\"Smart\" traffic lights are controlled by loops of wire embedded in the road. T
ID: 2263697 • Letter: #
Question
"Smart" traffic lights are controlled by loops of wire embedded in the road. These "loop detectors" detect the change in the Earth's magnetic field as a large metal object - such as a car or truck - moves over the loop. Suppose a lop detector is circular with a diameter of 3.0 feet (convert). As a car passes over the detector, the downward magnetic field within the loop increases from 1.5 x 10^-5 T to 2.4 x 10^-5 T in 0.52 seconds. a.) As viewed from above, the what is the direction of the induced current in the loop? Explain. b.) If the loop has 120 turns, what is the induced emf in the loop?
Explanation / Answer
d = 3 feet = 3*12*2.54 = 91.44 cm = 0.9144 m
r = d/2 = 0.4572 m
B1 = 1.5*10^-5 T, B2 = 2.4*10^-5 T
t = 0.52 s
a) counterclockwise
b) induced emf = N*A*(B2-B1)/t
emf = 120*3.14*0.4572^2*(2.4-1.5)*10^-5/0.52
emf = 1.36*10^-3 volts
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