Tuesday, May 31, 2016
Tuesday, May 24, 2016
Some reading before class
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
(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).
Tuesday, May 10, 2016
Saturday, May 7, 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, after you've had some class time to work on it. 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. We're talking about a couple of introductory paragraphs or so.
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) - percent error.
10. General concluding remarks
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. We're talking about a couple of introductory paragraphs or so.
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) - percent error.
10. General concluding remarks
It is ok to have 7-9 in one large section.
Tuesday, May 3, 2016
Practice problems
Here are two more practice problems for the equations of motion:
1. A car starts from rest and accelerates at 5 m/s/s. After 8 seconds of continual acceleration:
a. how fast is it moving?
b. how far has it gone?
(40 m/s, 160 m)
2. This one may require a bit of algebra. If you were to drop a ball (from rest) from a 50-m tall tower, find:
a. how long it takes to hit the ground?
b. how fast it will be traveling right before it hits?
(3.19 seconds, 31.3 m/s)
Don't forget to review the new lab stuff, too! (It is just below this on the blog.)
Also, answers to the recent quiz:
1. 3.65 days
2. approx. 9, 612,173
3. on the order of 140 million (or somewhere in the 10^8 range)
4. 27,273
1. A car starts from rest and accelerates at 5 m/s/s. After 8 seconds of continual acceleration:
a. how fast is it moving?
b. how far has it gone?
(40 m/s, 160 m)
2. This one may require a bit of algebra. If you were to drop a ball (from rest) from a 50-m tall tower, find:
a. how long it takes to hit the ground?
b. how fast it will be traveling right before it hits?
(3.19 seconds, 31.3 m/s)
Don't forget to review the new lab stuff, too! (It is just below this on the blog.)
Also, answers to the recent quiz:
1. 3.65 days
2. approx. 9, 612,173
3. on the order of 140 million (or somewhere in the 10^8 range)
4. 27,273
The New Lab!
Hey everybody!
As discussed in class, the new lab will have you trying to determine the acceleration due to gravity. We have heard that this should be close to 9.8 m/s/s, but how can we be sure? Here are some techniques for finding this value.
1. The dropped object method.
Drop an object from a measured height (d) and record the time (t) for it to fall. Calculate the acceleration (a) from this. You can try a large height (such as in the atrium, using a stopwatch), or you can use a small height with a photogate (and shut-off switch) set-up.
2. The pendulum method.
You know from earlier in the year that the period of a pendulum (T) is given by an equation:
In this equation, T is period/time for one swing (in seconds), L is length (in meters), and g is the acceleration due to gravity.
3. The ticker-tape method.
This is maybe the most novel method, and it gives you a lot of data. However, the analysis is probably the longest. A ticker-tape timer will vibrate 60 times per second (60 Hz is the frequency), which will give 60 dots per second on a piece of "ticker tape." By measuring the distance between dots and calculating the increasing speeds - details to be discussed in class - you can, in principle, determine the acceleration.
4. The single photogate method.
In this method, you will release a ball from rests and allow it to fall a measured distance (d). It will achieve a certain velocity (Vf) at the bottom of the fall - the photogate should give you the final velocity, if used correctly.
So there you have it - 4 methods for finding g. There are others, of course, but these 4 can be done in our classroom.
Consider these 4 methods and decide which one is most interesting to you. You will perform the experiment during the next class.
In preparation, start to write-up a tentative outline of how you plan to collect data. It is ok to leave out the technical details at this point, since you probably don't know how to use the new equipment yet.
In your final lab report, you will have a detailed procedure - something that a total stranger could conceivably follow - and a materials list. You will have other lab report things as well.
As discussed in class, the new lab will have you trying to determine the acceleration due to gravity. We have heard that this should be close to 9.8 m/s/s, but how can we be sure? Here are some techniques for finding this value.
1. The dropped object method.
Drop an object from a measured height (d) and record the time (t) for it to fall. Calculate the acceleration (a) from this. You can try a large height (such as in the atrium, using a stopwatch), or you can use a small height with a photogate (and shut-off switch) set-up.
2. The pendulum method.
You know from earlier in the year that the period of a pendulum (T) is given by an equation:
In this equation, T is period/time for one swing (in seconds), L is length (in meters), and g is the acceleration due to gravity.
3. The ticker-tape method.
This is maybe the most novel method, and it gives you a lot of data. However, the analysis is probably the longest. A ticker-tape timer will vibrate 60 times per second (60 Hz is the frequency), which will give 60 dots per second on a piece of "ticker tape." By measuring the distance between dots and calculating the increasing speeds - details to be discussed in class - you can, in principle, determine the acceleration.
4. The single photogate method.
In this method, you will release a ball from rests and allow it to fall a measured distance (d). It will achieve a certain velocity (Vf) at the bottom of the fall - the photogate should give you the final velocity, if used correctly.
So there you have it - 4 methods for finding g. There are others, of course, but these 4 can be done in our classroom.
Consider these 4 methods and decide which one is most interesting to you. You will perform the experiment during the next class.
In preparation, start to write-up a tentative outline of how you plan to collect data. It is ok to leave out the technical details at this point, since you probably don't know how to use the new equipment yet.
In your final lab report, you will have a detailed procedure - something that a total stranger could conceivably follow - and a materials list. You will have other lab report things as well.
Friday, April 29, 2016
Equations of motion HW
The equations of motion are given above. Try these problems. Solutions will be coming over the weekend. Thanks!
1. (From last homework.) How far will a freely-falling body fall in 4 seconds? There is no air resistance.
2. A car starts from rest and achieves a speed of 30 m/s in 4 seconds. Find:
a. the acceleration
b. the distance that it goes in the 4 seconds
Now the same car applies the brakes once it has achieved 30 m/s. If the car stops in 2 seconds, find:
a. the acceleration
b. how far the car travels in the 2 seconds
3. Consider throwing a ball up in the air with an initial speed of 30 m/s. I suggest that you call up positive (which makes gravity negative).
a. How long will it take for the ball to reach apogee (the top)?
b. How high will it go?
b. How high will it go?
Wednesday, April 27, 2016
for next class (Friday)
1. Find a definition for acceleration: in words and in equation form.
2. What are the units for acceleration?
3. If an object starts from rest and achieves a velocity of 8 m/s in 10 seconds, what is the acceleration?
4. If a car has an acceleration of 1.5 m/s/s, how fast is it traveling after 7 seconds (assuming that it starts from rest)?
5. What is gravitational acceleration all about? (Hint: recall 9.8.....)
6. If a ball is dropped from rest, how fast will it be traveling after 4 seconds?
7. If a ball is dropped from rest, how far will it have fallen after 4 seconds? (This is tricky - find the average velocity first. We'll soon have equations to make this all easier.)
8. See if you can find anything about "equations of motion" for homework.
Thanks!
Friday, April 22, 2016
Wednesday, April 13, 2016
HW practice
Hey there,
FIRST THINGS FIRST: Quiz in 2 classes - it will cover units, unit conversions and speed, etc.
Come up with a couple of conversions on your own and work through them.
Try a speed conversion first: something like m/s to furlongs per fortnight, etc. Be creative or weird.
If you're feeling ambitious, try a volume/time conversion: something like liters/second to gallons/hour.
Also, find a definition for acceleration - in words, and in equation.
Thanks!
SL
FIRST THINGS FIRST: Quiz in 2 classes - it will cover units, unit conversions and speed, etc.
Come up with a couple of conversions on your own and work through them.
Try a speed conversion first: something like m/s to furlongs per fortnight, etc. Be creative or weird.
If you're feeling ambitious, try a volume/time conversion: something like liters/second to gallons/hour.
Also, find a definition for acceleration - in words, and in equation.
Thanks!
SL
Monday, April 11, 2016
For next class (Weds)
Work out your average speeds in m/s.
Think closely about all the errors in your methodology. Write down all of the errors you can think of.
Also, some of you are still struggling with unit conversions. Try to come up with 2-3 more of your own: m/s to some other unit. Work them all out.
Got it?
Thanks!
Think closely about all the errors in your methodology. Write down all of the errors you can think of.
Also, some of you are still struggling with unit conversions. Try to come up with 2-3 more of your own: m/s to some other unit. Work them all out.
Got it?
Thanks!
Friday, April 8, 2016
Fun problems
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. A good problem to think about. If we gathered ALL of the people in the world together for the world's largest party, how large a social hall would we need to build? Think about how to calculate this, and try it. You'll have to make a couple of assumptions, of course.
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. A good problem to think about. If we gathered ALL of the people in the world together for the world's largest party, how large a social hall would we need to build? Think about how to calculate this, and try it. You'll have to make a couple of assumptions, of course.
For fun:
https://en.wikipedia.org/wiki/List_of_humorous_units_of_measurement
Friday, April 1, 2016
HW
HW for B block class:
Review the ideas of basic unit conversions. Do these, looking up equalities as needed:
1. 10 inches = ____ m
2. 5 m = ____ km
3. 30 ft = ____ miles
4. 4 seconds = ____ hours
Now come up with your own "conversion factors", using the technique illustrated in class:
5. 1 m/s = ____ feet/year
6. 1 mile/hour = ____ feet/fortnight
* A fortnight is 14 days.
>
HW for C block class.
Play around with this. It simulates the length of days (of sunlight) based on the day of the year. It is related to the conversation we had today.
http://astro.unl.edu/naap/motion3/animations/sunmotions.swf
Also, we will be discussing unit conversions (now that you know a little about units). Convert these units:
1. 30 inches = ____ feet
2. 30 inches = ____ yards
3. 30 inches = ____ miles
Recall that there are 5280 feet per mile.
4. Now try this. Usain Bolt is widely regarded as the fastest man alive. He ran 200 m in 20.40 seconds, which is an average speed of 9.8 m/s. Try to convert this speed to miles/hour.
You may need to know that there are 1609 meters in 1 mile.
Review the ideas of basic unit conversions. Do these, looking up equalities as needed:
1. 10 inches = ____ m
2. 5 m = ____ km
3. 30 ft = ____ miles
4. 4 seconds = ____ hours
Now come up with your own "conversion factors", using the technique illustrated in class:
5. 1 m/s = ____ feet/year
6. 1 mile/hour = ____ feet/fortnight
* A fortnight is 14 days.
>
HW for C block class.
Play around with this. It simulates the length of days (of sunlight) based on the day of the year. It is related to the conversation we had today.
http://astro.unl.edu/naap/motion3/animations/sunmotions.swf
Also, we will be discussing unit conversions (now that you know a little about units). Convert these units:
1. 30 inches = ____ feet
2. 30 inches = ____ yards
3. 30 inches = ____ miles
Recall that there are 5280 feet per mile.
4. Now try this. Usain Bolt is widely regarded as the fastest man alive. He ran 200 m in 20.40 seconds, which is an average speed of 9.8 m/s. Try to convert this speed to miles/hour.
You may need to know that there are 1609 meters in 1 mile.
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
>
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
Friday, March 11, 2016
Homework related to the mini-projects
Try to find an explanation for how your device works (or is supposed to work). This means, you should be looking up what pertains to your device:
- how do speakers work
- how do microphones work
- how do motors work
- how do telegraphs work
Or whatever makes sense. Bring some kind of explanation to class next time and we can talk it through so it makes sense to you. A picture might be really helpful, too!
Keep in mind that you'll be teaching the class a little about your device, so you should understand as much of the basic operation as possible. You'll have time to compare notes with others who built the same type of device.
Thanks!
- how do speakers work
- how do microphones work
- how do motors work
- how do telegraphs work
Or whatever makes sense. Bring some kind of explanation to class next time and we can talk it through so it makes sense to you. A picture might be really helpful, too!
Keep in mind that you'll be teaching the class a little about your device, so you should understand as much of the basic operation as possible. You'll have time to compare notes with others who built the same type of device.
Thanks!
Wednesday, March 9, 2016
Project ideas
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 today's Google doodle...)
And in honor of Clara Rockmore's birthday:
https://www.youtube.com/watch?v=pSzTPGlNa5U
(Also see today's Google doodle...)
Monday, March 7, 2016
If you missed class today (Monday)...
Brainstorm a list of things about magnetism:
- what you know (or think you know)
- what you don't know and/or might want to know
Also, look into "magnetite" - what is it, etc.?
Also, look into "magnetite" - what is it, etc.?
Write down your list and bring it to Wednesday's class. Thanks!
Thursday, February 25, 2016
Thursday, February 18, 2016
Quiz next class
Be able to:
- solve a series circuit
- solve a parallel circuit
- find resistance using the new formula: (1/Rtotal = 1/R1 + 1/R2 + 1/R3)
Practice:
1. Resolve this circuit: 4 resistors (1, 2, 3, 4 ohms) in series with 12 volt battery.
2. Resolve this circuit: 2 resistors (3 and 6 ohms) in parallel with a 24 volt battery.
3. Check the value of the resistance in #2 above, using the new resistance formula.
- solve a series circuit
- solve a parallel circuit
- find resistance using the new formula: (1/Rtotal = 1/R1 + 1/R2 + 1/R3)
Practice:
1. Resolve this circuit: 4 resistors (1, 2, 3, 4 ohms) in series with 12 volt battery.
2. Resolve this circuit: 2 resistors (3 and 6 ohms) in parallel with a 24 volt battery.
3. Check the value of the resistance in #2 above, using the new resistance formula.
Tuesday, February 16, 2016
Thursday, February 11, 2016
Wednesday, February 3, 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.
Monday, February 1, 2016
Circuit lab
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 a graph of current (I) vs. resistance (R).
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.
Thursday, January 28, 2016
Hypothesis HW
For our next formal lab, which we will begin next week, you will ask the question:
How does resistance affect electrical current?
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 a graph of current (y-axis) vs. resistance (x-axis) in the lab, show (in your hypothesis) what you think the graph will resemble (with a sketch).
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.
Friday, January 22, 2016
From class today
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:
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
Wednesday, January 20, 2016
HW for Friday
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?
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?
Tuesday, January 12, 2016
HW to be turned in next class
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 proton?
4. How many protons
does it take to make 1 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
double 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. What is the
difference between the charge of a proton and the charge of an electron?
Wednesday, January 6, 2016
For Friday's and Tuesday's classes - revised HW
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 classes today, gang.
Also, have a look at how the Van de Graaff generator works.
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 classes today, gang.
Also, have a look at how the Van de Graaff generator works.
Monday, January 4, 2016
First homework of the new year - yay!!!
Come up with a definition that you understand for: Charge
You may need to come up with a definition that is like 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).
OK? OK!
Welcome back, physics phriends!
You may need to come up with a definition that is like 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).
OK? OK!
Welcome back, physics phriends!
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