Simulation of the greenhouse effect with silvered and unsilvered glass bottles.
What it shows:
Heat energy readily escapes from a clear glass flask, but is trapped inside a silvered flask which rapidly heats up.
How it works:
Two 2L flat bottom Florence flasks, one clear and one silvered (see reference), have identical 10Ω, 25W resistors placed inside them connected in series to a DC supply 1 These resistors act as good sources of infrared radiation. The clear flask readily transmits the IR, but the silvered surface of the second flask reflects the IR back in and traps it, raising the air temperature. Temperature measurement inside the flasks is made by thermocouple wire (not shown in figure 1), revealing a dramatic temperature rise in the silvered flask (5 degrees in as many minutes with this apparatus), but no change in the clear flask.
figure 1. flask and heater setup
Setting it up:
The resistor and thermocouple wires are suspended down inside the flask and held in place by the flask stoppers. These stoppers also have the role of cutting down on convection losses. Be careful not to have the thermocouple junction in contact with the resistor (trickier to judge in the silvered flask). It is more convenient to have both thermocouples connected to a common meter; with the Omega thermometer we use, 2 T1, T2and (T1-T2) can be displayed.
Although the mechanism for the planetary greenhouse effect is absorption of IR by "greenhouse gasses" in the atmosphere, primarily water vapor and CO2, this demo gives a clear indication of the effects of heat trapping in a closed system. Although developed as a model for the Greenhouse effect, this demo could also be used to show one of the main design features of the Dewar flask. The technique for silvering flasks is given in the reference below.
B. Z. Shakhashiri, Chemical Demonstrations, volume 4, p. 240-243 (University of Wisconsin Press, 1992).
1 HP E3611A DC power supply
1 Omega HH23 dual digital thermometer