Tuesday, May 31, 2016
Tuesday, May 24, 2016
Some reading.
Brief history
From last class:
Ancient science highlights:
Epicycles
Precession
From class:
http://astro.unl.edu/naap/ssm/animations/ptolemaic.swf
http://astro.unl.edu/naap/motion3/animations/sunmotions.swf
The most important things to get out of this were:
- Epicycles were a very useful way to (wrongly) explain why retrograde motion happened with planets.
- Precession (the wobbling of the Earth) causes us to have different North Stars (or no North Star) at various points over the course of thousands of years. Thus, star maps are not accurate after several hundred years. However, this was not understood until the time of Newton and others.
From class:
http://astro.unl.edu/naap/ssm/animations/ptolemaic.swf
http://astro.unl.edu/naap/motion3/animations/sunmotions.swf
The most important things to get out of this were:
- Epicycles were a very useful way to (wrongly) explain why retrograde motion happened with planets.
- Precession (the wobbling of the Earth) causes us to have different North Stars (or no North Star) at various points over the course of thousands of years. Thus, star maps are not accurate after several hundred years. However, this was not understood until the time of Newton and others.
Scientific Revolution
N. Copernicus, d. 1543
De Revolutionibus Orbium Celestium
Galileo Galilei, 1564-1642
Siderius Nuncius
Dialogue on Two World Systems
http://galileo.rice.edu/sci/observations/sunspot_drawings.html
(J. Kepler, C. Huygens, R. Descartes, et. al.)
Isaac Newton, 1642-1727
Principia Mathematica, 1687
Newton and his laws of motion.
Newton, Philosophiae naturalis principia mathematica (1687) Translated by Andrew Motte (1729)
Lex. I. Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare.
Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon.
Projectiles persevere in their motions, so far as they are not retarded by the resistance of the air, or impelled downwards by the force of gravity. A top, whose parts by their cohesion are perpetually drawn aside from rectilinear motions, does not cease its rotation, otherwise than as it is retarded by the air. The greater bodies of the planets and comets, meeting with less resistance in more free spaces, preserve their motions both progressive and circular for a much longer time.
Lex. II. Mutationem motus proportionalem esse vi motrici impressae, & fieri secundum lineam rectam qua vis illa imprimitur.
The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.
If any force generates a motion, a double force will generate double the motion, a triple force triple the motion, whether that force be impressed altogether and at once, or gradually and successively. And this motion (being always directed the same way with the generating force), if the body moved before, is added to or subtracted from the former motion, according as they directly conspire with or are directly contrary to each other; or obliquely joined, when they are oblique, so as to produce a new motion compounded from the determination of both.
Lex. III. Actioni contrariam semper & aequalem esse reactionent: sive corporum duorum actiones in se mutuo semper esse aequales & in partes contrarias dirigi.
To every action there is always opposed an equal reaction; or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.
Whatever draws or presses another is as much drawn or pressed by that other. If you press a stone with your finger, the finger is also pressed by the stone. If a horse draws a stone tied to a rope, the horse (if I may so say) will be equally drawn back towards the stone: for the distended rope, by the same endeavour to relax or unbend itself, will draw the horse as much towards the stone as it does the stone towards the horse, and will obstruct the progress of the one as much as it advances that of the other.
Lex. I. Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare.
Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon.
Projectiles persevere in their motions, so far as they are not retarded by the resistance of the air, or impelled downwards by the force of gravity. A top, whose parts by their cohesion are perpetually drawn aside from rectilinear motions, does not cease its rotation, otherwise than as it is retarded by the air. The greater bodies of the planets and comets, meeting with less resistance in more free spaces, preserve their motions both progressive and circular for a much longer time.
Lex. II. Mutationem motus proportionalem esse vi motrici impressae, & fieri secundum lineam rectam qua vis illa imprimitur.
The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.
If any force generates a motion, a double force will generate double the motion, a triple force triple the motion, whether that force be impressed altogether and at once, or gradually and successively. And this motion (being always directed the same way with the generating force), if the body moved before, is added to or subtracted from the former motion, according as they directly conspire with or are directly contrary to each other; or obliquely joined, when they are oblique, so as to produce a new motion compounded from the determination of both.
Lex. III. Actioni contrariam semper & aequalem esse reactionent: sive corporum duorum actiones in se mutuo semper esse aequales & in partes contrarias dirigi.
To every action there is always opposed an equal reaction; or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.
Whatever draws or presses another is as much drawn or pressed by that other. If you press a stone with your finger, the finger is also pressed by the stone. If a horse draws a stone tied to a rope, the horse (if I may so say) will be equally drawn back towards the stone: for the distended rope, by the same endeavour to relax or unbend itself, will draw the horse as much towards the stone as it does the stone towards the horse, and will obstruct the progress of the one as much as it advances that of the other.
>
And now, in more contemporary language:
1. Newton's First Law (inertia)
An object will keep doing what it is doing, unless there is reason for it to do otherwise.
The means, it will stay at rest OR it will keep moving at a constant velocity, unless acted on by an unbalanced force.
2. Newton's Second Law
An unbalanced force (F) causes an object to accelerate (a).
That means, if you apply a force to an object, and that force is unbalanced (greater than any resisting force), the object will accelerate.
Symbolically:
That's a linear relationship.
Greater F means greater a. However, if the force is constant, but the mass in increased, the resulting acceleration will be less:
a = F / m
That's an inverse relationship.
We have a NEW unit for force. Since force = mass x acceleration, the units are:
kg m / s^2
which we define as a newton (N). It's about 0.22 lb.
There is a special type of force that is important to mention now - the force due purely to gravity. It is called Weight. Since F = m a, and a is the acceleration due to gravity (or g):
W = m g
Note that this implies that: weight can change, depending on the value of the gravitational acceleration. That is, being near the surface of the Earth (where g is approximately 9.8 m/s/s) will give you a particular weight value, the one you are most used to. However, at higher altitudes, your weight will be slightly less. And on the Moon, where g is 1/6 that of the Earth's surface, your weight will be 1/6 that of Earth. For example, if you weight 180 pounds on Earth, you'll weight 30 pounds on the Moon!
3. Newton's Third Law
To every action, there is opposed an equal reaction. Forces always exist in pairs. Examples:
You move forward by pushing backward on the Earth - the Earth pushes YOU forward. Strange, isn't it?
A rocket engine pushes hot gases out of one end - the gases push the rocket forward.
If you fire a rifle or pistol, the firearm "kicks" back on you.
Since the two objects (m and M, let's say) experience the same force:
m A = M a
That's a little tricky to convey in letters but, the larger object (M) will experience the smaller acceleration (a), while the smaller object (M) experiences the larger acceleration (A).
And now, in more contemporary language:
1. Newton's First Law (inertia)
An object will keep doing what it is doing, unless there is reason for it to do otherwise.
The means, it will stay at rest OR it will keep moving at a constant velocity, unless acted on by an unbalanced force.
2. Newton's Second Law
An unbalanced force (F) causes an object to accelerate (a).
That means, if you apply a force to an object, and that force is unbalanced (greater than any resisting force), the object will accelerate.
Symbolically:
F = m a
That's a linear relationship.
Greater F means greater a. However, if the force is constant, but the mass in increased, the resulting acceleration will be less:
a = F / m
That's an inverse relationship.
We have a NEW unit for force. Since force = mass x acceleration, the units are:
kg m / s^2
which we define as a newton (N). It's about 0.22 lb.
There is a special type of force that is important to mention now - the force due purely to gravity. It is called Weight. Since F = m a, and a is the acceleration due to gravity (or g):
W = m g
Note that this implies that: weight can change, depending on the value of the gravitational acceleration. That is, being near the surface of the Earth (where g is approximately 9.8 m/s/s) will give you a particular weight value, the one you are most used to. However, at higher altitudes, your weight will be slightly less. And on the Moon, where g is 1/6 that of the Earth's surface, your weight will be 1/6 that of Earth. For example, if you weight 180 pounds on Earth, you'll weight 30 pounds on the Moon!
3. Newton's Third Law
To every action, there is opposed an equal reaction. Forces always exist in pairs. Examples:
You move forward by pushing backward on the Earth - the Earth pushes YOU forward. Strange, isn't it?
A rocket engine pushes hot gases out of one end - the gases push the rocket forward.
If you fire a rifle or pistol, the firearm "kicks" back on you.
Since the two objects (m and M, let's say) experience the same force:
m A = M a
That's a little tricky to convey in letters but, the larger object (M) will experience the smaller acceleration (a), while the smaller object (M) experiences the larger acceleration (A).
Thursday, May 19, 2016
Fun practice
1. A rock is dropped from a 100-m high cliff. How long does it take to fall the first 50-m, and the second 50-m?
2. A juggler usually tosses balls vertically to a height H. To what height must they be tossed if they are to spend twice as much time in the air?
3. Two objects are begin a free fall from rest from the same height, 1 second apart. How long after the first object begins to fall are the 2 objects 10 m apart?
4. A hot air balloon is ascending at a rate of 12 m/s and is 80-m above the ground when a package is dropped over the side. How long does it take the package to hit the ground, and with what speed does it hit the ground?
2. A juggler usually tosses balls vertically to a height H. To what height must they be tossed if they are to spend twice as much time in the air?
3. Two objects are begin a free fall from rest from the same height, 1 second apart. How long after the first object begins to fall are the 2 objects 10 m apart?
4. A hot air balloon is ascending at a rate of 12 m/s and is 80-m above the ground when a package is dropped over the side. How long does it take the package to hit the ground, and with what speed does it hit the ground?
Tuesday, May 10, 2016
Test and Lab due dates
May 16: Lab draft due
May 18: Complete lab due
May 20: Test on Motion
Also, try this fun problem: The Drowsy Cat
A drowsy cat is sleeping on a window sill (2-m tall). Someone throws a ball up past the window, and the cat (being a keen observer, as all cats are) notices that the ball is in view for a total of 0.5 seconds (up and down). How far ABOVE the top of the window does the ball actually go?
May 18: Complete lab due
May 20: Test on Motion
Also, try this fun problem: The Drowsy Cat
A drowsy cat is sleeping on a window sill (2-m tall). Someone throws a ball up past the window, and the cat (being a keen observer, as all cats are) notices that the ball is in view for a total of 0.5 seconds (up and down). How far ABOVE the top of the window does the ball actually go?
Monday, May 2, 2016
the new Lab
OK, it's time to start thinking about your lab write-up. This will ultimately be due next week some time. Here is what your lab will need:
1. Introduction - with purpose and discussion of the "problem" - this is more than just the purpose. Give some background to what you are trying to do/measure.
2. Detailed equipment list
3. Detailed procedure, as you did it - make sure that a total stranger can follow your method.
4. Data table - with all calculations done
5. Graphs, if relevant
6. Sample of calculation - you need to do them all, but you only need to SHOW one sample
7. Discussion of error - the things that you really can not control
8. Discussion of other errors - things that could conceivably be eliminated (and how to do so)
9. Discussion of how close you are to the accepted value of g (9.8 m/s/s)
10. General conclusion
It is ok to have 7-9 in one large section.
1. Introduction - with purpose and discussion of the "problem" - this is more than just the purpose. Give some background to what you are trying to do/measure.
2. Detailed equipment list
3. Detailed procedure, as you did it - make sure that a total stranger can follow your method.
4. Data table - with all calculations done
5. Graphs, if relevant
6. Sample of calculation - you need to do them all, but you only need to SHOW one sample
7. Discussion of error - the things that you really can not control
8. Discussion of other errors - things that could conceivably be eliminated (and how to do so)
9. Discussion of how close you are to the accepted value of g (9.8 m/s/s)
10. General conclusion
It is ok to have 7-9 in one large section.
Thursday, April 21, 2016
Tuesday, April 12, 2016
HW for Thursday
1. Consider a car starting from rest. If it has an acceleration of 4 m/s/s, find the following:
a. how fast the car is moving after 7 seconds
b. how far the car has gone in this time
2. Consider a falling object, subject only to gravity (a = 9.8 m/s/s, with no air resistance).
a. How fast is a falling ball moving after 3 seconds of freefall?
b. How far will it have fallen in this time?
3. If a ball is dropped from 15 m, how long does it take to hit the ground? Assume that the acceleration is due only to gravity and that there is no air resistance.
4. Consider a baseball pitch, starting from rest and accelerating up to 45 m/s in a 0.5 m pitching arc. Find:
a. the acceleration of the ball
b. the time that the pitcher is in contact with the ball
5. Try this: Derive a "5th equation of motion," in which initial velocity (Vi) is absent.
ALSO - WE WILL HAVE A QUIZ IN 2 CLASSES. Units, unit conversions, speed....
a. how fast the car is moving after 7 seconds
b. how far the car has gone in this time
2. Consider a falling object, subject only to gravity (a = 9.8 m/s/s, with no air resistance).
a. How fast is a falling ball moving after 3 seconds of freefall?
b. How far will it have fallen in this time?
3. If a ball is dropped from 15 m, how long does it take to hit the ground? Assume that the acceleration is due only to gravity and that there is no air resistance.
4. Consider a baseball pitch, starting from rest and accelerating up to 45 m/s in a 0.5 m pitching arc. Find:
a. the acceleration of the ball
b. the time that the pitcher is in contact with the ball
5. Try this: Derive a "5th equation of motion," in which initial velocity (Vi) is absent.
ALSO - WE WILL HAVE A QUIZ IN 2 CLASSES. Units, unit conversions, speed....
Friday, April 8, 2016
HW for Tuesday
First, some Fermi questions to play with:
1. How many hairs are on your head?
2. How many times does one of the tires on a car rotate in its lifetime?
3. How many toilets are there in all of the major league football stadiums, baseball parks, and basketball arenas in the USA?
Now some motion math.
4. What is the difference between traveling at an average speed vs. a constant speed (for the same distance)? In other words, if you were traveling at an average speed of 5 m/s vs. traveling at a constant speed of 5 m/s, would you go a greater distance in either case (after, say, 10 minutes)? Discuss.
5. Find the definition of acceleration and write it down.
6. Consider a car traveling at a constant 10 m/s in a straight line. What would a graph of distance (or position) vs. time look like for 50 seconds of travel time? What would a graph of speed vs. time look like for the same time period?
1. How many hairs are on your head?
2. How many times does one of the tires on a car rotate in its lifetime?
3. How many toilets are there in all of the major league football stadiums, baseball parks, and basketball arenas in the USA?
Now some motion math.
4. What is the difference between traveling at an average speed vs. a constant speed (for the same distance)? In other words, if you were traveling at an average speed of 5 m/s vs. traveling at a constant speed of 5 m/s, would you go a greater distance in either case (after, say, 10 minutes)? Discuss.
5. Find the definition of acceleration and write it down.
6. Consider a car traveling at a constant 10 m/s in a straight line. What would a graph of distance (or position) vs. time look like for 50 seconds of travel time? What would a graph of speed vs. time look like for the same time period?
Monday, April 4, 2016
FUN Homework!
Here are some unit problems to play with:
1. Create a factor to convert from m/s to light-years (LY) per millennium. 1 LY = 9.4607 x 10^12 km.
2. Create your own conversion factor - m/s to something else interesting.
3. How long is a micro-century? Give your answer in units that are appropriate. Also, micro equals 1 millionth.
4. How long is a nano-year. Nano is a billionth.
5. Roughly, what is the growth rate of your hair in m/s?
6. What is your average speed on the way to school - in miles per hour (and m/s)?
For fun:
https://en.wikipedia.org/wiki/List_of_humorous_units_of_measurement
1. Create a factor to convert from m/s to light-years (LY) per millennium. 1 LY = 9.4607 x 10^12 km.
2. Create your own conversion factor - m/s to something else interesting.
3. How long is a micro-century? Give your answer in units that are appropriate. Also, micro equals 1 millionth.
4. How long is a nano-year. Nano is a billionth.
5. Roughly, what is the growth rate of your hair in m/s?
6. What is your average speed on the way to school - in miles per hour (and m/s)?
For fun:
https://en.wikipedia.org/wiki/List_of_humorous_units_of_measurement
Thursday, March 31, 2016
SI Units - notes
Some comments on standards. We generally use SI units in physics. To inform you:
Mass is measured based on a kilogram (kg) standard.
Length (or displacement or position) is based on a meter (m) standard.
Time is based on a second (s) standard.
How do we get these standards?
Length - meter (m)
- originally 1 ten-millionth the distance from north pole (of Earth) to equator
- then a distance between two fine lines engraved on a platinum-iridium bar
- (1960): 1,650,763.73 wavelengths of a particular orange-red light emitted by atoms of Kr-86 in a gas discharge tube
- (1983, current standard): the length of path traveled by light during a time interval of 1/299,792,458 seconds
That is, the speed of light is 299,792,458 m/s. This is the fastest speed that exists. Why this is is quite a subtle thing. Short answer: the only things that can travel that fast aren't "things" at all, but rather massless electromagnetic radiation. Low-mass things (particles) can travel in excess of 99% the speed of light.
Long answer: See relativity.
Time - second (s)
- Originally, the time for a pendulum (1-m long) to swing from one side of path to other
- Later, a fraction of mean solar day
- (1967): the time taken by 9,192,631,770 vibrations of a specific wavelength of light emitted by a cesium-133 atom
Mass - kilogram (kg)
- originally based on the mass of a cubic decimeter of water
- standard of mass is now the platinum-iridium cylinder kept at the International Bureau of Weights and Measures near Paris
- secondary standards are based on this
- 1 u (atomic mass unit, or AMU) = 1.6605402 x 10^-27 kg
- so, the Carbon-12 atom is 12 u in mass
Volume - liter (l)
- volume occupied by a mass of 1 kg of pure water at certain conditions
- 1.000028 decimeters cubed
- ml is approximately 1 cc
Temperature - kelvin (K)
- 1/273.16 of the thermodynamic temperature of the triple point of water (1 K = 1 degree C)
- degrees C + 273.15
- 0 K = absolute zero
For further reading:
http://en.wikipedia.org/wiki/SI_units
http://en.wikipedia.org/wiki/Metric_system#History
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In addition, we spoke about the spherocity of the Earth and how we know its size. I've written about this previously. Please see the blog entries below:
http://howdoweknowthat.blogspot.com/2009/07/how-do-we-know-that-earth-is-spherical.html
http://howdoweknowthat.blogspot.com/2009/07/so-how-big-is-earth.html
Thursday, March 10, 2016
EM project ideas, etc.
Projects?
Speaker
Microphone
Motor
(Generator)
Guitar pickup
Telegraph
Circuit? Amplifier circuit? LED blinking circuit?
Arduino?
And in honor of Clara Rockmore's birthday:
https://www.youtube.com/watch?v=pSzTPGlNa5U
(Also see the recent Google doodle...)
And in honor of Clara Rockmore's birthday:
https://www.youtube.com/watch?v=pSzTPGlNa5U
(Also see the recent Google doodle...)
Tuesday, March 8, 2016
For today (Tuesday)
Hello everyone,
Sorry to be out sick today. Please do the following:
Gather in small groups and come up with 2 lists - what you know (or believe to be true) about magnetism, and what questions you have about magnetism. Then join with another group and compare notes.
After that, do some research by inquiring about these questions (online, etc.):
1. What is magnetite?
2. What are North and South poles?
3. What is true about opposite poles and like poles in magnetism?
4. How do we discuss Earth's magnetic field? Where are the magnetic poles?
5. How does a compass work? That is, how is it "polarized?"
6. How are magnetic fields similar to electric fields? (Or different?)
7. Research some of the questions you generated in your list(s) above.
See you Thursday.
Sean
Tuesday, February 23, 2016
Monday, February 22, 2016
Also, FYI regarding quiz tomorrow
Topics are:
- series and parallel circuit
- how to find resistance using 1/R equation
There will be NO full combination circuit questions (unless it is a bonus).
OK? OK!
- series and parallel circuit
- how to find resistance using 1/R equation
There will be NO full combination circuit questions (unless it is a bonus).
OK? OK!
SAT physics prep
Thank you to those who made it today. If you didn't, but still want to prep for the test, see me asap and I'll give you the packet of information. We will be meeting at 1 PM on Mondays.
Here's the homepage for the test itself:
https://collegereadiness.collegeboard.org/sat-subject-tests/subjects/science/physics
Note that we will NOT be covering thermodynamics or modern topics in class, so we will spend the first few sessions covering these ideas.
Here's the homepage for the test itself:
https://collegereadiness.collegeboard.org/sat-subject-tests/subjects/science/physics
Note that we will NOT be covering thermodynamics or modern topics in class, so we will spend the first few sessions covering these ideas.
Friday, February 19, 2016
Combination circuit notes
The two examples from class today:
We will have a quiz next class, focusing on series and parallel circuits. There will be no combo circuit problems, unless it is a bonus question.
Wednesday, February 17, 2016
Saturday, February 13, 2016
Monday, February 8, 2016
Lab Homework
For next class, make 2 graphs:
current vs. resistance
voltage vs. resistance
Start to think about what these graphs suggest, maybe even writing down your thoughts.
There will be a few lab questions that you might want to start thinking about as well. Here they are:
1. What is Ohm's law?
2. Are there things that do not "obey" Ohm's law?
3. What are sources of error in this experiment?
4. Would you expect to have the same readings if you left things running for a few minutes? How about after an hour?
5. What exactly is "internal resistance" and how is it relevant in this experiment?
6. Somewhere, maybe in your conclusion, be sure to address why the graphs look as they do.
Don't forget that you'll also need these things in your lab:
Purpose
Hypothesis (copied from the earlier homework - don't change it, and include the graph you predicted)
Data table with correct units
Graphs with correct units
Questions
Conclusion
Thanks everybody! You'll have some time in the next class to work on the lab.
current vs. resistance
voltage vs. resistance
Start to think about what these graphs suggest, maybe even writing down your thoughts.
There will be a few lab questions that you might want to start thinking about as well. Here they are:
1. What is Ohm's law?
2. Are there things that do not "obey" Ohm's law?
3. What are sources of error in this experiment?
4. Would you expect to have the same readings if you left things running for a few minutes? How about after an hour?
5. What exactly is "internal resistance" and how is it relevant in this experiment?
6. Somewhere, maybe in your conclusion, be sure to address why the graphs look as they do.
Don't forget that you'll also need these things in your lab:
Purpose
Hypothesis (copied from the earlier homework - don't change it, and include the graph you predicted)
Data table with correct units
Graphs with correct units
Questions
Conclusion
Thanks everybody! You'll have some time in the next class to work on the lab.
Thursday, February 4, 2016
Pre-Lab homework
For our next formal lab, which we will begin next week, you will ask the question:
How does resistance affect electrical current and voltage?
Do NOT do any formal research on this question. Instead, think about it and put your ideas into a testable hypothesis. And since you will be making graphs of current (y-axis) vs. resistance (x-axis) AND voltage vs. resistance in the lab, show (in your hypothesis) what you think the graphs will resemble (with sketches).
Important info:
Current (I) is the rate at which charge "flows" in a circuit.
Resistance (R) is a measure of "push back" against current.
So bring your hypothesis and expected graph sketch to next class.
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The lab set-up:
>
The lab set-up:
Current and Resistance - a lab!
In this lab, you will determine the relationship (if there is one) between electrical current and resistance.
At this point, you should have a hypothesis - including the graph you expect will represent the relationship between current and resistance (I vs. R).
1. Set up the circuit depicted in the pictures below:
Two batteries in series with the "resistance box" and a meter set to measure current (in A - use the 20A setting and socket for the red wire). Connections are made with alligator wires - wires with "alligator" clips on each end.
Have a separate meter set up to measure voltage. To do this, the meter needs to be "in parallel" with the resistance box. See the picture below.
2. Change the resistance in small increments, starting at around 4 ohms. Write down the following data: resistance (in ohms), current (in A), and voltage (in V). Take at least 20 trials. If you get to a point where the current is staying the same (or reading zero), try switching the dial to the mA setting (and move the red wire to the mA socket as well).
3. For homework, plot 2 graphs: current (I) vs. resistance (R), and voltage (V) vs. resistance.
4. A few questions will be forthcoming.
In the last picture, the yellow meter is measuring current - it is IN SERIES with the batteries and resistance box. The red meter is measuring voltage - it is IN PARALLEL with the resistance. You will notice that a student is holding the leads from the meter - you may need to do the same.
Friday, January 29, 2016
Notes from Today - batteries and whatnot
https://phet.colorado.edu/en/simulation/circuit-construction-kit-dc
Play around with this. If you can't get it to run (since it uses Java), maybe try it on a different browser - Firefox instead of Chrome.
You can build virtual circuits and basically see how they work. The moving balls are supposed to represent electrons.
The battery is a source of voltage. Recall that voltage is:
V = E/Q
- the amount of available energy per charge. The unit is the joule per coulomb, also known as a volt (V).
It can be a little confusing that the symbol for voltage (V) is the same as the symbol for its units (V). Hopefully the context will make it clear what the V is supposed to represent in data or an equation.
Battery image:
Play around with this. If you can't get it to run (since it uses Java), maybe try it on a different browser - Firefox instead of Chrome.
You can build virtual circuits and basically see how they work. The moving balls are supposed to represent electrons.
The battery is a source of voltage. Recall that voltage is:
V = E/Q
- the amount of available energy per charge. The unit is the joule per coulomb, also known as a volt (V).
It can be a little confusing that the symbol for voltage (V) is the same as the symbol for its units (V). Hopefully the context will make it clear what the V is supposed to represent in data or an equation.
Battery image:
Friday, January 22, 2016
Charge notes FYI
Charge
- as fundamental to electricity & magnetism as mass is to mechanics
Charge is a concept used to quantatively related "particles" to other particles, in terms of how they affect each other - do they attract or repel? If so, with what force?
Charge is represented by letter Q.
The basic idea - likes charges repel (- and -, or + and +) and opposite charges attract (+ and -).
Charge is measured in units called coulombs (C). A coulomb is a huge amount of charge, but a typical particle has a tiny amount of charge:
- the charge of a proton is 1.6 x 10^-19 C. Similarly, the charge of an electron is the same number, but negative, by definition (-1.6 x 10^-19 C). The negative sign distinguishes particles from each other, in terms of whether or not they will attract or repel. The actual sign is arbitrarily chosen.
The charge of a neutron is 0 C, or neutral.
But what IS charge?
Charge is difficult to define. It is property of particles that describes how particles interact with other particles.
In general, the terms are negative and positive, with differing amounts of each, quantified as some multiple of the fundamental charge value (e):
e = 1.6 x 10^-19 C
That's hard to visualize, since a coulomb (c) is a huge amount of charge. One coulomb, for example, is the charge due to:
1 coulomb = charge due to 6.3 x 10^18 protons
A typical cloud prior to lightning may have a few hundred coulombs of charge - that's an enormous amount of excess charge.
If the charge is negative (-), the excess charge is electrons.
If the charge is positive (+), the excess charge is protons - however, we can NOT easily move protons. That usually takes a particle accelerator. Typically, things are charged positively by REMOVING electrons, leaving a net charge of positive.
Other things to remember:
Neutral matter contains an equal number of protons and electrons.
The nucleus of any atom contains protons and (usually) neutrons (which carry no charge). The number of protons in the nucleus is called the atomic number, and it defines the element (H = 1, He = 2, Li = 3).
Electrons "travel" around the nucleus in "orbitals." See chemistry for details. The bulk of the atom is empty space.
Like types of charge repel. Opposite types of charge attract.
The proton is around 2000 times the mass of the electron and makes up (with the neutrons) the bulk of the atom. This mass difference also explains why the electron orbits the proton, and not the other way around.
Protons in the nucleus of an atom should, one would imagine, repel each other greatly. As it happens, the nucleus of an atom is held together by the strong nuclear force (particles which are spring-like, called gluons, keep it together). This also provides what chemists called binding energy, which can be released in nuclear reactions.
COULOMB'S LAW
How particles interact with each other is governed by a physical relationship called Coulomb's Law:
F = k Q1 Q2 / d^2
Or, the force (of attraction or repulsion) is given by a physical constant times the product of the charges, divided by their distance of separation squared. The proportionality constant (k) is used to make the units work out to measurable amounts.
Note that this is an inverse square relationship, just like gravity.
The "big 3" particles you've heard of are:
proton
neutron
electron
However, only 1 of these (the electron) is "fundamental". The others are made of fundamental particles called "quarks""
proton = 2 "up quarks" + 1 "down quark"
neutron = 2 "down quarks" + 1 "up quark"
There are actually 6 types of quarks: up, down, charm, strange, top, & bottom. The names mean nothing.
Many particles exist, but few are fundamental - incapable of being broken up further (so far as we know).
In addition, "force-carrying" particles called "bosons" exist -- photons, gluons, W and Z particles.
The Standard Model of Particles and Interactions:
http://www.pha.jhu.edu/~dfehling/particle.gif
HW
HW for Monday
Investigate how a basic battery works.Some questions you might want to think about:
- What parts are needed?
- What types of batteries exist and what are the differences?
- How is Volta connected to the battery?
- What is voltage?
Friday, January 15, 2016
To play around with....
Recall the definition of electric field (E-field):
- region surrounding a charged object (or chunk of charge)
- represented by arrows that indicate the direction a "tiny positive test charge" (like a proton) would take if placed in the field
Applets to play with:
https://phet.colorado.edu/en/simulation/charges-and-fields
Maybe these:
http://www.falstad.com/emstatic/
http://www.dgp.toronto.edu/~mjmcguff/research/electrostatic/applet1/main.html
http://surendranath.tripod.com/Applets/Electricity/FieldLines/EFL.html
- region surrounding a charged object (or chunk of charge)
- represented by arrows that indicate the direction a "tiny positive test charge" (like a proton) would take if placed in the field
Applets to play with:
https://phet.colorado.edu/en/simulation/charges-and-fields
Maybe these:
http://www.falstad.com/emstatic/
http://www.dgp.toronto.edu/~mjmcguff/research/electrostatic/applet1/main.html
http://surendranath.tripod.com/Applets/Electricity/FieldLines/EFL.html
Wednesday, January 13, 2016
HW for Friday - to turn in
Electrostatics homework – to be turned in Thursday
1. Define charge.
2. Explain why a
charged balloon will stick to a (neutral) wall.
3. What is the charge
(in coulombs) of a electron?
4. How many protons
does it take to make 15 coulomb of charge?
5. In any atom, which
particle(s) are fundamental and which are composite (made of smaller
particles)?
6. You have two
clusters of charge: 10 C and 20 C,
separated by 1-m of distance.
a. Use Coulomb’s law
to calculate the force that exists between the charges.
b. Is this force
attractive or repulsive?
c. If you were to quadruple
the distance between the charges, what exactly would happen to the amount of
force between the charges?
7. Carbon is element
number 6.
a. What does the 6
represent?
b. What do you
suppose is the difference between Carbon-12 and Carbon-14?
8. Why do you think
that electrons orbit protons (and not the other way around)?
Friday, January 8, 2016
For Monday's class
Come with a definition of the coulomb, a unit of charge.
Also come with a definition (or equation) for Coulomb's law.
If you're feeling ambitious, try to find out what an "inverse square" relationship (or law) is all about.
Thanks! Fun class today, gang.
Also, the standard model chart, in high resolution:
http://www.pha.jhu.edu/~dfehling/particle.gif
http://www.cpepphysics.org/images/2014-fund-chart.jpg
And others:
http://www.u-tokyo.ac.jp/content/400020908.png
https://thetimedok.files.wordpress.com/2014/11/standard-model.jpg
Also come with a definition (or equation) for Coulomb's law.
If you're feeling ambitious, try to find out what an "inverse square" relationship (or law) is all about.
Thanks! Fun class today, gang.
Also, the standard model chart, in high resolution:
http://www.pha.jhu.edu/~dfehling/particle.gif
http://www.cpepphysics.org/images/2014-fund-chart.jpg
And others:
http://www.u-tokyo.ac.jp/content/400020908.png
https://thetimedok.files.wordpress.com/2014/11/standard-model.jpg
Wednesday, January 6, 2016
Tuesday, January 5, 2016
First HW of the new year - yay!!!
Come up with an actual definition that you understand for: Charge
Recall the definition for mass - oh wait, how do we define mass exactly? Well, it's the amount of "stuff" that an object has, compared to a standard (the kilogram, which is precisely defined).
I want you to have a definition for charge that makes sense to you.
OK? OK!
Welcome back, physics phriends!
Recall the definition for mass - oh wait, how do we define mass exactly? Well, it's the amount of "stuff" that an object has, compared to a standard (the kilogram, which is precisely defined).
I want you to have a definition for charge that makes sense to you.
OK? OK!
Welcome back, physics phriends!
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