I did a simple experiment in class with my students that has proven to be a bit of a head scratcher.
We started with two weighing boats (used to weigh chemicals on a balance). One was the natural shiny white of a standard weighing boat but the other I had spray painted a flat black. In each we placed 25 mL of water and a thermometer. Next we set an incandescent lamp above each container and timed the temperature each minute. I expected the temperatures of both thermometers to increase with the black increasing more than the white. Simple right?
Instead, I got a mish mash of results. Some students actually got a greater increase with the white container.
I wonder if it is possible that the white plastic is actually a good absorber of the lamp’s infrared light causing its temperature to increase.
I welcome all hypotheses.
Way to many uncontrolled variables.
Were all the lamps the same?
Same reflector?
Same distance?
Same angle of incidence?
Also, water itself absorbs IR pretty well, so if the light was aimed at the water, that effect my swamp any other.
Thanks. Our only variable was the colour of the weighing boat. I did not know that water was an IR absorber.
The black boat is absorbing more energy than the white boat in the visible light frequencies … but you’re taking the temperature of the water … this will require that the extra energy be conducted into the water … which tends to be a very slow process and even slower since water is a fairly good convector of energy …
Are the air currents about the classroom all exactly the same … that would make a big difference regarding the temperature of the water …
I suggest painting the thermometer bulbs black and white and use sunlight …
Thanks. I have a minimal amount of water in the boats. Only 25 mL. Just enough to cover the bulb. Good points though.
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I’ve changed my mind, I will say this …
It takes 100 Joules of energy to raise the temperature of 25 mL of water 1ºC … however, that same amount of energy is lost with just 50 µL of evaporation … so unless your classroom was uncomfortably muggy, your results will be all over the place … as you found …
Incandescent lamps radiate most of their energy in the infrared. You don’t really know how good an absorber each of your surfaces is in the infrared. Try the experiment with LED lamps. As beuwulff points out there are many possible issues, but this particular one might be a good first place to look.
Why use water? Measure the temperature of the boats or other materials using an IR thermometer.
At first, I thought this was a brilliant solution to the problem. But, the more I thought…
The IR thermometers work by measuring the intensity of IR radiation emitted by an object. Usually, on a narrow window of IR wavelengths, the actual wavelengths are part of the design of the thermometer and affect the accuracy and precision of the instrument. So, the thermometer looks at a narrow band of IR radiation and, depending on the intensity, says the temperature is XX degrees.
Now, the intensity of IR radiation given off by an object is dependent on its temperature. It is also dependent on a property known as emissivity. Emissivity is the compliment of reflectivity; illuminate an object with light, IR or visible, and a portion will be absorbed and the rest will be reflected. The portion being absorbed is equal to the object’s emissivity. So, for two objects at the same temperature but with different values for emissivity, the object with the higher emissivity will register as being hotter with the IR thermometer.
Now, the purpose of painting one object black and leave the other white is to change the amount of light it absorbs, or, in other words, to change its emissivity. So, trying to measure the temperature with an IR thermometer to illustrate the differences in absorption rates is now complicated by the fact that if you did, in fact, change the absorption by painting, you also changed the emissivity and you now need to figure out if the different temperature readings are due to differences in emissivity, differences in temperature, or both.
Paint just one side of your test objects (the side towards the lamp), and then measure the temperature by looking at the other side (which is the same for both). You might still have some systematic error from the emissivity of that other side, but it’ll be the same for both, so you should at least be able to say which one is hotter.
Or, if you wanted the students to experience a little more complexity and the importance of control groups, use the IR gun to measure the temperatures of the black and white objects when they’re both sitting at room temperature with the lamps off. There’s not going to be a big difference in emissivity at room temperature and at room temp plus 10 to 20 degrees, so you should be able to correct the readings at higher temps.
The emissivity might not change, but the way you do the corrections might.
Also use one bulb and one thermometer. Put the objects next to each other so they’re receiving the same level heat. And to be sure switch the locations and measure again to check for error.
IR thermometers allow you to calibrate for emissivity. You could get each boat to a known temperature and then calibrate the thermometer to it. Once you have the calibrations, you can measure the temperature of each boat accurately regardless of color.
I did repeat this experiment with a fluorescent bulb and I did not see a temperature change in either container. Maybe a few degrees but not significant and the same for both.
Here’s a cute diagram that explains why fluorescent is the worst choice … it has very narrow bands where it emits visible light and almost none at all in the IR …
Try a halogen light next time … or like I mentioned up-thread, use sunlight …
Emitting almost none at all in the IR is exactly what the OP wants, because he’s afraid that the IR emissivity of the paint was different from its visible emissivity.
Which is logical. The problem is that since the fluorescent doesn’t emit any IR, it emits almost no power compared to the incandescent (at about the same brightness).
To get the same heating power as a single 60W incandescent bulb, he’d need huge banks of fluorescent bulbs (and even more LED bulbs).
Plus, I think visible light is reflected better than IR, so if all the heating is through visible spectrum, you need even more power.
Wrong time of year to do it, but a special feature of sunlight is that most of the energy at the earths surface is in the visible wavelengths (handy that). So objects that /look/ black absorb more sunlight than objects that /look/ white. And the sun is major power source putting out a lot of energy (even after a lot of the energy has been filtered out by the atmosphere). If you get clear summer days, maybe try it again just before or after summer break.