Answers to discussion topic questions in Unit 3

Please make sure you understand the answers before you attempt the practice quiz or unit test.  Many of these answers came from your fellow students.

Basics of Electricity

1. Early scientists used an electroscope to detect static electric charges.  Briefly explain what the device looked like and how it worked.
   
   This device is quite easy to understand.  Two thin metal strips are attached to a conducting rod and hang down due to gravity.  Once the device is given either a negative or a positive charge, the leaves move apart because like charges repel each other.
   animation 
    
2. Sailors often reported observing a phenomenon  known as St. Elmo's fire on the tip of the sail mast.  What is this? Explain the cause.
   
   Scientists have discovered that once an object acquires a static charge, the charge resides on the surface and tends to concentrate at sharp edges and pointed surfaces.  Now consider what happens when a cloud (with a large net electric charge) is directly above a boat.  The animation of the triboelectric motion sensor (in the class material) suggests that the cloud can induce a charge separation on the boat.  Since the mast of the ship comes to a point, that is where you can expect a large charge build-up.  It is often enough to allow a discharge to the atmosphere which shows up as a faint glow known as St. Elmo's fire as the surrounding air becomes ionized.
   
   Ben Franklin knew these facts when he introduced the lightning rod.  His original thought was this process relieves some of the "electric pressure" that could lead to a lightning strike, and thus, prevent the likelihood of a strike. Controversy still exists as to the theory behind the lightning rod mainly because it is difficult to "conduct" conclusive experiments (sorry ...bad pun). 
    
3. What is a Faraday cage?  Give one practical application of it.
   
   The last question shows that when an object is charged, the charge concentration tends to build at sharp edges and points (all on the outside of the object).  However, it was discovered that when any conducting object holds a charge, the interior of the object remains electrically neutral.  That is, the interior is shielded from any electric fields that might seriously effect the outside surface of the conducting object.  This is useful information if you are inside your metal car during a lightning storm.  Electrical engineers use this idea by placing sensitive electronics in the interior of metal boxes and why your TV cables have a metal covering (which protects signals from exterior electric interference).  Watch the video here and see a demo ... or perhaps this link which is a bit more entertaining (note: the Tesla coil is a step up transformer).
    
4. Robert Millikan designed a famous experiment in 1909 that involved static charges and several principles discussed in this section.  What was the purpose of his experiment?  Briefly explain the procedure, and the result.
   
   Millikan wanted to discover the (magnitude of) charge carried by an electron.  He used tiny drops of ink to accomplish this.  As ink droplets were ejected from an atomizer, they would pick up a small static electric charge.  If these droplets were placed in an electric field, it would be possible to suspend them in air if the strength of the electric field was enough to match gravity.  That is, the force of gravity (weight) downward was balanced by electric forces acting in the upward direction.  If you knew the weight of the drop and the strength of the applied electric field, it would be possible to determine the (magnitude of) charge on any individual drop.  By repeating this experiment many times, he found that the charges found on all drops were a multiple of one number - 1.60217646 × 10^-19 coulombs.  This number represented the charge of one electron (don't worry what the units mean).
    
5. In many situations the buildup of static charges is undesirable.  What procedures should you follow to avoid a potentially deadly accident at the gas pump (related to static charges) when filling a gas can or when pumping your own gas?  Explain.
   
   One student wrote: As your car goes rushing down the highway it picks up free electrons and builds up a static charge (aided by the fact that the tires insulate the car from the ground). When you get out to pump gas into your car, flammable gas fumes escape from the filling opening. These can be ignited by the release of this static charge.
   
   The best thing to do is to discharge the static, once you step out of the vehicle, by touching the metal body of the car. That way you are making a connection between the car and the ground to equal the static charge as it flows through you. My car regularly gives me a jolt like this when I get out of it. It is important not go get back into the car again while fueling as you can reacquire the static charge. This has led to accidents like this one (click here).
   
   If the gas vapors do ignite it is important to leave the nozzle alone and hit the emergency stop on the pump.
    
6. Cathode ray tubes (CRT) use a beam of electrons to produce an image.  How are electrons initially put in motion?
   
   Typically a filament is heated by an electric current and free electrons "boil" off the surface into a partial vacuum.  To make them move, a nearby grid is given a positive charge.  Since unlike charges attract, the electrons accelerate toward the grid and their momentum carries them past the grid to the picture tube where they produce light when interacting with a phosphor coating..
   
   This idea was the basis for a very important early device in electronics - the vacuum tube.  Instead of projecting electrons to hit a phosphor coating on a screen, the electrons would race toward a (positively charged) plate.  The number of electrons hitting the plate depended on the strength and polarity of the voltage applied to the grid.  That is, the measured current from filament to plate depended upon the voltage signal applied to the grid.  In this way a vacuum tube could act as an amplifier.  Any electric signal (applied to the grid) was reproduced as a much larger current flow between filament and plate.
    
7. Explain how electrostatic precipitators make use of static charges to purify your household air.
   
   First, a filament (known as a corona) acts as a source of free electrons which are picked up by particulate matter in the air.  These (now negatively charged) particles of dust flow past positively charged plates which act as collectors.
   
   This idea is used extensively in coal fired power plants to remove fly ash (smoke) that would normally pollute the atmosphere.  Similar plates are used in the cement industry, paper & pulp mills, and the steel industry.
   
    
8. Explain why using extremely long extension chords can damage a motor rated for 115 volts.  Hint: You will need to understand how voltage changes as it goes through 2 or more resistors placed in series.
   
   As stated in the class material, you can think of voltage as "electrical pressure" and use the analogy of  water pressure to demonstrate the point.  If you have a very short garden hose attached to the outside faucet, the pressure at the outlet will be fairly high.  However, if you use a very long garden hose, the pressure at the outlet will be less because water encounters some friction as it moves through the long hose.  The same thing happens in electricity.  The power company tries to deliver voltage at 115 volts to your appliances but the household wires leading to the appliance will offer some degree of electrical resistance.  Ohm's law says there is a voltage drop across any resistor so it becomes a matter of how the voltage supplied by the power company gets divided up.  Let's say the extension wires are very long and offer enough resistance to cause a 15 volt drop.  This leaves only 100 volts to the appliance you intend to power.  Maybe enough to damage it!
   
   Note: Since the electric company has no idea how far you are from the supply line, they rate household voltage as 110-120 volts.  I've heard stories of houses very close to the power substation often have problems with light bulbs burning out prematurely because the supply voltage is closer to the 120 volt range.
    
9. The ventilation fan motor in your car has settings that range from low to high.  What is happening behind the dashboard that permits these different blower settings?
   
   The last answer should help explain what is happening.  Each blower setting is associated with a different electrical resistor.  The 12 volt supply is fed through one of these resistors and then to the blower motor (in series).  If the resistor setting is high, a large voltage drop occurs ... leaving less voltage for the blower motor.  This would correspond to the "low" setting.
    
10. Explain how a "bubble jet" printer is able to force ink from a reservoir to your paper.
    
    The bubble jet printer got its name from the way the ink forms their droplets.  The inventors at Canon discovered that when a hot soldering iron accidentally touched the needle of an ink-filled syringe, it caused the ink to spray from the needle's tip.  Bubble jet printers uses tiny resistors to create heat.  The heat then vaporizes the ink to create a bubble.  When the bubble expands some of the ink gets pushed out of a nozzle, then onto the paper.  Once the bubble has broken onto the paper a vacuum is created.  The vacuum pulls more ink into the print head of the cartridge.  A typical bubble jet print head has 300 to 600 tiny nozzles, which all can drop ink droplets at once.
    
    animation
     
11. GM cars have ignition keys with embedded chips in them to deter theft.  Describe how this system works.
    
    The "chip" is actually a resistor.  When you put the key in the ignition, the resistor completes an electric circuit that the computer monitors.  Only a key with the correct resistance can start the car.
     

12. How does a clothes dryer moisture sensor work?  That is, how does the controller know when your clothes are dry enough to turn itself off?

    Moist air conducts electricity much better than dry air.  Two electrodes constantly monitor the electrical resistance of the air between.  As your clothes get dryer, the resistance goes up letting the controller know when to shut down.

    There are two other answers (which don't fit this topic).  One is a simple thermostat that measures temperature.  Wet cloths absorb lots of heat to turn liquid water into a vapor (which you learned about in the last unit).  As the cloths dry, less heat is lost to this process and the air gets warmer.  Once all the cloths are dry, the temperature rises rapidly which the thermostat picks up and shuts the dryer off.  A second type of sensor uses rotation.  Wet cloths are heavier and denser than dry cloths ... which then to put greater centrifugal force on the spinning drum.  Once the cloths are dry, this force lessens, which opens an electric contact on a centrifugal switch.  This is a bad solution since someone could overload the dryer ... in which case the thing might not shut off.

Use of static charges

Here is another variation to the electrostatic precipitator introduced above.  Certain automobile manufactures use static electricity to paint cars. First, they prepare the car by putting it in a paint booth and give the car a positive electric charge.  The paint particles (which are charged negatively) come out of a paint sprayer and are attracted to the car where they stick.  This also ensures an even paint coat.  As the negatively charged paint droplets race to the positively charged car, the individual droplets repel each other and try to create distance.  This same idea has been successfully deployed in other areas.  Farmers may employ the same idea to deliver herbicides or insecticides to their crops (no, the crops are not charged up but at least you get an even distribution).  In another example, one can spray metal ions onto circuit boards to form conducting paths.  Obviously, the beam of metal ions can be controlled to a high degree of accuracy.

Have you ever noticed that your TV screen (CRT) seems to collect dust?  This is because dust is often electrically charged (and the screen acts as a collector plate).  The Sharper Image's Ionic Breeze is used to accelerate the process by placing a negative electric charge on dust.  This dust then collects on surfaces and not in your lungs.  I have no idea how effective these devises are.

Miners have used electrostatics for ore separation in the same way you separate plastics and paper from solid wastes (for recycling).  If you want to get an idea how this works, grind up some of those packing peanuts (or some Styrofoam) and let the pieces fall a few inches in front of your TV screen.  Many of the particles will stick to the screen.  Don't ask me to come over and clean up the mess you made.

Ever hear of an electrostatic loudspeaker?  I hadn't until recently.  A diaphragm with a electrostatic charge is placed between two stationary plates (called stators).  By altering the charges on the stators, the forces of attraction and repulsion force motion of the diaphragm. This produces sound. BTW, this is not how most speakers work (which is covered in a later unit).

Your portable flash drive (thumb drive) works by using static charges applied to the gate of a transistor.  More about this in units 4 & 7.

The Dawn spacecraft has made a trip to the asteroid belt using an ion-propelled engine.  In conventional rockets, a combustible fuel is ignited and the action-reaction of the exhaust gasses propel the rocket forward.  This craft carried just a fraction of the "fuel" because solar panels continually converted xenon gas to ions (charged particles).  An electric field (also solar powered) then accelerated these particles out as exhaust ... producing the necessary forward thrust.

Researchers at Temple University found a way to increase fuel efficiency in a car by using static charges.  The idea is to allow the fuel to pass between two oppositely charged plates just before it enters the fuel injectors.  This causes a change in the way the molecules of the fuel cluster, reducing the overall viscosity of the fuel.  Fuels that flow easier are also easier to disperse in the fuel injectors.  Thus, the overall efficiency of the combustion process increases by 15% .  Source

Flooring in operating rooms are "earthed" to the ground using static electricity. Electricity must be able to discharge without building up so it does not cause failures in sensitive equipment. The same idea is used in places where sensitive electronics are built and stored.

Electrochemistry

Potato Clock

Also, many of the conducting paths on printed circuits (integrated circuits) are electroplated.

Electroplating is used to apply a layer of tin to aluminum wire. This is because exposed aluminum surface oxidizes very quickly making it a poor conductor of electricity. Today some electrical feeders are aluminum with a tin plating and it is growing because aluminum is more cost effective than copper.

The write heads on your hard drive are typically electroplated.  This allows the generated magnetic fields to be fairly uniform.

Chrome Plating: Chrome is actually Chromium (atomic #24) and oddly enough, not the element which is electroplated in shiny cars and motorcycles. This is usually a thicker layer of nickel (electroplated) with a very thin layer of chromium to give the piece a bluer hue.  However, chrome is used to coat steel parts inside the engine (camshaft, crankshaft, ball bearings, etc.) to reduce wear and enhance lubrication.

Zinc is often electroplated on nuts and bolts to prevent corrosion.

You usually call the container holding your soup a "tin can".  However, it is a steel can electroplated with tin.

Electroplating is being used on plastics now. For example, in the automotive industry they are able to take plastic parts, electroplate them, and then they were able to have plastic trims that were nickel/chrome plated on the vehicles, which allowed them to have the look they wanted to achieve while still achieving good aerodynamic shape.  Source: http://www.azom.com/details.asp?ArticleID=525

Electromagnetism

1. Consider the image taken from a bubble chamber (see image in the discussion board).  Particle accelerators get subatomic particles moving at near light speed and then let them smash into each other in a bubble chamber.  Why are some tracks straight, some slightly curved, and others spirals?
   
   There is an external magnetic field within the bubble chamber.  Charged particles moving in this field are deflected by this field.   The key is realizing that the magnetic deflection force is at right angles (90 degrees) to the direction of the moving particle.  It is as if you were given a sideways push every time you took a step forward.   The reason for this is simple.  A moving charged particle generates a magnetic field of its own.  This field interacts with the external field and the particle is given a magnetic push (to the side).  If the particle has no electric charge, it moves in a straight line.  Positively charged particles are deflected in one direction (say clockwise) and negatively charged particles are forced in the other direction (counter-clockwise).  The magnitude of deflection force depends on the magnitude of the charge (+1, +2, etc), the speed of the particle and the strength of the external magnetic field.  These variable are usually known quantities.  The behavior of the particle (how tight the spiral is) under any given force depends on the mass of the particle.  If we assume all particles within the chamber have unit charges (+1 or -1), the deflection forces are all equal so the more massive particles only deflect slightly and the lower mass particles spiral in tight circles. 
    
2. Why do charged particles which move through space surrounded by an extended, uniform magnetic field spiral in corkscrew motion?
   
   See the last answer.  Basically the key is knowing that the deflection force acts at right angles to the direction of motion.
    
3. Another type of motor, called an electrostatic motor, runs off static charges.  Briefly explain how this works.  Does this motor have any practical applications?
   
   A motor converts electrical energy into mechanical energy, but you may be surprised that the first attempt to do this was in 1742 when Andrew Gordon devised a motor that runs off electrostatics.  American genius, Benjamin Franklin took this idea to construct a device known as Franklin's Bells.  The idea was to provide a warning when an electric storm was approaching.  The animation below is a crude attempt to show how it works.

   animation

   One bell is attached to a lightning rod (another Franklin invention) which became electro-statically charged during a storm.  This charge on the bell induces a charge separation within the clapper (similar to the triboelectric motion detector) and is attracted to the left.  Once the clapper touches the bell, it acquires the same positive charge and is now repelled until it hits (and touches) a grounded bell and loses its charge.  The process repeats.

   In this case, the motion is lateral (in 2 dimensions) vs. rotational (3 dimensional in a traditional motor).

   I thought this was more-or-less a novelty item.  Then I saw this page and I got interested. I believe it works on the same principal as the Franklin bell.  If you look it over, they show a demonstration video how this idea could become a practical device.  For example, this motor could be used produce motion in a paper feeder.  This type of motor many be very useful where slight lateral motion is required on a small scale.
   
   Want to go further?  Someone found a way to make a motor so small that it is not constructed from individual parts, but as part of the etching/masking process used on circuit boards.  These "micro-motors" run on electrostatic forces rather than magnetic forces.    Scroll to the bottom of this page to read more.

    

4. Has anyone invented a brushless DC motor (no commutator)?  If so, how does it operate and who would use it?
   
   
   
   In a standard DC motor, contacts with the rotating commutator and the brushes act as electrical switches as a coil spins inside a stationary magnetic field .  As stated in the class materials, the rotor is a coil of wires (electromagnet) that spins and the stator is the permanent magnet that resides on the outside and just "stays there" (see eBook).  In the brushless DC motor, a permanent magnet is the rotor that spins around a stationary set of electromagnets (stator).  Now here is the tricky part.  If you were to feed DC current into the winding it would not go very far (AC would work great however).  Why?  With DC, the magnet would spin until it reached the position shown above ... and then stop.  So you need to reverse the flow of current in the windings to reverse the polarity of the electromagnets.  This is accomplished outside the housing by transistors (that we cover in the next unit).  Hall sensors are used to identify the position of the rotor so the transistor knows when to flip the direction of current in the windings. And away the motor spins.
   
   All this adds cost to the motor, but you don't have any mechanical wear found on traditional DC motors (brushes do wear out) .  It also eliminates sparks which are a common by-product of motors with commutator & brushes.  This is useful where flammability is an issue.  In addition, one annoying by-product of sparks is unwanted radio noise which could interfere with radio frequency (RF) controlled devices.  Brushless DC motors run quieter and tend to last longer as well.
   
   Brushless DC motors have many applications such as computer hard drives, CD/DVD players, and PC cooling fans.
    

5. Consider two pieces of iron that appear identical but one is permanently magnetized (a magnet) and the other is not.  What is the difference between the two pieces to account for this property?

   If you look at both pieces of iron at the microscopic level there is a big difference.  When you examine iron on this scale you discover that there are many tiny regions (called domains) that individually act like permanent magnets.  In the piece of iron that is not magnetized, the domains are oriented randomly so that all those tiny magnets cancel themselves out.  However, if you can get all the domains to line up together, the piece becomes one big permanent magnet.  If you have ever taken an iron nail and rubbed it with a strong magnet (in the same direction) you would discover that the nail then becomes a weak magnet for a short time.  The magnetic field acting on the nail is able to convince some of the domains to line up.
   
    

6. A magnetostrictive material is a metal (such as iron, nickel or cobalt) that changes its shape in the presence of a changing magnetic field.   Find one specific example where this property is put to practical use.
   
   Please read the answer to the last question.  Let’s first understand why the shape changes.  An un-magnetized block of iron with no magnetic field surrounding it will have randomly oriented domains … which takes up the smallest space.  If an external magnetic field now starts building around this block of iron, more and more domains will start lining up with this field.  Magnetic domains that line up will take up more space.  The result is the entire length of the iron, therefore, increases.

    Place a piece of iron inside a coil (like a solenoid …. except the iron is held in place).  Now allow current to flow in the coil, creating a magnetic field.  As the current builds, so does the magnetic field.  The result is more and more domains in the iron start lining up and the iron bar increases in length.  When the current drops, the magnetic field decreases, and the iron shrinks.

    Now imagine a situation where you want to apply precise tiny pushes on something.  The gadget described above becomes a perfect actuator for the job.  We are getting a bit ahead of the story to come (next unit) but think back to this discussion when we cover piezoelectric crystals and what they can do … like act as solid state speakers or become ultrasound machines. 

    It gets better.  Like most things in this class, this idea works in the other direction.  If you have a strong magnet and apply a force to it, the magnetic field will decrease.   That is, the magnet gets weaker.  This is because the outside force is able to shove the magnetic domains to a more random state.  This change in magnetism can be picked up by a Hall sensor and you just made a force sensor.

Applications of Solenoids

This one is too numerous to list so I'll just let you teach me something.  Here is one I just recently learned.  I knew there were solenoids galore in my car but learned about one when I suddenly had trouble putting gasoline in my car.  The nozzle shut the gas flow off at about half full.  I learned there is a "purge solenoid" that allows air to escape while filling.  Mine got stuck in the closed position, preventing air from escaping through its designed outlet. The back flow of air through the fill inlet triggered the nozzle to shut off. 

Applications of Hall Effect sensors

Here are some ideas submitted by students

Hall Effect keyboard.  When a key is pressed, a magnet is moved and a Hall sensor detects that movement.  This is much more reliable than the manual switch method of detecting when a key is pressed.

Flip type cell phones. When the phone is closed a hall sensor detects a magnetic field and turns off the screen. Unfolding the phone causes the magnet to move away from the sensor.  The Sensor then turns the screen on. This helps to prolong the life of the battery.

I worked fixing appliances for years and I know of many places Hall sensors are commonly used.  For example, in a washer they are used to indicate if the lid is open or closed, to determine the water level, and to measure fluid flow.  In a dryer a Hall sensor is used to tell if the drum is not moving (from perhaps a broken belt) when the heating element is on.  This will avoid a potential fire hazard.  In the refrigerator a Hall switch knows if the door is open (so the light goes on) as well as a flow meter for the ice maker.  Come to think about it, a Hall sensor even tells you if the ice reservoir is full.  These sensors are very reliable and rarely need replacing because there are no moving parts.

An application of a Hall Sensor would be in a vending machine. When you select an item from a vending machine there is a sensor that tells the machine that the "corkscrew arm" has done a full rotation allowing the item being vended to fall. So we can blame the stupid hall sensor when our candy bar is just barely hanging there and forces you to make a scene fighting with the vending machine.

Induction

1. Using a magnet, a coil of wire, and a flexible diaphragm, how can you make a microphone?  Explain how it works.
   
   animation
   
   The mechanical vibrations of your voice are able to make the diaphragm vibrate as well.  The easiest way to see this is to blow up a balloon and grasp it firmly in your hand.  Now speak into the balloon.  You will be able to feel the vibrations in your hand.  The diaphragm is now connected to a coil of wire so that as the diaphragm vibrates, so does the coil of wire.  Place a strong permanent magnet near this coil.  As the coil moves, it will cut across the magnetic field lines of the magnet.  That is to say, the coil will be moving through a magnetic field ... and this will "induce" currents to flow within the wire.  Send the electric current to a set of earphones and you hear the voice.  Usually, transistors boost the signal and they end up coming out of speakers.
    
2. If a motor is also a generator, does this mean that when a motor is running it is also generating electrical energy at the same time?  Explain.
   
   Hybrid cars use electric motors to propel the car but these same motors can be used to generate electricity as well.  This can be seen when you apply the brakes.  In this case, you are spinning the motor so it acts like a generator (which can be used to recharge the battery).  However, this question is really asking if a running motor is generating electricity at the same time.
   
   This one is a bit tricky.  Electricians know that motors draw a huge current when they initially start and then the current draw drops dramatically once the motor reaches top speed.  Let's see why.  If you look at the generator and the motor they look the same.  The only difference is ... in the motor you feed in electricity and get out motion ... in the generator you feed in motion and get out electricity.  When a motor is running it is also generating electricity at the same time (called a back current ... or more correctly a back emf)!  You never see this electricity because it is fed back the utility grid.  Oddly enough, when a motor is running at full speed and NOT under a load, it is using very little electricity at all!!!!  But why have a motor spin unless you make use of it?  Any load you place under the motor will mean you have more electricity going in than going out so you are really only paying for the difference.  BTW, the directions of each current are in opposite directions.   When you start a motor up from rest, there is a huge current surge because there is very little back current and the "load" is inertia which must be overcome.  Also consider this.  If you place a motor under a great load (or even if it freezes up) there is a great IN current and a small OUT current. This is where you are in danger of overheating the motor because a large net current means large electrical resistance (lot of heat).  Many motors have bimetallic circuit breakers which pops the line if this happens.
    
3. Thomas Edison and Nikola Tesla engaged in a great battle whether power should be distributed to homes as AC or DC.  Tesla favored AC and we all know that is the standard.  Why did Edison favor DC (or what was his objection to AC)?
   
   Edison first established electricity in homes using DC current (using his patents) and anything that would take away from that system meant a loss of royalties.  Edison put up the argument that AC was unsafe and even demonstrated a public electrocution of an elephant to prove his point (Topsy had killed 3 circus workers and was scheduled for termination).  However, there was a lot of pride at stake between Edison and Tesla.  Edison was adamant that DC current was the way to go because it was his idea.  Tesla had a much better idea in AC (I hope you know why) but in this case, personalities got in the way.  Using DC would have produced huge voltage drops between point of generation and customers (and very thick copper wires) so the generation plants would have to be placed every mile or so.  Imagine that! 
    
4. Is it wise to leave phone chargers plugged into the wall even when your phone is not connected to the device?  Explain.
   
   Those little black boxes you plug in to the wall to charge your phone / iPod / notebook computer/ etc. are transformers.  They convert the 115 volt AC to a lower voltage (and rectify it to DC as well).  As long as you have them plugged into the wall, they are drawing current (even if the phone is not attached).  If it feels warm, you know why.  This is known as "vampire power drain"  and can waste 75% of the electricity used in common electronics according to this source.  The good news ... there are devices that do auto shutoff when not in use.
    
5. Household line voltage is advertised as 110 - 120 volts.  Why are there variations in this value?  That is, why isn't it exactly 115 volts!  Hint: It has something to do with why Edison lost to Tesla.
   
   I've answered this question above.   There is very little loss of power between the power plant and the substation which is closer to your home.  That all changes when the voltage is transformed for household use.  The low voltage current in your home offers lots of electrical resistance.  There will be a certain amount of voltage drop within the wires leading from the sub-station to your appliance.  These substations are relatively close to your house but people who live very close will measure in the 120 volt range and folks a bit farther away will only get 110 volts by the time it gets service to the home.
   
   Another student added:  It will also vary with type of wire used, temperature of the wires, corrosion on the connections between your outlet and the generator, etc...
   It will even vary slightly with time of day, depending on the load that the grid has during the day. Summer and Winter voltages will vary as well. 
    
6. Now that you understand the principles of induction, what is an induction motor?  Just state the basic principles.
   
   Induction motors are AC motors that don't need any permanent magnets.  The stator consists of coils of wires.  AC currents flow in the stator which induce currents to flow in the rotor windings.  The magnetic fields in both wires (rotor and stator) will repel each other and make the thing spin. You can think of this like a transformer .. the stator represents the primary windings and the rotor represents the secondary windings.  See the magnetic levitation animation below ... it is the same thing.
    
7. One way to "heat seal" is by a technique called inductive heating.  For example, the safety foil you have to unpeel on many food items is applied with this technique.  (The same idea is applied to brazing metals.)   Explain how that works.
   
   Look over the material on the metal detector and you should see how this works.  A coil of wires with pulses of DC or AC currents is placed just over the metal lid surface (which is an electrical conductor).  This induces eddy currents to flow within the seal.  This generates heat via electrical resistance which melts a glue like substance.  Inductive heating has many other practical applications such as inductive cooktops (stoves where the heating is done in the pan itself ... not the stovetop), to heat bearings so they expand for fitting, to weld plastics (doped with metal so they conduct currents), and to melt or temper harden metals (as done in a foundry).
    
8. Explain why a sheet of aluminum (a non magnetic material) may become very difficult to maneuver in a room with a functioning MRI.  That is, why does it offer significant resistance in any attempt to change its state of motion?
   
   Again, eddy currents are involved.  As you push the aluminum sheet through the magnetic field of the MRI, you are inducing currents to flow in tiny circles within the metal sheet.  These currents, in turn, generates a magnetic field that opposes the external field within the room.  It becomes very difficult to maneuver because it offers lots of resistance (I actually did this).  You can think of this as a way of converting KE into other forms of energy.  You can bet engineers are looking into ways of applying this as a braking system.  In a way, hybrid cars are doing that now as KE is dissipated to electrical energy in a process called regenerative braking.  This not only slows the car down, but the electricity produced is used to recharge the batteries.
    
9. Earlier cars had distributors that were used to ensure that a spark was delivered at the correct time.  How was a 12 volt battery able to produce the thousands of volts necessary to arc the gap?  In your answer, explain why the distributor required "breaker points" as a necessary part of the system.
   
   Older cars used a step up transformer to feed the spark plugs (it was called a "coil" in those days).  However, the battery runs off of 12 volt DC current.  A steady DC current will not work in a transformer because the secondary coil must experience a changing magnetic field.  The breaker points provide the on/off switch needed to chop up the DC current into pulses.  When the points break, the magnetic field collapses and it is this changing field that induces currents to flow in the secondary winding.  Modern cars use transistors to turn things on and off.   Basically they are doing the same thing the points did years ago.
    
10. Using principles described in this section, explain how a credit card reader is able to obtain information encoded on the magnetic strip of your Visa® or MasterCard®.
    
    Your credit card has a unique pattern of magnetic strips that the card reader must interpret.  The pick-up head in the card reader is much like the read heads in a tape player or hard drive.  As the card slides by, the unique magnetic patterns on the card induces unique electrical pulses in a coil of wires in the card reader.  Induction - pure and simple!  However, there are all kinds of innovative ideas out there to read the magnetic information imprinted on the card.  One uses Hall sensors .. which produce unique voltage patterns based on the unique magnetic patterns on the card when it is swiped.   Another idea uses magnetoresistive sensors.  Basically the card reader has tiny electric currents constantly running in ferrous (iron) wires at the read head.  However, the overall electrical resistance within these wires changes in the presence of an external magnetic field ... that being the magnetic patterns on the card as it whizzes by that read head.
     
11. Radio frequency identification (RFID) cards use a pulse of radio waves to identify you in automating toll collection and speed passes at the gas pump.  The pulse comes from the toll booth or the gas pump.  How do you think the ID card makes use of this pulse? Hint: Make a guess based on what you learned in this unit!
    
    Some cards have their own power source (a small battery) but most cheap RFID cards use the radio pulse as the power source.  A conducting loop imprinted on the card has currents induced in it when the pulse goes by (you should know why after you understand the basics of induction).  This becomes the energy source for the card.  This energy can be used to send encoded pulses back to the sender - identifying you as the card holder.  This is known as a "passive" system and usually have a limited range.  I've seen places where you are given a wrist bracelet upon entry.  Unbeknownst to you, your whereabouts can be traced as long as you wear the bracelet (a great marketing tool).  These are also utilized by many merchants to reduce shoplifting.

Magnetic Levitation

When AC current run through the coil of wires, the disk will magically rise up and hover in thin air.  Why?

animation

The reason this works is magnetic repulsion (the same reason an induction motor works).  You can think of this as a simple transformer.  AC currents in the wires generate changing magnetic fields in space.  These, in turn, induce electric currents to flow in the aluminum ring ... which, in turn, generates magnetic fields of their own.   In physics, the phenomena is an example of Lenz's Law which states that the magnetic field of any induced current opposes the change that produced it in the first place.  Therefore, the two magnetic fields will repel each other.  The key ideas are demonstrated here.

Want more?  You can click here to see a demonstration.  Now watch this video.  Can you see how it works?  What would you expect if you moved a strong magnet closer to a stationary flat sheet of copper?  As long as the magnet approaches the sheet (and the magnetic field is building), eddy currents are set up and there is a magnetic repulsion.  What if the magnet is held stationary and the flat sheet is moving?  That is (kind of) what is happening in the video.

Have you ever heard of a maglev train?  Click here to read the details.  Basically, it is a train that has no traditional engine.  There are several ideas being researched.  Here are a few:  Place huge magnets on the underside of the train.  Electromagnets in the guideway provide lift (1 - 10 cm) as well as forward thrust as currents in the coils are carefully adjusted. Speeds over 300 mph have been achieved.  You can even turn things around and put the electromagnets on train (which  levitate on a steel guideway applying Lenz's Law).  Japan is developing a train where the electromagnets are constructed from superconducting materials.  These systems create magnetic repulsion even when the power is shut off but are expensive.  There is even a design (called Inductrack) that utilized permanent magnets to achieve levitation. 

How about tossing away ball bearing (in big machines with moving parts)?  Magnetic bearing use the same principles of levitation which virtually never wear out.  Click here for more.

Within the past few years there have been heart pumps created that use the idea of magnetic levitation. The heart pump is a small device implanted in someone and connected to their weakened heart - it is an alternative to getting a heart transplant. Inside the device there is a revolving part that is suspended in a magnetic field and levitates in the middle of the pump, which pushes blood from the heart to the body.

Superconductivity:  There is a phenomena known as superconductivity.  Certain materials at very low temperatures show zero electrical resistance.  Since its discovery in 1911, scientists have been looking for new materials that super conduct at ever higher temperatures.  Superconducting materials exhibit strange magnetic properties whose details are beyond the scope of this class (see Meissner effect).  This leads to other way engineers could exploit magnetic levitation.  If scientists could develop a superconducting material at room temperatures, it would revolutionize transportation, communications and energy distribution.