Small Group Activity

30 min.

##### $|\pm\rangle$ Forms an Orthonormal Basis
Student explore the properties of an orthonormal basis using the Cartesian and $S_z$ bases as examples.
• Found in: Quantum Fundamentals course(s) Found in: Completeness Relations sequence(s)

Lecture

30 min.

##### A coarse-grained model for transportation
A short lecture introducing the idea that most of the energy loss when driving is going into the kinetic energy of the air.
• Found in: Contemporary Challenges course(s)

Small Group Activity

30 min.

##### A glass of water
Students generate a list of properties a glass of water might have. The class then discusses and categorizes those properties.
• Found in: Energy and Entropy course(s)

Small Group Activity

30 min.

##### Acceleration

In rectangular coordinates, the natural unit vectors are $\{\boldsymbol{\hat x},\boldsymbol{\hat y}\}$, which point in the direction of increasing $x$ and $y$, respectively. Similarly, in polar coordinates the natural unit vectors are $\boldsymbol{\hat r}$, which points in the direction of increasing $r$, and $\boldsymbol{\hat\phi}$, which points in the direction of increasing $\phi$.

The unit tangent vector to a parametric curve is the unit vector tangent to the curve which points in the direction of increasing parameter. The principal unit normal vector to a parametric curve is the unit vector perpendicular to the curve “in the direction of bending”, which is the direction of the derivative of the unit tangent vector.

1. Consider the parametric curve $\boldsymbol{\vec r} = 3\cos\phi\,\boldsymbol{\hat x} + 3\sin\phi\,\boldsymbol{\hat y}$ with $\phi\in[0,2\pi]$. Calculate the unit tangent vector $\boldsymbol{\hat T}$ and the principal unit normal vector $\boldsymbol{\hat N}$ for this curve in terms of $\boldsymbol{\hat x}$ and $\boldsymbol{\hat y}$.
2. Consider a circle of radius $3$ centered at the origin. Determine the unit tangent vector $\boldsymbol{\hat T}$ and the principal unit normal vector $\boldsymbol{\hat N}$ for this curve in terms of $\boldsymbol{\hat r}$ and $\boldsymbol{\hat\phi}$.

## Instructor's Guide

#### Main ideas

• Geometric introduction of $\boldsymbol{\hat r}$ and $\boldsymbol{\hat\phi}$.
• Geometric introduction of unit tangent and normal vectors.

#### Prerequisites

• The position vector $\vec{r}$.
• The derivative of the position vector is tangent to the curve.

#### Warmup

See the prerequisites. It is possible to briefly introduce these ideas immediately preceding this activity.

#### Props

• whiteboards and pens

#### Wrapup

Emphasize that $\boldsymbol{\hat r}$ and $\boldsymbol{\hat\phi}$ do not live at the origin! Encourage students to use the figure provided, which may help alleviate this confusion.

Point out to the students that $\boldsymbol{\hat r}$ and $\boldsymbol{\hat\phi}$ are defined everywhere (except at the origin), whereas $\boldsymbol{\hat{T}}$ and $\boldsymbol{\hat{N}}$ are properties of the curve. It is only on circles that these two notions coincide; $\boldsymbol{\hat r}$ and $\boldsymbol{\hat\phi}$ are adapted to round problems, and circles are round! Symmetry is important.

Emphasize that $\{\boldsymbol{\hat r},\boldsymbol{\hat\phi}\}$ can be used as a basis (except at the origin). Point out to the students that their answer to the last problem gives them a formula expressing $\boldsymbol{\hat r}$ and $\boldsymbol{\hat\phi}$ in terms of $\boldsymbol{\hat{x}}$ and $\boldsymbol{\hat{y}}$. When comparing these basis vectors, they should all be drawn with their tails at the same point.

### Details

We have had success helping students master the idea of “direction of bending” by describing the curve as part of a pickle jar; the principal unit normal vector points at the pickles!

#### In the Classroom

The easiest way to find $\boldsymbol{\hat{N}}$ is to use the dot product to find vectors orthogonal to $\boldsymbol{\hat{T}}$, then normalize. Students must then use the “direction of bending” criterion to choose between the two possible orientations.

Finding $\boldsymbol{\hat{N}}$ in this way requires the student to give names to the its unknown components. This is a nontrivial skill; many students will have trouble with this.

It may be important to draw some examples. Despite that, students still feel wary of embracing $\boldsymbol{\hat{r}}$ and $\boldsymbol{\hat{\phi}}$. People will feel more comfortable over the next few classes but emphasize the geometry: the circle is still the circle and the unit tangent remains the same regardless.

Some students are natural geometors and will realize what the desired vectors are. This is terrific. Certainly, it is worthwhile to explicitly demonstrate this, but the point is the geometry can often do the work for you. This will convince students of the value of smart coordinates.

#### Subsidiary ideas

• Dividing any vector by its length yields a unit vector.
• Using the dot product to find vectors perpendicular to a given vector.

#### Homework

• Some students will not be comfortable unless they work out the components of $\boldsymbol{\hat r}$ and $\boldsymbol{\hat\phi}$ with respect to $\boldsymbol{\hat{x}}$ and $\boldsymbol{\hat{y}}$. Let them.

#### Enrichment

• What units does a unit vector have? Do $\boldsymbol{\hat r}$ and $\boldsymbol{\hat\phi}$ have the same units?
• Found in: Vector Calculus II course(s)

Kinesthetic

10 min.

##### Acting Out Charge Densities
Students, pretending they are point charges, move around the room acting out various prompts from the instructor regarding charge densities, including linear $\lambda$, surface $\sigma$, and volume $\rho$ charge densities, both uniform and non-uniform. The instructor demonstrates what it means to measure these quantities. In a remote setting, we have students manipulate 10 coins to model the prompts in this activity and we demonstrate the answers with coins under a doc cam.
• Found in: Static Fields, AIMS Maxwell, Surfaces/Bridge Workshop, Problem-Solving course(s) Found in: Gauss/Ampere Sequence (Integral Form), Integration Sequence, E&M Ring Cycle Sequence sequence(s)

Kinesthetic

10 min.

##### Acting Out Current Density
Students, pretending they are point charges, move around the room so as to make an imaginary magnetic field meter register a constant magnetic field, introducing the concept of steady current. Students act out linear $\vec{I}$, surface $\vec{K}$, and volume $\vec{J}$ current densities. The instructor demonstrates what it means to measure these quantities by counting how many students pass through a gate.
• Found in: Static Fields, AIMS Maxwell, Problem-Solving course(s) Found in: Gauss/Ampere Sequence (Integral Form), Integration Sequence, E&M Ring Cycle Sequence sequence(s)

Kinesthetic

10 min.

##### Acting Out Effective Potentials
A student is invited to “act out” motion corresponding to a plot of effective potential vs. distance. The student plays the role of the “Earth” while the instructor plays the “Sun”.
• Found in: Central Forces course(s)

Small Group Activity

5 min.

##### Acting Out Flux
Students hold rulers and meter sticks to represent a vector field. The instructor holds a hula hoop to represent a small area element. Students are asked to describe the flux of the vector field through the area element.
• Found in: AIMS Maxwell, Static Fields, Surfaces/Bridge Workshop, Problem-Solving course(s) Found in: Gauss/Ampere Sequence (Integral Form), Flux Sequence sequence(s)

Kinesthetic

10 min.

Students are shown a topographic map of an oval hill and imagine that the classroom is on the hill. They are asked to point in the direction of the gradient vector appropriate to the point on the hill where they are "standing".
• Found in: Static Fields, AIMS Maxwell, Surfaces/Bridge Workshop, Problem-Solving course(s) Found in: Geometry of Vector Fields Sequence, Gradient Sequence sequence(s)

Problem

5 min.

##### Active transport

The concentration of potassium $\text{K}^+$ ions in the internal sap of a plant cell (for example, a fresh water alga) may exceed by a factor of $10^4$ the concentration of $\text{K}^+$ ions in the pond water in which the cell is growing. The chemical potential of the $\text{K}^+$ ions is higher in the sap because their concentration $n$ is higher there. Estimate the difference in chemical potential at $300\text{K}$ and show that it is equivalent to a voltage of $0.24\text{V}$ across the cell wall. Take $\mu$ as for an ideal gas. Because the values of the chemical potential are different, the ions in the cell and in the pond are not in diffusive equilibrium. The plant cell membrane is highly impermeable to the passive leakage of ions through it. Important questions in cell physics include these: How is the high concentration of ions built up within the cell? How is metabolic energy applied to energize the active ion transport?

You might wonder why it is even remotely plausible to consider the ions in solution as an ideal gas. The key idea here is that the ideal gas entropy incorporates the entropy due to position dependence, and thus due to concentration. Since concentration is what differs between the cell and the pond, the ideal gas entropy describes this pretty effectively. In contrast to the concentration dependence, the temperature-dependence of the ideal gas chemical potential will not be so great.

• Found in: Thermal and Statistical Physics course(s)

Small Group Activity

30 min.

Students learn about the geometric meaning of the amplitude and period parameters in the sine function. They also practice sketching the sum of two functions by hand.
• Found in: Oscillations and Waves course(s)

Small White Board Question

5 min.

Many students do not know what it means to add two functions graphically. Students are shown graphs of two simple functions and asked to sketch the sum.
• Found in: Static Fields, None course(s)

Problem

A diesel engine requires no spark plug. Rather, the air in the cylinder is compressed so highly that the fuel ignites spontaneously when sprayed into the cylinder.

In this problem, you may treat air as an ideal gas, which satisfies the equation $pV = Nk_BT$. You may also use the property of an ideal gas that the internal energy depends only on the temperature $T$, i.e. the internal energy does not change for an isothermal process. For air at the relevant range of temperatures the heat capacity at fixed volume is given by $C_V=\frac52Nk_B$, which means the internal energy is given by $U=\frac52Nk_BT$.

Note: in this problem you are expected to use only the equations given and fundamental physics laws. Looking up the formula in a textbook is not considered a solution at this level.

1. If the air is initially at room temperature (taken as $20^{o}C$) and is then compressed adiabatically to $\frac1{15}$ of the original volume, what final temperature is attained (before fuel injection)?

2. By what factor does the pressure increase?

• Found in: Energy and Entropy course(s)

Whole Class Activity

10 min.

##### Air Hockey
Students observe the motion of a puck tethered to the center of the airtable. Then they plot the potential energy for the puck on their small whiteboards. A class discussion follows based on what students have written on their whiteboards.
• Found in: Central Forces course(s)

Problem

5 min.

##### Angular Momentum and Kinetic Energy in the Center of Mass

(Messy algebra) Convince yourself that the expressions for kinetic energy in original and center of mass coordinates are equivalent. The same for angular momentum.

Consider a system of two particles of mass $m_1$ and $m_2$.

1. Show that the total kinetic energy of the system is the same as that of two “fictitious” particles: one of mass $M=m_1+m_2$ moving with the velocity of the center of mass and one of mass $\mu$ (the reduced mass) moving with the velocity of the relative position.
2. Show that the total angular momentum of the system can similarly be decomposed into the angular momenta of these two fictitious particles.

• Found in: Central Forces course(s)

Lecture

10 min.

##### Angular Momentum Commutation Relations: Lecture

We will show that the components of the angular momentum operator $\vec{L}$, written in differential operator form in rectangular components, satisfy the commutation relations: $$\left[L_x,L_y\right]=+i\hbar L_z \qquad(\text{and cyclic permutations})$$

First calculate the components of angular momentum classically: \begin{align} \vec{L}&=\vec{r}\times\vec{p}\\ &=(x\hat{x}+y\hat{y}+z\hat{z})\times(p_x\hat{x}+p_y\hat{y}+p_z\hat{z})\\ &=(yp_z-zp_y)\hat{x}+(zp_x-xp_z)\hat{y}+(xp_y-yp_x)\hat{z} \end{align}

Making the standard quantum substitutions, \begin{align} p_x&\rightarrow -i\hbar\partial_x\\ p_y&\rightarrow -i\hbar\partial_y\\ p_z&\rightarrow -i\hbar\partial_z\\ \end{align} we obtain the following operators for the components of angular momentum: \begin{align} \hat{L}_x&=-i\hbar(y\partial_z-z\partial_y)\\ \hat{L}_y&=-i\hbar(z\partial_x-x\partial_z)\\ \hat{L}_z&=-i\hbar(x\partial_y-y\partial_x)\\ \end{align}

To see the role of the product rule in the commutation relations, it is helpful to give the partial derivatives an arbitrary function $\psi$ to act on. \begin{align} \left[\hat{L}_x,\hat{L}_y\right]\psi &=\left[-i\hbar(y\partial_z-z\partial_y), -i\hbar(z\partial_x-x\partial_z)\right]\psi\\ &=-\hbar^2\left\{(y\partial_z-z\partial_y)(z\partial_x-x\partial_z) -(z\partial_x-x\partial_z)(y\partial_z-z\partial_y)\right\}\psi \end{align} Now, foil-like-mad. Make sure that all of the partial derivatives act on EVERYTHING to their right. Two of the terms above of the form \begin{align} y\,\partial_z(z\,\partial_x \psi) \end{align} require a product rule: \begin{align} y\,\partial_z(z\,\partial_x \psi) &=y((\partial_z z)(\partial_x\psi)+z(\partial_x\partial_x\psi))\\ &=y\partial_x\psi+yz(\partial_x\partial_x\psi) \end{align}

Continuing the calculation above, we see that all of the second derivative terms will cancel because the order of differentiation doesn't matter, leaving only the first derivative terms from the product rule. \begin{align} \left[\hat{L}_x,\hat{L}_y\right]\psi &=\left[-i\hbar(y\partial_z-z\partial_y), -i\hbar(z\partial_x-x\partial_z)\right]\psi\\ &=-\hbar^2\left\{(y\partial_z-z\partial_y)(z\partial_x-x\partial_z) -(z\partial_x-x\partial_z)(y\partial_z-z\partial_y)\right\}\psi\\ &=-\hbar^2\left\{\left(y\,\partial_z(z\,\partial_x \psi) -y\,\partial_z(x\,\partial_z \psi) -z\,\partial_y(z\,\partial_x \psi) +z\,\partial_y(x\,\partial_z \psi)\right)\right.\\ &\;\;\;\quad\quad\left.-\left(z\,\partial_x(y\,\partial_z \psi) -z\,\partial_x(z\,\partial_y \psi) -x\,\partial_z(y\,\partial_z \psi) +x\,\partial_z(z\,\partial_y \psi)\right) \right\}\\ &=-\hbar^2\left\{\left(\cancel{yz(\partial_z\partial_x\psi)} +y\partial_x\psi -\cancel{yx(\partial_z^2\psi)} -\cancel{z^2(\partial_y\partial_x\psi)} +\cancel{zx(\partial_y\partial_z\psi)}\right)\right.\\ &\;\;\;\quad\quad\left.-\left(\cancel{zy(\partial_x\partial_z\psi)} -\cancel{z^2(\partial_x\partial_y\psi)} -\cancel{xy(\partial_z^2\psi)} +\cancel{xz(\partial_z\partial_y\psi)} +x\partial_y\psi\right) \right\}\\ &=i\hbar\left(-i\hbar(-y\partial_x+x\partial_y)\psi\right)\\ &=i\hbar\hat{L}_z\, \psi \end{align} The other components are cyclic permutations of this calculation.

• Found in: Central Forces course(s)

Small Group Activity

10 min.

##### Angular Momentum in Polar Coordinates
Students learn how to express Angular Momentum as a vector quantity in polar coordinates, and then in Cylindrical and Spherical Coordinates
• Found in: Central Forces course(s)

Small White Board Question

5 min.

##### Angular Momentum SWBQ

Write something you know about angular momentum.

• Found in: Central Forces course(s)

Small Group Activity

30 min.

##### Applying the equipartition theorem
Students count the quadratic degrees of freedom of a few toy molecules to predict their internal energy at temperature $T$.
• Found in: Contemporary Challenges course(s)

Problem

##### Approximating a Delta Function with Isoceles Triangles

Remember that the delta function is defined so that $\delta(x-a)= \begin{cases} 0, &x\ne a\\ \infty, & x=a \end{cases}$

Also: $\int_{-\infty}^{\infty} \delta(x-a)\, dx =1$.

1. Find a set of functions that approximate the delta function $\delta(x-a)$ with a sequence of isosceles triangles $\delta_{\epsilon}(x-a)$, centered at $a$, that get narrower and taller as the parameter $\epsilon$ approaches zero.
2. Using the test function $f(x)=3x^2$, find the value of $\int_{-\infty}^{\infty} f(x)\delta_{\epsilon}(x-a)\, dx$ Then, show that the integral approaches $f(a)$ in the limit that $\epsilon \rightarrow 0$.

• Found in: Static Fields, AIMS Maxwell, Problem-Solving course(s)

Problem

##### Basic Calculus: Practice Exercises
Determine the following derivatives and evaluate the following integrals, all by hand. You should also learn how to check these answers on Wolfram Alpha.
1. $\frac{d}{du}\left(u^2\sin u\right)$
2. $\frac{d}{dz}\left(\ln(z^2+1)\right)$
3. $\displaystyle\int v\cos(v^2)\,dv$
4. $\displaystyle\int v\cos v\,dv$
• Found in: AIMS Maxwell, Static Fields, Problem-Solving course(s)

Lecture

10 min.

##### Basics of heat engines
This brief lecture covers the basics of heat engines.
• Found in: Contemporary Challenges course(s)

Small Group Activity

30 min.

##### Black space capsule
In this activity, students apply the Stefan-Boltzmann equation and the principle of energy balance in steady state to find the steady state temperature of a black object in near-Earth orbit.
• Found in: Contemporary Challenges course(s)

Computer Simulation

30 min.

##### Blackbody PhET
Students use a PhET to explore properties of the Planck distribution.
• Found in: Contemporary Challenges course(s)

Problem

##### Boltzmann probabilities
Consider a three-state system with energies $(-\epsilon,0,\epsilon)$.
1. At infinite temperature, what are the probabilities of the three states being occupied? What is the internal energy $U$? What is the entropy $S$?
2. At very low temperature, what are the three probabilities?
3. What are the three probabilities at zero temperature? What is the internal energy $U$? What is the entropy $S$?
4. What happens to the probabilities if you allow the temperature to be negative?
• Found in: Energy and Entropy, Thermal and Statistical Physics course(s)

Lecture

120 min.

##### Boltzmann probabilities and Helmholtz
These notes, from the third week of https://paradigms.oregonstate.edu/courses/ph441 cover the canonical ensemble and Helmholtz free energy. They include a number of small group activities.
• Found in: Thermal and Statistical Physics course(s)

Problem

5 min.

##### Bottle in a Bottle

The internal energy of helium gas at temperature $T$ is to a very good approximation given by \begin{align} U &= \frac32 Nk_BT \end{align}

Consider a very irreversible process in which a small bottle of helium is placed inside a large bottle, which otherwise contains vacuum. The inner bottle contains a slow leak, so that the helium leaks into the outer bottle. The inner bottle contains one tenth the volume of the outer bottle, which is insulated. What is the change in temperature when this process is complete? How much of the helium will remain in the small bottle?

• Found in: Energy and Entropy course(s)

Problem

##### Bottle in a Bottle 2

Consider the bottle in a bottle problem in a previous problem set, summarized here.

A small bottle of helium is placed inside a large bottle, which otherwise contains vacuum. The inner bottle contains a slow leak, so that the helium leaks into the outer bottle. The inner bottle contains one tenth the volume of the outer bottle, which is insulated.

The volume of the small bottle is 0.001 m23 and the volume of the big bottle is 0.01 m3. The initial state of the gas in the small bottle was $p=106$ Pa and its temperature $T=300$ K. Approximate the helium gas as an ideal gas of equations of state $pV=Nk_BT$ and $U=\frac32 Nk_BT$.

1. How many molecules of gas does the large bottle contain? What is the final temperature of the gas?

2. Compute the integral $\int \frac{{\mathit{\unicode{273}}} Q}{T}$ and the change of entropy $\Delta S$ between the initial state (gas in the small bottle) and the final state (gas leaked in the big bottle).

• Found in: Energy and Entropy course(s)

Small Group Activity

120 min.

##### Box Sliding Down Frictionless Wedge
Students solve for the equations of motion of a box sliding down (frictionlessly) a wedge, which itself slides on a horizontal surface, in order to answer the question "how much time does it take for the box to slide a distance $d$ down the wedge?". This activities highlights finding kinetic energies when the coordinate system is not orthonormal and checking special cases, functional behavior, and dimensions.
• Found in: Theoretical Mechanics course(s)

Problem

5 min.

##### Building the PDM: Instructions
In your kits for the Portable Partial Derivative Machine should be the following:
• A 1ft by 1ft board with 5 holes and measuring tapes (the measuring tapes will be on the top side)
• 2 S-hooks
• A spring with 3 strings attached
• 2 small cloth bags
• 4 large ball bearings
• 8 small ball bearings
• 2 vertical clamp pulleys
• A ziploc bag containing
• 5 screws
• 5 hex nuts
• 5 washers
• 5 wing nuts
• 2 horizontal pulleys
To assemble the Portable PDM, start by placing the PDM on a table surface with the measuring tapes perpendicular to the table's edge and the board edge with 3 holes closest to you.
1. one screw should be put through each hole so that the threads stick out through the top side of the board. Next use a hex nut to secure each screw in place. It is not critical that they be screwed on any more than you can comfortably manage by hand.
2. After securing all 5 screws in place with a hex nut, put a washer on each screw.
3. Slide a horizontal pulley onto screws 1 and 2 (as labeled above).
4. On all 5 screws, add a wing nut to secure the other pieces. Again, it does not need to be tightened all the way as long as it is secure enough that nothing will fall off.
5. Using the middle wingnut/washer/screw (Screw 4), clamp the shortest of the strings tied to the spring.
6. Loop the remaining 2 looped-ends of string around the horizontal pulleys and along the measuring tape.
7. Using the string as a guide, clamp the vertical pulleys into place on the edge of the board.
8. Through the looped-end of each string, place 1 S-hook.
9. Put the other end of each s-hook through the hole in the small cloth bag.
Here is a poor photo of the final result, which doesn't show the two vertical pulleys. If you would like, you could view a video of the building process.
• Found in: Energy and Entropy course(s) Found in: The Partial Derivatives Machine sequence(s)

Small Group Activity

5 min.

##### Calculating a Total Differential
Students are placed into small groups and asked to calculate the total differential of a function of two variables, each of which is in turn expressed in terms of two other variables.

Small Group Activity

30 min.

##### Calculating Coefficients for a Power Series

This activity starts with a brief lecture introduction to power series and a short derivation of the formula for calculating the power series coefficients.

$c_n={1\over n!}\, f^{(n)}(z_0)$

Students use this formula to compute the power series coefficients for a $\sin\theta$ (around both the origin and (if time allows) $\frac{\pi}{6}$). The meaning of these coefficients and the convergence behavior for each approximation is discussed in the whole-class wrap-up and in the follow-up activity: Visualization of Power Series Approximations.

• Found in: Theoretical Mechanics, AIMS Maxwell, Static Fields, Problem-Solving, None course(s) Found in: Power Series Sequence (Mechanics), Power Series Sequence (E&M) sequence(s)

Problem

##### Calculation of $\frac{dT}{dp}$ for water
Calculate based on the Clausius-Clapeyron equation the value of $\frac{dT}{dp}$ near $p=1\text{atm}$ for the liquid-vapor equilibrium of water. The heat of vaporization at $100^\circ\text{C}$ is $2260\text{ J g}^{-1}$. Express the result in kelvin/atm.
• Found in: Thermal and Statistical Physics course(s)

Problem

##### Carbon monoxide poisoning

In carbon monoxide poisoning the CO replaces the $\textsf{O}_{2}$ adsorbed on hemoglobin ($\text{Hb}$) molecules in the blood. To show the effect, consider a model for which each adsorption site on a heme may be vacant or may be occupied either with energy $\varepsilon_A$ by one molecule $\textsf{O}_{2}$ or with energy $\varepsilon_B$ by one molecule CO. Let $N$ fixed heme sites be in equilibrium with $\textsf{O}_{2}$ and CO in the gas phases at concentrations such that the activities are $\lambda(\text{O}_2) = 1\times 10^{-5}$ and $\lambda(\text{CO}) = 1\times 10^{-7}$, all at body temperature $37^\circ\text{C}$. Neglect any spin multiplicity factors.

1. First consider the system in the absence of CO. Evaluate $\varepsilon_A$ such that 90 percent of the $\text{Hb}$ sites are occupied by $\textsf{O}_{2}$. Express the answer in eV per $\textsf{O}_{2}$.

2. Now admit the CO under the specified conditions. Fine $\varepsilon_B$ such that only 10% of the Hb sites are occupied by $\textsf{O}_{2}$.

• Found in: Thermal and Statistical Physics course(s)

Problem

5 min.

##### Center of Mass for Two Uncoupled Particles

The center-of-mass motion is determined by the net external force, even when the particles are not interacting. Practice with center-of-mass coordinates.

Consider two particles of equal mass $m$. The forces on the particles are $\vec F_1=0$ and $\vec F_2=F_0\hat{x}$ (for this problem, ignore gravitational forces between the two particles). If the particles are initially at rest at the origin, find the position, velocity, and acceleration of the center of mass as functions of time. Solve this problem in two ways,

1. solve for the motion of each of the particles, separately, then see what happens to the center of mass
2. solve directly for the center of mass motion
3. Write a short description comparing the two solutions.

• Found in: Central Forces course(s)

Problem

##### Central Force Definition

(Quick) Purpose: Recognize the definition of a central force. Build experience about which common physical situations represent central forces and which don't.

Which of the following forces can be central forces? which cannot? If the force CAN be a central force, explain the circumstances that would allow it to be a central force.

1. The force on a test mass $m$ in a gravitational field $\vec{g~}$, i.e. $m\vec g$
2. The force on a test charge $q$ in an electric field $\vec E$, i.e. $q\vec E$
3. The force on a test charge $q$ moving at velocity $\vec{v~}$ in a magnetic field $\vec B$, i.e. $q\vec v \times \vec B$

• Found in: Central Forces course(s)

Problem

##### Central Force with no External Forces

(Quick) Purpose: Quickly recognize a consequence of central forces.

If a central force is the only force acting on a system of two masses (i.e. no external forces), what will the motion of the center of mass be?

• Found in: Central Forces course(s)

Lecture

5 min.

##### Central Forces Introduction Lecture Notes

In this course, we will examine a mathematically tractable and physically useful problem - that of two bodies interacting with each other through a central force, i.e. a force that has two characteristics:

Definition of a Central Force:

1. A central force depends only on the separation distance between the two bodies,
2. A central force points along the line connecting the two bodies.

The most common examples of this type of force are those that have $\frac{1}{r^2}$ behavior, specifically the Newtonian gravitational force between two point (or spherically symmetric) masses and the Coulomb force between two point (or spherically symmetric) electric charges. Clearly both of these examples are idealizations - neither ideal point masses or charges nor perfectly spherically symmetric mass or charge distributions exist in nature, except perhaps for elementary particles such as electrons. However, deviations from ideal behavior are often small and can be neglected to within a reasonable approximation. (Power series to the rescue!) Also, notice the difference in length scale: the archetypal gravitational example is planetary motion - at astronomical length scales; the archetypal Coulomb example is the hydrogen atom - at atomic length scales.

The two solutions to the central force problem - classical behavior exemplified by the gravitational interaction and quantum behavior exemplified by the Coulomb interaction - are quite different from each other. By studying these two cases together in the same course, we will be able to explore the strong similarities and the important differences between classical and quantum physics.

Two of the unifying themes of this topic are the conservation laws:

• Conservation of Energy
• Conservation of Angular Momentum
The classical and quantum systems we will explore both have versions of these conservation laws, but they come up in the mathematical formalisms in different ways. You should have covered energy and angular momentum in your introductory physics course, at least in simple, classical mechanics cases. Now is a great time to review the definitions of energy and angular momentum, how they enter into dynamical equations (Newton's laws and kinetic energy, for example), and the conservation laws.

In the classical mechanics case, we will obtain the equations of motion in three equivalent ways,

• using Newton's second law,
• using Lagrangian mechanics,
• using energy conservation.
so that you will be able to compare and contrast the methods. The third approach is slightly more sophisticated in that it exploits more of the symmetries from the beginning.

We will also consider forces that depend on the distance between the two bodies in ways other than $\frac{1}{r^2}$ and explore the kinds of motion they produce.

• Found in: Central Forces course(s)

Problem

##### Centrifuge
A circular cylinder of radius $R$ rotates about the long axis with angular velocity $\omega$. The cylinder contains an ideal gas of atoms of mass $M$ at temperature $T$. Find an expression for the dependence of the concentration $n(r)$ on the radial distance $r$ from the axis, in terms of $n(0)$ on the axis. Take $\mu$ as for an ideal gas.
• Found in: Thermal and Statistical Physics course(s)

Small Group Activity

30 min.

##### Chain Rule
This small group activity is designed to provide practice with the chain rule and to develop familiarity with polar coordinates. Students work in small groups to relate partial derivatives in rectangular and polar coordinates. The whole class wrap-up discussion emphasizes the importance of specifying what quantities are being held constant.
• Found in: Vector Calculus I course(s)

Small Group Activity

30 min.

##### Chain Rule Measurement
This small group activity using surfaces combines practice with the multivariable chain rule while emphasizing numerical representations of derivatives. Students work in small groups to measure partial derivatives in both rectangular and polar coordinates, then verify their results using the chain rule. The whole class wrap-up discussion emphasizes the relationship between a directional derivative in the $r$-direction and derivatives in $x$- and $y$-directions using the chain rule.
• Found in: Vector Calculus I course(s)

Small Group Activity

30 min.

##### Change of Variables

Consider the region $D$ in the $xy$-plane shown below, which is bounded by $u=9 \qquad u=36 \qquad v=1 \qquad v=4$ where $u=xy \qquad v={y\over x}$ If you want to determine $x$ and $y$ as functions of $u$ and $v$, consider $uv$ and $u/v$.

• List as many methods as you can think of for finding the area of the given region.

It is enough to refer to the methods by name or describe them briefly.

• For at least 3 of these methods, give explicitly the formulas you would use to find the area.

You must put limits on your integrals, but you do not need to evaluate them.

• Using any 2 of these methods, find the area.

One of these should be a method we have learned recently.

• Now consider the following integral over the same region $D$:

$\int\!\!\int_D {y\over x} \>\>dA$

• Which of the above methods can you use to do this integral?
• Do the integral.

#### Main ideas

• There are many ways to solve this problem!
• Using Jacobians (and inverse Jacobians)

#### Prerequisites

• Surface integrals
• Jacobians
• Green's/Stokes' Theorem

#### Warmup

Perhaps a discussion of single and double integral techniques for solving this problem.

#### Props

• whiteboards and pens

#### Wrapup

This is a good conclusion to the course, as it reviews many integration techniques. We emphasize that (2-dimensional) change-of-variable problems are a special case of surface integrals.

Here are some of the methods one could use to do these integrals:

• change of variables (at least 2 ways)
• Area Corollary to Green's Theorem (at least 2 ways)
• ordinary single integral (at least 2 ways)
• ordinary double integral (at least 2 ways)
• surface integral

### Details

#### In the Classroom

• Some students will want to simply use Jacobian formulas; encourage such students to try to solve this problem both by computing $\frac{\partial(x,y)}{\partial(u,v)}$ and by computing $\frac{\partial(u,v)}{\partial(x,y)}$.
• Other students will want to work directly with $d\boldsymbol{\vec{r}}_1$ and $d\boldsymbol{\vec{r}}_2$. This works fine if one first solves for $x$ and $y$ in terms of $u$ and $v$.
• Students who compute $d\boldsymbol{\vec{r}}_1$ and $d\boldsymbol{\vec{r}}_2$ directly can easily get confused, since they may try to eliminate $x$ or $y$, rather than $u$ or $v$.

Along the curve $v=\hbox{constant}$, one has $dy=v\,dx$, so that $d\boldsymbol{\vec{r}}_1 = dx\,\boldsymbol{\hat{x}} + dy\,\boldsymbol{\hat{y}} = (\boldsymbol{\hat{x}} + v\,\boldsymbol{\hat{y}})\,dx$, which some students will want to write in terms of $x$ alone. But one needs to express this in terms of $du$! This can be done using $du = x\,dy + y\,dx = x (v\,dx) + y\,dx = 2y\,dx$, so that $d\boldsymbol{\vec{r}}_1 = (\boldsymbol{\hat{x}} + v\,\boldsymbol{\hat{y}}) \,\frac{du}{2y}$. A similar argument leads to $d\boldsymbol{\vec{r}}_2 = (-\frac{1}{v}\,\boldsymbol{\hat{x}}+\boldsymbol{\hat{y}})\,\frac{x\,dv}{2}$ for $u=\hbox{constant}$, so that $d\boldsymbol{\vec{S}} = d\boldsymbol{\vec{r}}_1\times d\boldsymbol{\vec{r}}_2 = \boldsymbol{\hat{z}} \,\frac{x}{2y}\,du\,dv = \boldsymbol{\hat{z}} \,{du\,dv\over2v}$. This calculation can be done without solving for $x$ and $y$, provided one recognizes $v$ in the penultimate expression.

Emphasize that one must choose parameters, both on the region, and on each curve, and that $u$ and $v$ are chosen to make the limits easy.

• Take time before the activity to gauge students' recollection of single variable techniques and the Jacobian. After the activity, be sure to set up more than one approach. People will be fine after the first couple of steps but shouldn't leave class feeling stuck.

#### Subsidiary ideas

• Review of Green's Theorem
• Review of single integral techniques
• Review of double integral techniques

#### Enrichment

• Discuss the 3-dimensional case, perhaps relating it to volume integrals.
• Found in: Vector Calculus II course(s)

Small Group Activity

30 min.

##### Changes in Internal Energy
Students consider the change in internal energy during three different processes involving a container of water vapor on a stove. Using the 1st Law of Thermodynamics, students reason about how the internal energy would change and then compare this prediction with data from NIST presented as a contour plot.
• Found in: Warm-Up sequence(s)

Small Group Activity

10 min.

##### Changing Spin Bases with a Completeness Relation
Students work in small groups to use completeness relations to change the basis of quantum states.
• Found in: Quantum Fundamentals course(s) Found in: Completeness Relations sequence(s)

Problem

5 min.

##### Charge on a Spiral
(8pts) A charged spiral in the $x,y$-plane has 6 turns from the origin out to a maximum radius $R$ , with $\phi$ increasing proportionally to the distance from the center of the spiral. Charge is distributed on the spiral so that the charge density increases linearly as the radial distance from the center increases. At the center of the spiral the linear charge density is $0~\frac{\textrm{C}}{\textrm{m}}$. At the end of the spiral, the linear charge density is $13~\frac{\textrm{C}}{\textrm{m}}$. What is the total charge on the spiral?
• Found in: Static Fields, Problem-Solving course(s)

Small Group Activity

30 min.

##### Charged Sphere
Students use a plastic surface representing the potential due to a charged sphere to explore the electrostatic potential, equipotential lines, and the relationship between potential and electric field.

Lecture

120 min.

##### Chemical potential and Gibbs distribution
These notes from the fifth week of https://paradigms.oregonstate.edu/courses/ph441 cover the grand canonical ensemble. They include several small group activities.
• Found in: Thermal and Statistical Physics course(s)

Problem

5 min.

##### Circle Trigonometry

On the following diagrams, mark both $\theta$ and $\sin\theta$ for $\theta_1=\frac{5\pi}{6}$ and $\theta_2=\frac{7\pi}{6}$. Write one to three sentences about how these two representations are related to each other. (For example, see: this PHET)

• Found in: Quantum Fundamentals course(s)

Problem

5 min.

##### Circle Trigonometry and Complex Numbers

Find the rectangular coordinates of the point where the angle $\frac{5\pi}{3}$ meets the unit circle. If this were a point in the complex plane, what would be the rectangular and exponential forms of the complex number? (See figure.)

• Found in: Quantum Fundamentals course(s)

Problem

5 min.

##### Circle Trigonometry Practice
This activity gives links to some external resources (2 simulations and 1 video) that allow students to explore circle trigonometry. There are no prompts and nothing specific to turn in.
• Found in: Quantum Fundamentals course(s)