Download How Things Work Homework 10

How Things Work
Homework 10
Due: Tuesday, December 1st, 2015
NOTICE THAT THERE ARE TWO PAGES ON THIS ASSIGNMENT
Conceptual Questions:
1. Chapter 11. Exercise 20
2. The figure represents an electromagnetic brake that uses
eddy currents. An electromagnet hangs from a railroad car
near one rail. To stop the car, a large current is sent through
the coils of the electromagnet. The moving electromagnet
induces eddy currents in the rails, whose fields oppose the
change in the electromagnet’s field. The magnetic fields of
the eddy currents exert force on the current in the
electromagnet, thereby slowing the car. The direction of the
car’s motion and the direction of the current in the
electromagnet are shown correctly in the picture. Determine
whether one or the other or both of the eddy currents shown
on the rails is/are correct. Explain your answer.
3. Why does an iron core increase the magnetic field produced by a coil of wire? (I.e., the
iron is located in the middle of the coil.)
4. If you place a metal ring in a region in which a magnetic field is rapidly alternating, the
ring may become hot to your touch. Why?
5. Why is it dangerous to touch the terminals of a high-voltage capacitor even after the
voltage source that charged the capacitor is disconnected from the capacitor? (b) What
can be done to make the capacitor safe to handle after the voltage source has been
removed?
Quantitative Questions:
6. What is the current delivered by the
power supply for the circuit on the
right?
7. As a person moves about in a dry environment, electric charge accumulates on the
person’s body. Once it is at high voltage, either positive or negative, the body can
discharge via sparks and shocks. Consider a human body isolated from ground, with the
typical capacitance 150 × 10−12 𝐹. (a) What charge on the body will produce a potential
of 10,000V? (b) Sensitive electronic devices can be destroyed by electrostatic discharge
from a person. A particular device can be destroyed by a discharge releasing an energy of
0.25 J. To what voltage on the body does this situation correspond?