This is the last course guided experiment that you will perform, so let’s turn up the heat!
Time spent on lab = 2 hours
Materials needed = thermometer, human thermometer (you!), fan or other source of blowing air, 1 glass container (glass cup), 2 foam cups, oven mitt, electric kettle or access to boiling water, kitchen scale, small plastic, waterproof bag (ziploc bag), microwave
Learning Objectives:
- Define heat and heat transfer
- Predict the changes in matter given its physical properties and environment
Temperature:
Many people consider themselves to be excellent judges of temperature. “It’s cold outside” is something I’m sure we have all heard, but in physics we like to be fairly precise. A fair question to the person making the statement above might be, “what’s the temperature?”. Most people that have not looked at the weather station recently would likely get it wrong!
The reason being that most people are only able to accurately measure a “real feel” measurement of temperature. Given all the variables in weather, that can be quite far from the actual temperature! For example, imagine the weather is a perfect 25C and you are laying on the beach getting a tan. You might start to sweat and think that there is something wrong with your internal body temperature. Likely, you are just experiencing a higher “heat index” than the actual temperature states. The National Weather service does a great job of explaining heat index.
For cold temperatures, there is a similar concept called windchill, meaning it feels colder than it actually is.
Let’s try a few quick experiments to fine tune what we mean by temperature. First a cautionary note, DO NOT get too close to any boiling water! If you are ever unsure if the water you are experimenting with is too hot, test it by sticking a pen or something similar into the water, draw it out, and see if the object you placed in the water is very hot.
Mini-experiment #1:
- Begin by getting a warm foam cup of water, it doesn’t have to be TOO HOT, and one cold cup of water. Quickly pour ½ of each cup into a third, empty cup, to make room mild temperature cup. Place one index finger in the warm water, and one in the cold water. After 30 seconds, take each finger out, and put them BOTH in the mild temperature cup. Describe below what you experienced in your fingers throughout the mini experiment.
You may have found that despite your fingers belonging to the same person, YOU, they experienced vastly different temperatures when placed in water of a single moderate temperature cup. So are humans good thermometers? Not likely, but let’s try one more!
Mini-experiment #2:
2. This time we will only use the moderate temperature cup and a fan. Put your two index fingers in the cup. After 30 seconds, remove them both, and then quickly place one finger in front of a fan or other source of blowing air, and leave one well away from any source of blowing air. Below write down what you notice about the temperature between the finger in blowing air, and the finger that is not in blowing air.
Now that you are sufficiently suspicious if anyone were to tell you what temperature it is outside on any specific day, let’s bring it home with a quantifiable experience.
MIni-experiment #3:
3. Take your thermometer and place it in the moderate temperature cup. Leave it there until the temperature stabilizes, and then put it in front of the fan. Notice it takes some time for the temperature to get back to the room temperature. Repeat the procedure only now do NOT put the thermometer in front of the fan, but leave it sitting away from any heat or cold source. Write down your findings below.
You may have noticed that air flow did not have a significant change in the temperature of the thermometer. A blowing fan may seem to cool us off, but not by directly lowering our temperature. This cooling effect is more a matter of evaporating our sweat, and the evaporation lowers our temperature. A lot like heat index and wind chill!
Now that we are thoroughly confused about what temperature really is, let’s try to define it in physics terms. One often refers to temperature as a “Thermodynamic Temperature”, which is to say an average measure of the kinetic energy of the particles that make up an object. A rather clear picture is sketched out in Georgia State University’s web portal entitled “Hyperphysics”.
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/temper.html
So if several objects, made of a set of specific particles, are touching, but NOT transferring kinetic energy between particles, then they are at the same “temperature”. The transfer of kinetic energy can be thought of as part of what makes up the definition of heat in physics.
Heat Transfer Mechanisms:
So another logical question is how does one transfer heat? Clearly there must be some interaction of the particles that make up a substance, right?? As with all physics questions that answer would be both yes and no! The physics classroom does a great job of defining and giving examples of heat transfer.
http://www.physicsclassroom.com/class/thermalP/Lesson-1/Methods-of-Heat-Transfer
In summary, conduction is like you burn your hand by grabbing a hot pan. Convection is actually quite similar to windchill, only it does lower the temperature via transferring kinetic energy via evaporation. Radiation is quite succinctly, the transfer of heat from light!
Mini-Experiment #4:
4. Begin by filling a glass cup/container with a bit of water, 200 mL or so. Next place the cup in the microwave for 2-3 minutes. After the microwave has stopped, grab an oven mit and remove the cup from the microwave. Write down what you discover below, including comments on the apparent water temperature, how much water is left, and humidity in the microwave after you take the cup out. DO NOT put your hand in the water! Simply make not of the temperature when you bring your hand close to the mug, without touching it.
Mini-Experiment #5:
5. Wait 10 minutes with the microwave door open, and repeat the experiment with a room temperature glass mug/cup, and NO water! In the space below, write down answers to the same questions as in #4, including the temperature of the cup/mug. Again, for safety’s sake, take the cup out with an oven mit and test the temperature of the mug by bringing your fingers close to the mug, without actually touching it.
If all went well, you hopefully noticed that without water, the glass mug/cup did not heat up! That means no heat was transferred to it by the microwave, it was transferred to the water. How did this occur? Most of this explanation is appropriate for Physics 108, but in general terms, the microwave radiation generated by your microwave oven, is of the perfect energy to heat up water! This process of heating up, changing the temperature, is an example of radiative heat transfer.
Calorimetry:
It is assumed here that you have read the textbook on calorimetry experiments and examples and so it is advised that you wait to perform this experiment until you have read that segment in the textbook.
If you have read ahead, or are comfortable with the concepts thus far, you are ready for a calorimetry experiment at home! The question we want to answer is “what is the temperature of our freezer?”.
6. To begin answering this question, start by imagining what you supplies you have already used in this experiment, to make a homemade calorimeter. Write down how you will build this calorimeter below. The picture below the textbox may help you visualize how to do this.
7. Now if we want to only use frozen water (ice) and room temperature water (room temperature water of four times the mass of the ice), write out the various forms of heat transfer that will occur between frozen water and room temperature water, when the come into contact. Hint: There will be one term of heat transfer from the room temperature water, and three terms of heat transfer in the ice/cold water.
8. It’s time to perform the experiment! Be sure to measure out a set mass of water, say 50g, put it into a freezer bag, or plastic dish, let it freeze solid, and then place that resulting piece of ice into the 200g of room temperature water, inside your calorimeter. Seal that top with some piece of wood, cardboard, or plastic container top, but leave a spot for your thermometer to poke out! Once the temperature on your thermometer stabilizes, and you are certain the ice has melted, write down the final temperature below and solve for the initial temperature of the ice cube. That just happens to be the temperature of your freezer! After your derivation, be sure to include a note on how accurate you believe your answer to be, and where any source of errors may have come from.
9. You may have noticed that you have a thermometer and a freezer, why not do this experiment the easy way and simply put your thermometer in the freezer and wait until the temperature stabilizes. Good point! If your thermometer goes below freezing, then try this out now. If not, google around for your model of freezer and see if you can find a specification for average freezer temperature. Write either number below and calculate how far off you are in your prediction. This calculation can take on various forms, so be sure to explain what you are calculating, percent error, percent difference, difference, etc.
Please download this document as a PDF only! Then upload this PDF and your video to Blackboard under the assignment “Lab 07” for grading/review.
