A word about the final exam

I'm astonished and disappointed at the number of students who felt entitled to google the bonus question.  That is generally called cheating.  Typically, exams get disqualified for such an infraction.  

On the other hand, I truly appreciate the effort put in by those of you who attended class and studied.  All the best to you.

Good luck with everything.

Exam - don't forget

It's at 7:30 PM in our normal location.  I'll be here by 6:30, but we can't start until 7:30.

And as before, YOU ARE PERMITTED TO HAVE A NOTESHEET, front and back.

Good luck, and email me any questions you have (seanplally@gmail.com).

And thank you all for a great semester!  Best of luck to you!!

Sean Lally

Exam topics

Folks:

The topics for the final exam are:

interference of light and holography (first topic after test 2)
electrical charge
proton, neutron, electron, quark - particles
atomic number and elements
charging things - what happens
voltage
current
resistance
units of V, I and R
series circuit
parallel circuit
basics of circuits
bulb brightness predictions - it's related to current
V = I R
basic electrical schematics (and symbols)
magnetism
electromagnetism
electromagnetic induction
motors
engines (the very basics of a 2-stroke engine)
compasses
finding north
generators (vs. motors)

Magnetism questions for review

Magnetism

1. In general, what causes magnetism?

2. What is electromagnetism?

3. What is the peculiarity involving magnetic north?

4. How would you find true (geographic) north?

5. What is a motor and how does it basically work?  How does it differ from an engine?

6. What is electromagnetic induction?

7. What is a generator?

8. How do compasses respond to magnetic fields?

Motors are not engines.

Motors and engines are words that are often used interchangeably.  They are, in fact, two very different devices.  Yes, both rotate, but engines typically use gasoline/fuel, whereas motors are typically all electric.



http://www.animatedengines.com/

We talked briefly about the "4 stroke" engine


This depicts a "two stroke" engine, typically found in small gas-powered devices:  mopeds, chainsaws....


http://science.howstuffworks.com/transport/engines-equipment/two-stroke1.htm

http://science.howstuffworks.com/transport/engines-equipment/two-stroke2.htm

The more complicated case of the 4-stroke engine:

http://auto.howstuffworks.com/engine1.htm

http://auto.howstuffworks.com/engine2.htm

More info, FYI:

http://auto.howstuffworks.com/engine4.htm

Study group?

Several folks have expressed interest in a study group for the final exam.  If this is you, send me an email about this:  seanplally@gmail.com

I will then connect all of you together.

Thanks!

SL

Magnetism images

Magnetism

Some ideas from the Magnetism classes:

Similar to the case of charge, magnetic poles are divided into North and South poles.

A North magnetic pole is one that points toward the Earth's magnetic north pole.  This means that the Earth's magnetic north is ACTUALLY A SOUTH POLE (magnetically speaking).

Also:

- Like poles repel
- Opposite poles attract
- Each magnet must have at least one North and one South pole (though they may have more than one of each).  There is NO such thing as a magnetic monopole.
- Magnetic fields are real, but the lines are imaginary - Field lines indicate the direction that a compass needle would take in the vicinity of the magnetic field.

Magnetic north on the Earth is near Ellesmere Island in Northern Canada, several hundred miles from true (geographic) North (the North Pole).  It is moving toward Russia at several miles per year.

For gory detail:

http://en.wikipedia.org/wiki/North_Magnetic_Pole

To find True/Geographic north, it is easiest to find Polaris (the current north star).  Polaris is actually not all that bright, though in the top 50 brightest stars in the night sky.  You need to find the Big Dipper (asterism at the rear end of Ursa Major).  Follow the “pointer stars” at the end of the dipper.  These visually lead you to Polaris.  [If you were to follow the “arc” of the handle, you’d come to a bright star, Arcturus – “Follow the arc to Arcturus.”]

FYI:

http://skymaps.com/

How do we get magnetism?

Magnetic fields are related to electrons spins.  Electrons act like tiny magnetic  spinning tops.  There is a tiny magnetic element associated with each electron spin.  If the spins align, more or less, the object is said to be somewhat magnetic.  More spin alignments (domains) means more magnetism.  Materials that do this easily are generally said to be ferromagnetic.  

As it happens, metals do this best (free electrons).  In the core of the Earth, molten metal convects (rises and falls), giving the Earth a good magnetic field – measurable from the surface and beyond.  Several planets have magnetic fields.

In general, the motion of charges leads to magnetic fields.  If you have charge traveling through a wire, electrons can be thought of as moving together – this causes a magnetic field, also known as electromagnetism.  The magnetic field caused by a current passing through a wire is often small, but if you coil the wire upon itself, the magnetic fields “add up”.  Several hundred turns of wire (with current running through it) can produced quite a strong electromagnet. 

A coil with current running through it can naturally react to a permanent magnet – if this is engineered well, we have a motor.  See illustrations and demos in class.

Electromagnetic Induction

Current causes magnetism – something shown in the early 19^th century by Hans Oersted.  As it happens, the reverse is also true – magnetism can cause current, but there must be some relative CHANGE in the magnetic field or location of conductor.   There must be relative change – either coil or magnet must move, relative to the other.

This phenomenon, wherein a change in magnetic field relative to a conductor, generates electric current is called “electromagnetic induction.”  It is the secret to understanding generators.  If something, say moving water from Niagara Falls, can cause a coil of wire (in a turbine) to spin, current is generated.  More spins of wire means more current.

It’s all about moving conductors in magnetic fields

In conclusion:

Electromagnetism:

Current (moving charges) à  Magnetic Field

Electromagnetic Induction:

Change in magnetic field (through conductor), or vice versa à electric current

Series and Parallel

Voltage (V) - amount of available energy per coulomb of charge.  The unit is volt (also V).

Current (I) - how quickly charge travels (or charge per time, q/t).  The unit (a coulomb per second) is called the ampere (or amp, A). 

Resistance (R) - a way of expressing how much charge is resisted through a device.  It is expressed as a ratio of applied voltage to the resulting current (V/I).  The unit (a volt per amp) is called an ohm (represented as the Greek symbol omega).

Often, the relationship between V, I and R is expressed as Ohm's Law:

V = I R

Batteries and other sources (such as wall sockets) "provide" voltage, which is really a difference between TWO points (marked + and - on a battery).  A wall outlet is a bit more complex - there are 2 prongs, but often also a third prong (the "ground", for safety purposes).

Some folks like analogies.  Consider the water analogy discussed in class.  Voltage is like a tank of water (how much water).  Resistance is provided by a drain or faucet.  The rate at which water comes out is the current.  It's only an analogy, but it gets the gist of circuit terminology ok.

What exactly *IS* a circuit?

An electrical circuit can be thought of as a complete "loop" through which charge can travel. Therefore, it actually has to be physically complete - there can be no openings. That is, the current actually has to have a full path to take.

But there is an exception:

If the supplied voltage is high enough, charge can "jump" an "open circuit." This is clearly a dangerous situation, and one way in which a person can get shocked. Think of the unfortunate situation of sticking your finger (or a paper clip, etc.) into an electrical outlet (or something like a toaster, for that matter). You would "bridge" the circuit, becoming in effect, a resistor.

That's bad.

OK, so about regular circuits:

The images represent SERIES CIRCUITS and PARALLEL CIRCUITS.

In a series circuit, the current is constant and is set by the total resistance of the circuit (the sum of the resistors). If you remove one resistor (or light bulb, as in the first image), the current stops. If the resistors were identical bulbs, having more bulbs would result in dimmer bulbs, since the battery voltage is distributed among them.  Note that the sum of the voltages "over" the bulbs is equal to the total voltage provided by the battery (give or take some minor losses).  Identical bulbs (or resistors) have identical voltages "over" them - 3 identical bulbs connected to a 9-V battery would have roughly 3-V each over them.

In parallel circuits, current has multiple paths to take, so the total resistance of the circuit is actually LESS than if the resistors were alone or in series with other resistors. Since the bulbs are connected equally to the battery, they experience the same as the battery voltage - they are, therefore, of equal brightness (and the same brightness they would have if there were only ONE bulb connected). Of course, bulbs in parallel draw more current and thus cause a battery to die sooner.  You could have 10 bulbs or resistors connected in parallel to a battery - each will be as bright as if only 1 were connected to the battery (same voltage over each), though 10 bulbs will kill the battery 10 times faster.

Does this have anything to do with holiday lights?

What I've written above is primarily geared toward identical bulbs. In series, add up the resistances to get the total resistance. In parallel, it is more complicated. There is a formula one can use (1/Rp = 1/R1 + 1/R2 + ...), but we will only concern ourselves with the case of identical resistors in parallel. In that case, divide the value of the resistor by the number of resistors to get the total effective resistance. For example, two identical 50-ohm resistors in parallel is the same as one 25-ohm resistor. This seems strange, but it's a little like toll booths - when one toll booth is open, it can get crowded (the current is small). With multiple toll booths open, the resistance is effectively less, so the current can be greater. 

In the images below, the first graphic represents the schematic view of a parallel circuits, with 2 resistors.  Note that 2 possible paths are available for current to take - current runs through EACH path, though there will be more current where there is less resistance.  The total current from the battery is equal to the sum of the currents through the 2 resistors.  It follows V = I R, though the V over each R is the same.  The I through each will therefore be V/R.

The second image illustrates the series circuit concept:  identical resistors in series will effectively give MORE resistance (the sum of the resistances, actually) to the battery, so the current will be LESS (and exactly the same in each resistor or bulb).  It also easily follows V = I R, with more R yielding less I (when V is constant).  Think of V = I R this way:  I = V/R.  More R, less I.