In many apartment buildings visitors must call the person they are visiting from the lobby to be "buzzed" in. The person in the apartment can push a button that will unlock the bolt on the front door of the building so that the visitor can enter. Consider the simple circuit and mechanical system shown below in the following problems.
1. When the switch is closed, current can flow through the wire of the electromagnet. When the current passes through the battery it obtains energy. When the current travels through the long wire forming the electromagnet it loses most of this energy. Compare the voltage at the point where current enters the electromagnet (point A) to the voltage where it leaves the electromagnet (point B).
2. When the switch is closed, compare the current (the number of electrons per second) entering the electromagnet at point A to the current leaving it at point B.
3. When the switch is open, is there a net charge on either side of the switch? If so, is it positive or negative? Be sure to answer for both sides of the switch.
4. When the switch is closed, the current flowing through the coil of wire will turn on the electromagnet. Assume that the bolt is a magnet with its north (labeled N) and south (labeled S) poles positioned as shown in the figure. Where should the north and south poles of the electromagnet be in order to attract the bolt toward the electromagnet and open the door?
5. With the battery positioned as shown in the figure, the bolt will open when the switch is closed. If the battery were put in backward, so that the positive terminal was on the bottom and the negative terminal on the top, would the bolt open when the switch was closed? Why?
6. If you move the electromagnet to the right, so that it is farther from the bolt, it may not be able to open the bolt when the switch is closed. Briefly explain why.
7. If the electromagnet shown in the figure were not quite strong enough to open the bolt, you might consider adding a second battery to the circuit. To make the electromagnet stronger you need to increase the current through the coil. Increasing the voltage difference between its two ends (points A and B) will increase the current through the coil, making the electromagnet stronger. How would you add a second battery to increase the voltage difference across the coil?
8. Adding lots of batteries in the proper way would make both the voltage drop across the coil and the current through the coil large when the switch is closed. Although this would make the automatic lock very reliable since the electromagnet would be very strong, it may also be a fire hazard. Briefly explain why by describing where most of the energy is deposited in the circuit and what form that energy takes.