case, the current goes from b to a , so V ba =Ldi/dt<0 . This means V b -V a <0 , so V a >V b . The inductor acts like a battery with its + terminal facing a to fight the change in current. 30.10. [Inductor] In this problem, we apply   L di dt . Since i is changing at a constant rate, di i t dt . (a) The induced emf points from low potential to high potential across the inductor. In this case, the induced emf points from b to a , in the direction of the current. Therefore, the current is decreasing and the induced emf is directed to oppose this decrease. (b) L i / t ,so i / t V ab / L (1.04 V) / (0.260 H) 4.00 A/s. In 2.00 s the decrease in i is 8.00 A and the current at 2.00 s is 12.0 A 8.0 A 4.0 A.

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HW10solutions - Physics 213 Q30.2 Homework 10 Solutions...

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