Using the Specific Heat Formula, I found that I need to apply 612J of heat to melt 18g of marshmallow. Marshmallow has a specific heat of 2 J/(g*C), melting point of 98.6°F, and a starting room temperature of 68°F. I found a heat lamp generates up to 150 watts of power via radiation, which means it can melt the marshmallow in 4 seconds (according to Time (s) = energy required (J) / power (W)).

5 seconds seems too good to be true. I am assuming perfect transfer of heat from lamp to marshmallow, but even after considering 50% efficiency, it only takes 8 seconds.

Does this make sense or did I miss something obvious? Thanks!!

  • 2
    $\begingroup$ Tip: use HTML entities for degrees: °. (Also available for Ω, μ, etc.) $\endgroup$
    – Transistor
    Aug 13 '20 at 21:24
  • $\begingroup$ That "perfect heat transfer" may be about 10% when you take losses into account. air currents etc $\endgroup$
    – Solar Mike
    Aug 14 '20 at 6:34

You are missing several things.

  • You need to know the emissivity of marshmallow. That will tell you how much of the 150W of radiation is absorbed, and how much is reflected.

  • You need to know the latent heat of fusion, which tells you how much heat is needed to change the marshmallow from "solid" to "liquid". That may not be a very well defined quantity for marshmallow compared with melting a block of ice for example, but there will probably be a significant amount of heat required.

  • You also need to know the thermal conductivity. If the conductivity is low and you apply too much heat, you will burn the surface of the marshmallow before the material below it has been heated enough to melt it.

  • $\begingroup$ And you need to know the area of marshmellow under the rays of lamp. Surely some radiakion escapes by nissing altogether (which based on cirumstance is probably greater than the reflectivity) $\endgroup$
    – joojaa
    Aug 14 '20 at 4:07
  • $\begingroup$ thanks for the feedback! i see i missed some things now. do you agree that it will take 612J to melt marshmallow? i assume that marshmallow will be melted once it teaches melting temperature. $\endgroup$
    – Wei Xun He
    Aug 15 '20 at 13:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.