Due Monday, February 26, In Class
Please Answer Each Question As Briefly As Possible
You May Work Together, But Write Up Your Answers Separately
Question 1: A nerve cell is an electrostatic device that operates very differently from a wire. It's a fluid-filled tube that's surrounded by another fluid. Both fluids contain ions (electrically charged atoms) of sodium, potassium, and chlorine. Each sodium or potassium ion has one elementary unit of positive charge while each chlorine ions has one elementary unit of negative charge. The wall of the nerve cell normally prevents these ions from passing through it.
a. When the nerve is in its resting state, the fluid outside it has slightly more sodium plus potassium ions than chlorine ions. The fluid inside it has slightly more chlorine ions than sodium plus potassium ions. What are the net electric charges of these two fluids?
b. Explain why the resting nerve has electrostatic potential energy.
c. To produce its electrostatic potential energy, the nerve pumps sodium ions out of the nerve cell. Show that the nerve must do work on the sodium ions during this transfer.
d. Which part of the nerve has a positive voltage? which part has a negative voltage?
e. When the nerve cell "fires," it abruptly begins to allow sodium ions to pass through its walls. Which way do the sodium ions move and what happens to the voltages of the various parts of the cell?
Question 2: Your local market has an electric eye that rings a bell as you walk in door. Your passage through a light beam is what activates the bell. The light beam is emitted at the left side of the door and normally strikes a small photoconductor on the right side of the door.
a. When the light beam is turned off, no light strikes the photoconductor. A battery pumps negative charges onto one side of the photoconductor and positive charges onto the other. These opposite charges exert attractive forces on one another so why don't they move together?
b. Because the photoconductor is in the dark, the battery soon stops sending charges to it. What makes it stop?
c. When the light beam is on and you're not in its way, charges can move through the photoconductor. What has happened inside the photoconductor that allows it to conduct electricity so that those charges can move together?
d. Because the photoconductor is exposed to light, the battery can continue to send charges to it. How has light made it possible for the battery to keeps sending charges?
e. You bend down to play with the electric eye. You block the market's light beam with your back and shine your own flashlight onto the photoconductor. The bell turns off, as though you were out of the way. But when you shine red light from your bicycle taillight onto the photoconductor, it doesn't respond. Why doesn't red light (which has relatively low energy photons) affect the photoconductor?
Question 3: A magnetic resonance imaging (MRI) machine uses an enormous and extremely strong magnet to study a patient's body. The magnet, which has its north pole at the patient's head and its south pole at their feet, is actually a coil of superconducting wire through while electric charges flow.
a. This fancy electric system seems unnecessary; why can't the technicians simply put a large number of north magnetic poles near the patient's head and an equal number of south magnetic poles near their feet?
b. The needle of your magnetic compass has its north magnetic pole painted red and its south pole painted white. Why does the white end turn toward the patient's head?
c. The compass is a magnetic dipole, with no net magnetic pole. So why do you feel it pulled toward the patient's head more and more strongly as you get closer to the magnet?
d. Aluminum isn't normally magnetic and a refrigerator magnet won't stick to it. But as you carry a large aluminum tray up to the magnet, you find that the magnet repels the aluminum. Explain.
e. Eventually you manage to get the aluminum tray up to the magnet. As long as the tray doesn't move, it experiences no magnetic forces. But when you drop it, it falls past the magnet remarkably slowly. What slows down its fall?
Question 4: Electric shavers come in two different types: reciprocating and rotary.
a. When you turn on a corded reciprocating shaver, AC current from the power company travels through a coil of wire inside the shaver. A permanent magnet attached to the cutting blades vibrates back and forth near this coil, taking the blades with it. Why does the permanent magnet vibrate back and forth?
b. How many complete cycles (back and forth) do the blades complete each second?
c. A cordless reciprocating shaver doesn't just send current from its battery through a coil of wire. Why wouldn't that arrangement work?
d. A corded rotary shaver uses a small universal motor to turn its circular blade. Why doesn't the shaver run backward if you plug it in backward?
e. A cordless rotary shaver uses a small DC motor. This shaver runs backward if you reverse its batteries. Why?
Question 5: A home burglar alarm uses various sensors to detect an intruder. These sensors are connected to the main unit by wires.
a. Each sensor is attached to the main unit via two wires, rather than just one. Why?
b. One of the simplest sensors is just a thin strip of metal foil that runs along the edge of a window. If a burglar breaks the window, the foil strip will be severed. How can the main unit determine that the strip has broken, using its two wires?
c. A more sophisticated sensor can tell when a door is opened. It uses a magnet attached to the door and a small device called a reed switch attached to the door frame. This switch contains two iron strips that are arrange head to tail in a line, but normally don't quite touch-they're bent slightly apart and must bend back to make contact. When the magnet's north pole is near the end of one of the iron strips, that strip becomes magnetic. Why?
d. The first iron strip, now magnetic, attracts the other the other strip and they pull together and touch. Why does it attract the other strip?
e. When the door is closed, the magnet is near the reed switch. How can the main unit tell when the door opens?