Monday, February 27, 2012

Friday/Monday - Electricity to Magnetism, HMWK

Homework for Tuesday:  Read p. 424-430.  P. 430#3-7
Thursday's exam is now moved to Monday, March 5th.

On Friday we had a quiz that ended up taking the majority of class.  You graded your own (only had a few dishonest people...) and it was only 7 points.  I will be handing those back tomorrow since I am waiting for a few to make theirs up after school today or before tomorrow (absences).
Today we made headway on notes involving magnetism (I posted online last week) - we discussed why/what things are magnetic (electron spins in some metals), how temporary magnets come to be, where magnetic field lines go (similar to electric field lines), and that charges in motion (electric current) makes a magnetic field, and tested this out with a compass while learning a basic right hand rule for magnetic field around a current carrying wire.
The Right Hand Rule:
1. visualize grabbing wire of interest with your right hand.
2. Your thumb must point to the (-) post of the battery (or that part of the circuit)
3. Fingers wrap around wire and show direction of magnetic field (N follows field lines)
Tomorrow we'll try to apply the right hand rule to making some electromagnets and predicting their poles.


Friday, February 24, 2012

Thurs/Fri: Review,problems, quiz, new date for exam

Thursday most classes finished the video on AC /DC and electric power/generation/history.  After the video, we discussed answers to homework problems and questions and corrected answers, and did independent review - some classes had time enough to look through some old test questions for Electricity.

Today we had a quiz covering several concepts in Electricity, from the basis of charge (the atom's components) to static electricity, charging, electric current, and electric power.  Grades were mixed, but most people felt comfortable with understanding why they missed what they did.
After the quiz, we started notes on Magnetism (get a copy here) and talked about safety with / care of magnets (not letting them fly together!).  Magnets are available on Monday for 50 cents apiece, and if you don't have money, you can stay after school on Monday to work on making lab equipment for Freeman and earn a Magnet Scholarship!  :)

Monday we'll continue with notes on Magnets, what they are, and how they interact, and everyone will have time to do some basic experimentation with their magnets.  We'll have a lab on Tuesday and Wed/Thurs (we have the PLAN test which will mean some classes will have a higher teacher:student ratio) and I am planning on moving the exam (due to the PLAN test) to Monday, 3/5.  If I can get it graded and into the gradebook on time, it will count for 3rd quarter grades, since it's over material you learned in 3rd quarter.

Wednesday, February 22, 2012

HMWK Tomorrow! ...LEDs and DC vs. AC, History of Electric Power

HMWK due Thursday: p. 418#1-14 (sketch graph only on #14 - doesn't have to be perfect), 17,19,21-25.
Quiz Friday: Electricity
Exam Next Thursday: Electricity and Magnetism (last day of 3rd quarter, no school Friday)

Today in class: L.E.D.s : Light Emitting Diodes - (A diode is a one-way "valve" for electrons) and DC/AC use.

We used the LEDs to determine that the generator in class produces alternating current.

Classes finished today with a video, clips from "Coal to Coltrane" , a history of electric power.  Included lots of limitations, demonstrations, and illustrations of AC and DC.

Tuesday, February 21, 2012

AC/DC plus Friday HMWK/Quiz, exam date, Electric Power


Today in class we discussed direct current and alternating current.  (notes below) 

We also went over answers to the weekend homework and you had the chance to correct answers.
We have three optional assignments expiring tomorrow at midnight and one that is good until next Monday; last minute emails may be sent to my school email address.  See older posts on this page for more info.

Homework that will be posted tomorrow and due Thursday:  p. 418#1-14 (sketch cartoon of graph on #14), 17,19, 21-25.  
There will be a quiz on Electricity this Friday.  We will learn some topics and concepts with magnetism next week and have an exam over electricity and magnetism next Thursday 

Alternating and Direct Current 

Direct Current (DC)

•      Continuous current in one direction

•      Electrons move from the - side of a circuit to +

•      e- move slowly, often < 1mm per min.

•      Batteries provide DC (& are charged by DC)

Alternating Current (AC)

•      Charge moves back and forth

•      No net movement of electrons

•      In the US, our electricity cycles 60x per second.

–  “120VAC, 60Hz” means 120 Volts of AC, alternating at 60 Hertz (cycles/sec)

•      AC can be used with transformers to change voltages up or down.

•      Most of our appliances use DC, but AC is used for transmission to our homes

–  High voltage = lower power loss through resistance - made possible by alternating current.


Electric Power: how fast electrical energy is converted into other forms  

Units are Watts  (Joules/sec)

Equation:  P = I * V

A 60W light bulb uses 60J of electricity per second, and makes thermal E and light.


 

Thursday, February 16, 2012

Tuesday HMWK, today in class, Another optional assign.

HMWK due Tuesday (no school Monday):  READ p. 407-413; p. 411#1-3, p. 413#1-7.  
In addition to the optional assignments due next Wednesday, this one: Electric Power & Ohm's Law is now available, and is due Monday, the 27th.  

Today in class we presented results/findings from the lab: trends that you noticed commonly were:
As bulbs are added in series, the current decreases and the bulbs dim.
As bulbs are added in parallel, the current increases and each light added is about equal in brightness.
As voltage is increased, current increases and bulb brightness increases.
Resistance of a light bulb is higher when it's lit; the increase in temperature that makes the bulb glow also causes it to resist current.
Some specifics were also noted, such as the specific shapes and trends within trends of graphs for advanced students.  

We then tried out a ranking task activity based on Electric Force, and had to apply three concepts:  
1. Opposite charges attract, like charges repel.
2. The electric force on two charges is proportional to the square of the distance between them  (double distance, force drops by 2^2, or 4.  Quadruple distance and force drops by 4^2, or 16.)
3. The Net Force on an object is the sum of the individual forces acting on an object.

Tomorrow in class we'll finish the back side of the ranking task worksheet (and use hands-on circuits to test our ideas!) and have notes/discussion of electric power.
I got an invite I figured I'd share with you too; it's for Wednesday, 3.14...

Monday, February 13, 2012

Snow day Optional Assignments & Ohm's Law Lab Part 1

Homework for Monday:  READ p. 407-413.  p. 411#1-3, p.413#1-7  (#4 is asking for you to argue for either series or parallel)

Optional Assignments currently available:
Balloons & Static Electricity simulation
Ohm's Law simulation
And this new one:  Electric Field Hockey

The due dates for these is next Wednesday, 2/22.  I will have a couple other related assignments that come online around then, plus we'll need some magnetism assignments for that part of the unit we're studying.
Today (hours 1-4) started a lab using Ohm's law where resistance, voltage, and current were measured and manipulated.  Classes will work to finish on Wednesday.  Be prepared to present/explain your results to the rest of the class and discuss any interesting findings on Thursday.



Friday, February 10, 2012

Series/Parallel Circuits - Lab postponed to Monday, New OP Assign.

** bring lab journals monday/tuesday**
New optional Assignment: Ohm's Law
Today in class:  Reviewed notes from Wednesday (see wednesday's post).
New material: Circuit types:

Open – has a break in the path
Closed – complete path
Series – one path for current
If one part of a series circuit is opened, current stops
Parallel – multiple paths for current
Most circuits have series and parallel parts within themWe drew different circuit elements such as resistors, batteries, switches, and meters, and a couple of examples of series and parallel circuits. (below)



  
A shows 3 resistors in series.   b shows 3 resistors in parallel.  C and D are combinations of series and parallel circuits.
 Finally, we discussed circuit protection - Fuses and Circuit breakers.  They're overcurrent protection, so if a circuit is short circuited or overloaded, they act to open the circuit and stop current.  A fuse or circuit breaker would be placed in line with a branch of parallel circuits.  In the diagrams above, the location of the resistor R1 in (c) would be an appropriate place to locate a fuse/circuit breaker.  Some examples were passed around, and I explained how they work.  (how fuses work)   (how circuit breakers work)  After notes, we watched demonstrations involving Ohm's law on the laserdisc player and a couple of classes had a quiz.

 
 


Thursday, February 9, 2012

Lab Journals needed Friday and Monday!

http://lightyears.blogs.cnn.com/2012/02/08/americas-asia-will-fuse-to-form-amasia/?hpt=hp_c3
Cool look back in the past regarding plate tectonics...

Today in class we worked with meters, batteries, lights as resistors, and wire to learn how to measure Ohms, Volts, and Amps.  Eventually everyone got the chance to measure resistance of at least an individual light bulb and see what happened to resistance of 2 light bulbs in series (R increased) and parallel (R decreased)..., measure voltage in different scenarios of adding batteries in series +-+-+- or having one in reverse... dropping voltage more than not having the battery there at all.  Finally, we measured current in a short circuited battery vs. with a resistor, and some classes got to see what happened with series and parallel resistors to compare.
Tomorrow we'll start a lab on Ohm's law.  I don't anticipate finishing until Monday, especially since we didn't get to discuss answers to today's homework.

Wednesday, February 8, 2012

HMWK tomorrow, Lab Friday! Resistance, Voltage, Current


Lab Friday - Bring Lab Journal to class.
Homework due Thursday:   READ p. 400-405; p. 405#1-7.  (#1 is a bit confusing; you are to compare/contrast      Static discharge         vs        electric current   (one is a specific case of the other!))

Notes – Resistance, Voltage, Current
(R) Resistance – opposition to charge movement (e- movement)
Resistance is dependent on 4 things:
            Material type – conductor vs. insulator – Conductors have less R
            Temperature – Higher temp à higher R    (discuss how digital thermometers use this principle)
            Length of wire – longer wire à higher R – discuss extension cord used on vacuum cleaner – bad, heats                                                                         wire rather than working vacuum
            Thickness of wire – thicker wire à lower R  discuss thick wires better if high current demand, not needed                                                                     for little electronics like cell phone though.
            Unit for resistance: Ohms (  Ω  )  ß greek letter omega

(V) Voltage:  Electric Potential
Voltage is the “Push” that makes charges move.
-related to the concentration of charge on an object.
            Units for voltage:  Volts ( V )

( I )  Current (the I is for intensity): flow of charge.
Current is the quantity of charge that passes a point per second (rate of charge flow)
Units for current:  Amperes ,  or Amps.  ( A ).

Next, we related Newton’s 2nd law to Ohm’s law:
                                                (using some slight variations….)
F = m • a   Where Force is the push, mass (inertia) is the resistance to acceleration, and acceleration is “movement”
V = R • I   (usually people write it V = I•R)  or your book shows I = V / R.  All the same.
The above relationship is Ohm’s law, a basic electrical relationship.

We tried examples of using Ohm’s law to find unknown values:
Ex:  V = 6V   R = 3Ω.  Find I:    (we found I to be 2A)

Ex: you see that your toaster says “10A”.  Find its resistance.   Now since you know you’re in North America, you can assume you have 120V coming out of the wall outlet.     You should find the answer to be 12 Ω. 

Last, we graphed the effect of resistance on current present in a 10V circuit.  This showed an inverse relationship:  As resistance increases, current decreases, approaching zero.    (the image below is 1V, same idea though)

As resistance decreases, current increases, approaching infinity.  ß this is what happens in a “short circuit”  - resistance is effectively eliminated from a circuit and a huge amount of current is present, causing even good conductors to heat up and create a fire risk.  
We talked about short circuits with batteries in class and what not to do (eliminating resistors and cooking batteries)

Finally, we had a demonstration with series and parallel circuits and 25W (high resistance) and 300W (low resistance) light bulbs.  
Tomorrow we’ll play with circuits more as well as multimeters to test voltage, current, and resistance, and we’ll have a lab on Friday.  Bring your lab journal.

Tuesday, February 7, 2012

Static Electricity & Electric Current.. Thurs HMWK

Sorry for the delay in posting; after school clubs and my conference period vaporized my time for a few days.
Thursday's HMWK:  Read p. 400-405; p. 405#1-7.  again, some classes will have random quizzes.
Thursday, 2-3, we went over answers from last week's homework.  Some people refused to correct answers as we went over them; this explains any less-than-perfect grades.  
In class, we played with the Wimshurst machine again; this time inducing a metallic ball and having it move according to electric forces on it, which helped us visualize the electric field around charges.

Notes: electric field lines - Electric field is the area where electric forces act, and the field lines are a visual representation of the strength and direction of the field.   To draw field lines, just visualize the direction of the force on a positive charge at a point, then draw an arrow.  Some excellent explanations and images are here (we drew several examples in class).

On Friday we discussed how the Van de Graaff generator works and how lightning rods work, and several people from each class got to see the difference between a pointed object that dissipates charge gradually (a hissing sound is heard) or a round object that charge accumulates on until a large discharge occurs.

I re-demonstrated charging by induction with a conductive balloon near the Van de Graaff; the balloon initially was attracted because the electric field of the Van de Graaff moved around electrons in the balloon, then the attractive (opposite) charge was nearest the Van de Graaff, and caused the balloon to attract.  When the balloon touched the Van de Graaff, electrons were transferred and both objects obtained the same charge, and they repelled.  Afterward I held my hand out and the balloon induced a charge on me, attracted to me, and bounced back and forth from me to the Van de Graaff as an electron shuttle.

Monday's class was used for playing with the Van de Graaff; you don't get many opportunities to to so otherwise and I'm not about to deprive you.
Highlights:
-charging people who are on an insulating platform (plastic stool) and having their hair stand on end; it was the same charge as they were so it repelled.
-Inducing charges on passersby and feeling lightning pass from one person to another (ouch!)
-Using a fluorescent tube light to pass sparks through as a path to ground, and watching the excited electrons make the light glow like a light saber from Star Wars... Not to mention feeling the impact of the electric field's force.

Today (Tuesday) we discussed the bicentennial (200yrs) since New Madrid's last of 4 big quakes (Dec 1811 to Feb 7, 1812) (7.7 estimated Richter - Mercalli must be used since Richter wasn't born until ~1900).  Shook most of the midwest and was felt on the East Coast.  Would devastate Memphis (and much of STL) if it happened again today.  Be prepared!

Our activity was involving some basic electric concepts and experimentation: we played with Christmas tree lights and AA cells ("batteries").  Students were able to light a single bulb, find other conductors with the bulb/cell combination, light multiple bulbs with 2 or more cells, and increase the voltage for each light by using parallel circuits.  A few folks were able to make REALLY bright displays; some even burned out lights!
Tomorrow we'll have notes and more playing with circuits to back up what you will learn.  

Wednesday, February 1, 2012

Electrostatics practice, HMWK Thurs, New Optional Assignment

Homework due tomorrow: READ p. 392-399, and p. 399#1-7.

New optional assignment:  Balloons & Static Electricity - 15 points if done well.  Please take time to do it well.

Today in class: Electrostatics review: Charging by contact/induction practice.  I will post images to illustrate the lesson on here soon.

Besides the electrostatics review lesson and practice, classes watched a video on electricity concepts and everyone got to see their test grades for the essay questions.  Some were rough; remember you can re-take one test per semester and I will give you a different version of the test.  Grades were posted in class and online today.