In this remote-friendly activity, students use a microwave oven (and optionally a thermometer) to measure the latent heat of melting for water (and optionally the heat capacity). From these they compute changes in entropy. See also Ice Calorimetry Lab.
In this lab, we will be measuring how much energy it takes to melt ice and heat water. I have modified this lab to work with a microwave oven that you're likely to have at home.
Before the day of the lab, you will need to collect your equipment. If you do not have a microwave oven, or do not have a liquid measuring cup, you will need to use the data from another student in your group. If you do have a microwave and a way to measure the volume or mass of water, please be ready to do this lab during class.
You will need the materials below. Ideally the day before the lab you will put about 500 mL (or even more) of water into a container (not a glass jar, which could break) and put it into the freezer so you will have a large chunk of ice. This will make it easier to separate the ice from the melt water when you melt it. If you do not do this you can still use ice from an ice cube tray, but then it will really help to also have a collander or seive to help separate ice from water.
This is the home version of Ice Calorimetry Lab for when there is a pandemic, or if you want students to do the activity at home. The accuracy students can get with a microwave oven is surprising, so I may adapt this to be a "home lab" even when we are teaching in person.
While the measurements are being taken, I asked the students a couple of small-whiteboard-questions, “What is heat?” and “What is entropy?”. I then lecture on what the heat capacity \(C_p\) is, and how they could extract it from their data, and on how they can calculate entropy from their measurements: \(\Delta S = \int \frac{dQ}{T}\).
We will be using a microwave oven to measure how much energy it takes to melt ice and how much energy it takes to boil water. If you have a thermometer, you can also measure how much energy it takes to raise its temperature. Our experimental unit of energy will be “seconds in the microwave.” It's not a great unit of measure. A microwave may not always output equal power independent of what is in it, so we'll try not to change too much the amount of ice or water in the oven.
In the analysis, you can use the nominal power of the microwave (as written somewhere on the inside or outside of your oven) to approximately convert this to Joules, but keep in mind that this conversion is dubious on many fronts. There will be energy wasted in the electronics of the microwave, which is not transmitted to the ice. Also the documented power is probably rounded up, because it is used by electricians to determine whether the device is safe to put on a circuit, and for that purpose it is acceptable to use less than the documented power, but not more. We could do better by measuring the current drawn (and the line voltage), but I doubt you have that equipment in your kitchen.
face Lecture
30 min.
latent heat heat capacity internal energy entropy
This short lecture introduces the ideas required for Ice Calorimetry Lab or Microwave oven Ice Calorimetry Lab.group Small Group Activity
60 min.
assignment Homework
assignment Homework
group Small Group Activity
30 min.
face Lecture
120 min.
phase transformation Clausius-Clapeyron mean field theory thermodynamics
These lecture notes from the ninth week of Thermal and Statistical Physics cover phase transformations, the Clausius-Clapeyron relation, mean field theory and more. They include a number of small group activities.group Small Group Activity
30 min.
assignment Homework
Use integration to find the total mass of the icecream in a packed cone (both the cone and the hemisphere of icecream on top).
assignment Homework
(Synthesis Problem: Brings together several different concepts from this unit.) Use effective potential diagrams for other than \(1/r^2\) forces.
Consider the frictionless motion of a hockey puck of mass \(m\) on a perfectly circular bowl-shaped ice rink with radius \(a\). The central region of the bowl (\(r < 0.8a\)) is perfectly flat and the sides of the ice bowl smoothly rise to a height \(h\) at \(r = a\).
group Small Group Activity
30 min.