AP Physics 2 - fundamentals review #3

With three weeks to go before the AP physics exams, it's worth remembering that our students don't need MORE practice problems; rather, they need to pay careful attention to the practice problems they do.  This is my application of a John Dewey principle, that we don't learn by doing; we learn by paying attention to what we do.  

My first-year students in physics 1 are in a cycle of AP problem / quiz based on AP problem / corrections to AP problem if their quiz or problem shows they didn't get it the first time.  In AP Physics 2, which is a second-year course, students have already internalized that they must pay attention to what they do.  And, P2 students have a level of earned confidence in their skills that my 9th grade P1 folks lack.  So truly all we're doing is these daily quizzes, in-class experimental and problem solving work, and each week one take-home 25 minute "quiz" with 5 multiple choice and one free response.  Less is more when dealing with upperclasspeople in the spring.

P2 Fundamentals Review #3

21. A battery is connected to two resistors in series.  The resistors each take 20 V of voltage across them.  What is the voltage of the battery?

22. Write the first law of thermodynamics, which is an expression for the change in internal energy.

23. Two waves are initially in phase with one another.  One wave has traveled a small extra distance than the other.  Under what conditions does constructive interference occur?

24. Define the period of a wave.

25. What is the equation relating the image and object distances for a convex mirror?

26. Two light waves undergo constructive interference.  What physical effect will be observed?

27. Name two items that can produce a magnetic field.

28. A battery is connected to two resistors in series.  The resistors each take 20 mA of current through them.  What is the current provided by the battery?

29. A gas consists of molecules moving around.  What feature of these molecules’ behavior causes the macroscopic effect called pressure?

30. What is the physical quantity that means energy produced in one second?

AP Physics 2 - fundamentals review #2

My AP Physics 2 class is an ungraded honors course.  There's not even a contract.  There is a careful selection process - students are selected not based on their previous grades, but holistically based on their demonstrated authentic interest in the subject.  Basically, if a student passed P1, wasn't a jerk, and put forth reasonably consistent effort, we take them into the P2 class.  

Even without published grades, the ground state of our class is to begin with a 4-5 minute fundamentals quiz.  We grade the quiz; students tell me their scores.  The quiz grades have no extrinsic meaning, won't be seen by parents or counselors or universities.  So what!  Motivated students still care about getting things right.  But, with no published grades, the students are insulated from the shame or world-ending dread of receiving a published grade that is not an A.  If someone gets 3/10 - which happens somewhat regularly! - they don't have to fear that someone is waiting with a (hopefully figurative) cane for their poor performance.  They just should try to do better next time.

P2 Fundamentals Quiz #2

11. Light is incident on a thin film.  Under what conditions does the light change phase?

12. An ammeter measures ____.  It is connected in _____ with a resistor.

13. Write the equation for induced emf.  

14. And electric field points →.  A positron moves ← in the electric field.  Sketch the path of the positron, and describe briefly how it moves.

15. An electric field points from position A to position B.  Which position is at higher electric potential?

16. A gas expands adiabatically.  What is the sign of the work done on the gas during this process?

17. Write the units of electric field.

18. Kirchoff’s loop rule is a statement of conservation of _____.

19. A concave mirror has radius 50 cm.  What is the focal length of this mirror?

20. For a convex (diverging) mirror, how does a ray parallel to the principal axis reflect?

(Solutions will be in the comments in a few days!)

Fundamentals checks for AP Physics *2*

Last year I wrote a bunch of fundamentals checks in preparation for the AP physics 1 or C-mechanics exams.  This year, I'm teaching a physics 2 section.  So, I'm writing daily quizzes for P2 now!  I'll post the quizzes here, then follow up a couple of days later with the answers in the comments.

Quiz 1:

1.       A substance has index of refraction 1.5.  What is (or how do I figure out) the speed of light in this substance?

2.       Write the equation relating focal length, image distance, and object distance for a mirror.

3.       Write the equation for the energy carried by a photon.

4.       Under what conditions is the image distance d_i positive, and under what conditions is it negative?

5.       The diagram shows light hitting a mirror.  What is the angle of incidence as the light hits the mirror?

6.       A resistor is connected to a battery, and the current in the circuit is measured.  The voltage of the battery is increased, and the current is measured again.  Which Ohm’s Law variable(s) remain unchanged after the voltage is increased?

7.       Under what conditions is the electric field given by the equation E = kQ/d²?

8.       One mole of monotomic ideal gas begins at pressure P₁ and volume V_1­.  Next, the pressure is increased to P₂ without changing the volume.  Write an expression (including a + or – sign) for the work done on the gas.

 9.       Write the equation for the capacitance of a parallel plate capacitor.

 10.   A photon initially has wavelength 500 nm.  The photon collides with a free electron which was initially at rest, giving the electron 3.0 eV of kinetic energy.  The photon bounces back opposite the direction it was initially moving.  What is the speed of the reflected photon?

2024 Conceptual Physics Summer Institute - July 19-20, registration is OPEN!

This summer's session will be Friday-Saturday July 19-20.  We need a minimum of ten participants to run the institute.  In the unlikely event that we don't get ten, I'll refund all fees and you'll keep access to the shared files.  So sign up now, and I'll post here when we're confirmed!

If you're close enough to Woodberry Forest School, you're welcome to attend in person rather than online (though I'm prioritizing the online experience).  Two people chose this option last summer, and enjoyed it!

Folks, I'm already teaching several AP summer institutes - see the left-hand sidebar for details.  But what if you are looking for physics professional development that is NOT aimed at college-level physics?  I mean, I meet so many of you each year who teach on-level, honors, college-prep, Regents... to all ages, to all varieties of student.  And in my personal mission to spread physics knowledge to as wide an audience as possible, these sub-college courses represent a critical first point of contact with our discipline.  I focus as much energy on my conceptual course as on my AP course each year.  So I'd like to focus some of my summer professional development expertise on those who teach these first-level courses.

We've done this for four years now.  See the comments at the end of this flyer for participant reviews.  They all say, this institute was an amazing, friendly, and productive experience.

I'm offering a two-day institute on July 19-20, 2024.  It will be online, broadcasting via Zoom from my lab.  Skip past the institute description for fees and registration instructions.  The single session will be limited to the first 40 who sign up.  The daily agenda is included here at the bottom of the page.

Jacobs Physics

Conceptual Physics Institute Description
July 19-20, 2024

All levels of high school physics can be taught conceptually – where verbal and experimental reasoning is prioritized over mathematical problem solving.  While mathematics are used extensively, they are used as a tool to create predictions about the workings of the natural world.  Whether you teach “general”, “on-level”, “honors”, “Regents”, or “college-prep” physics, a conceptual approach can be adapted to most any introductory physics topic – and to most any state or district standards. 

In our institute, we will discuss, practice, and share methods of teaching common physics content in a conceptual style.  I will be broadcasting from my laboratory via zoom.  Time will be devoted to experimental methods that are especially useful at the sub-college level; to course planning on a year-long and a unit basis; and to best-practices physics pedagogy, which differs substantially from pedagogy in other disciplines.

Participants will be given a full-year’s set of classroom-ready materials, including fact sheets, in-class and laboratory activities, assessments, and planning documents.  More importantly, through their interactions with the instructor and with their colleagues, participants will develop skills and ideas for adapting these materials to their specific classroom environment.  Those attending will also earn a certificate indicating their participation in 15 hours of physics professional development.

 

How much does it cost:  $200 for the weekend.  The schedule of events is listed below.

How do I register?  Paypal, Venmo, or even a check!

For Paypal:
(1) Use this link, or the button in the left column of the blog.  It will take you to paypal.
(2) Enter $200 as the donation amount, either through paypal or credit card
(3) Click "Add special instructions to seller" or "click here to provide contact info"
(4) In the note, please include your name, preferred contact email, and institution
(5) Fill in payment info and click "donate now"

For Venmo, last four digits 3550, scan the code to the right here -->
Then email me greg underscore jacobs @ woodberry dot org with your name, contact email, and institution.

If you'd like to pay by check, just email me as above; I'll send an invoice that you can forward to your school for payment!

That's all - I'll be back to you within a day or two confirming your registration, and sending you links to the classroom-ready materials.

Cancelation issues: If you register then can't attend, contact me via email.  As long as I can replace your spot, I'll send a full refund; if I can't replace your spot, I'll refund all but $25.

We do need a minimum of 10 participants to run the session!  In the event that we don't get that minimum, I'll refund your registration fee; yet you will retain access to all the institute materials.   

Schedule: Each session will include both whole-group presentation/discussion, and breakout groups for activities.  In between sessions and during breaks, Greg will be available for informal conversation. 

Friday 19 July (all times eastern time)

10:00               Introductions

                        What does “conceptual” mean – defining levels of physics

Different levels of physics: Culture building in the physics classroom

                        Different levels of physics: developing your resources

11:30               Eight styles of physics laboratory activities

            Including the two best-adapted for conceptual physics

My first day activity – reflection experiment

My first group laboratory experiment – refraction

1:00-1:30    break

1:30                 Sequencing your course

Starting the year right: the most important physics teaching skill

Justifying answers with facts

Simple ray diagrams for optics in conceptual physics

Justifying answers with equations

In-class laboratory exercises: circuits

                       

3:00                 The daily “quiz”

Tests and quizzes, targeted to different levels

                        Other sorts of assessment

                        Preparing for the trimester/semester exam

                        Adapting a conceptual course to external standards

                        In-class laboratory exercises: motion graphs

Afternoon – asynchronous

                        Read through the shared files

                        Read through the Jacobs Physics blog

                        Adapt to your district or state standards

                        Bring questions and ideas for the social or for Sunday

7:30                 Optional Zoom social: Dinner, dessert, drinks, and conversation.  BYOB, obviously. 

Saturday 20 July

10:00               Building and creating experiments with whatever you’ve got

                        Developing your own in-class lab exercises

                        Using or substituting inexpensive equipment

                        In-class laboratory exercises: direction of force and motion

11:30               Methods to speed your grading

                        In-class laboratory exercises: forces in 2-d

                        In-class laboratory exercises: motion in 2-d

                       

1:00-1:30    break

1:30                 The final third of the year – once skills are built

                        How I teach impulse/momentum

                        Energy bar charts at the conceptual level      

                        Laboratory exercises with harmonic motion

                       

3:00                 Sharing: Any Other Demos

Online simulations:

                                    The Physics Classroom

The Physics Aviary

                         Ending the year: the Physics Fight

Energy of various systems in an inelastic collision

 

I was asked about the situation above, in which two carts of different masses are released from rest and roll down frictionless ramps.  The carts collide and stick together on the flat surface.

Let's treat this as a goal-less problem:

Edna, Bertha, and Anthony by @Aldescary

Which way do the carts move after they collide?  

Well, Anthony (whom Edna calls a mean hippopotamus) says that the carts don't move after collision - they both have the same energy but in opposite directions, which cancel.

Oy.  So many things wrong with Anthony's answer.  Let's start with the fact that energy is a scalar quantity - energy can't have direction at all, let alone "cancel" other forms of energy.  And whenever we see a collision, energy should never be the first port of call - momentum should be.

It is true that the gravitational energy of each earth-cart system is converted to kinetic energy at the bottom.  And since the gravitational energy 2mgd is the same for both earth-cart systems, each cart will have the same kinetic energy before collision.

However, the momentums of each cart will be different.  I like to use the shortcut equation K = p^2 / 2m in this case to see that with the same kinetic energy, the cart with a greater mass will also have greater momentum.  You could also convert gravitational energy to kinetic energy to show that the speed at the bottom will be the same for both carts with mass canceling; then by p = mv the bigger mass with the same speed has bigger momentum.

So the cart moving left has larger momentum than the cart moving right, meaning the two-cart system has a leftward momentum.  System momentum must be the same after collision as before, so the momentum is still leftward after collision... and that's the way the stuck-together carts will move.

Is mechanical energy of the both-carts-and-earth system conserved from release to just BEFORE the collision?

Anthony says mechanical energy is potential plus kinetic energy, and is always conserved, so yes.

Well, even a blind squirrel, or hippopotamus, finds a nut once in a while.  Anthony is pretty much correct.  Mechanical energy is conserved when no work is done by external forces and when no internal energy conversion occurs.  Here, the only external force acting on the carts-earth system is the normal forces of the surfaces on the carts.  These forces are perpendicular to the carts' motion, and so do no work.  Mechanical energy is, in fact, conserved here!

Is mechanical energy of the both-carts-and-earth system conserved from release to just AFTER the collision?

Anthony is perturbed... he already answered this question!  Mechanical energy is conserved, period, full stop, end of sentence.  Hemph.

Oh, Anthony... when carts collide and stick together, they undergo an inelastic collision by definition.  Mechanical energy may never be conserved in an inelastic collision - rather, some mechanical energy must be converted to internal energy.  

Is mechanical energy of the both-carts-only system conserved from release to just AFTER the collision?

Anthony says he's done answering these tricky questions involving systems.  He's gonna put his head down on his desk and listen to Edna for a change.

Edna thinks this one is pretty simple... because an object by itself can only possess kinetic energy!  (Potential energy can only exist when a spring or the earth is incl
uded in the relevant system.)  So the mechanical energy of the carts is just their kinetic energy.  On release from rest, the carts have no speed and therefore no kinetic energy.  After collision, the carts are moving, so they have kinetic energy.  The KE has gone from zero to not-zero, and therefore has increased.