Science Buddies: "Ask an Expert"

So, Ferrofluid is used in cooling the voice coils in some speakers, and my science fair expiriment was to test that. I heated up water, Ferrofluid and Vinegar to 40 degrees celcuis and recorded their time to return to room tempature. But, I am not sure what the independent and dependent varibles are in my experiment. If anyone can help, then thanks.

In the experiment you describe, the independent variable is the coolant. It is a discreet variable with three values: water, vinegar, and ferrofluid.

I am sorry to tell you that I think you may be doing the wrong experiment. The time to cool from 40 C to room temperature depends on the amount of thermal energy stored in the fluid, which will be proportional to the heat capacity per unit volume, and the rate of heat loss to the environment, which will depend in a rather complex way on the details of heat transport by means of convection, conduction, and possibly vaporization (if the vessel is not sealed, and it pretty much has to be unsealed for the sake of safety as heating a sealed container of fluid will be likely to cause the container to explode!!). On the other hand, the ability of the fluid to cool the voice coil will, I would guess, be determined by the thermal conductivity of the fluid. My guess is based on an assumption that the main cooling mechanism is conduction over a short gap from the voice coil to the walls of the fluid containment vessel. In this situation my intuition is that convection in the fluid will transport much less heat than conduction -- but I could be wrong. The experiment that will answer your question would be to fill the fluid reservoir of a loudspeaker with each of the fluids, then generate a known amount of power in the voice coil by driving the speaker with an amplifier and a signal source, that is by playing a steady tone through the system, and measuring the voltage across the voice coil and the current through the voice coil, and, finally, measuring the heat rise in the voice coil using a small temperature sensor. I am rather worried that implementing this test would involve a lot of work modifying a speaker voice coil assembly to add the heat sensor and to allow changing the fluids. Also, the vinegar might not work because it is a conductor and could tend to short out the voice coil. An alternative to modifying a speaker would be to replicate the geometry of the heat source and the heat sink to be similar in area and separation to the actual voice coil assembly in some sort of test fixture that would provide better access for experimentation than the real thing. It might be a good idea to contact a manufacturer of ferrofluid cooled loudspeakers to see if you can find out how they went about testing their product during development.

Another idea might be to change your question to be: which fluid has the highest heat capacity. That question would be a lot easier to answer experimentally.

A final suggestion would be to add more kinds of fluid, for example vegetable oil and methyl alcohol.

I'm very sorry to be a wet blanket, I do wish you best of luck. Feel free to ask more questions any time.