The Mystery of Rising Temperatures When Heating is Turned Off

[Wasif Shah]

When a heating experiment for example when measuring the specific heat capacity of an aluminum block, why is it practical to turn the heater off and wait for the highest temperature reading on the thermometer and take that reading.
Why does the reading even rise when the heater is turned off? Shouldnt it directly start falling as soon as the heater is turned off?

 

[Electric to be]

I am assuming this is likely so that the block reaches thermal equilibrium with itself (not it's surroundings). In other words, so any temperature gradients throughout the material diffuse and the temperature becomes roughly constant throughout as that is what you're really hoping to measure.

This equation models how temperature spreads:
https://en.wikipedia.org/wiki/Heat_equation

 

[ZapperZ]

When a heating experiment for example when measuring the specific heat capacity of an aluminum block, why is it practical to turn the heater off and wait for the highest temperature reading on the thermometer and take that reading.
Why does the reading even rise when the heater is turned off? Shouldnt it directly start falling as soon as the heater is turned off?

How are you heating the block?

One typically heat the block from the OUTSIDE. So there is a temperature gradient between the outside of the block and the center. So the whole block is not at the same temperature. The temperature on the surface is higher than the center.

After the heat is turned off, while the surface temperature may start to drop, the inside will still continue to rise because it is still at a lower temperature than the surface. That is why if you take the temperature of the middle of the block, it will continue to rise.

This, btw, is directly applicable to culinary science. It is why if one is roasting something in the oven, and one wants to aim at a particular internal temperature, one often stops cooking when the temperature is about 5 degrees F below that target temperature. This is because even after the roast (turkey, chicken, etc.) is taken out of the oven, the temperature of the center will continue to rise.

Zz.

 

[Wasif Shah]

Yeah i am taking the temperature from the middle. Two holes were drilled into the aluminum block, in one hole thermometer was placed, in the other my heater was placed. The whole aluminum block was lagged to reduce heat losses. (I didnt actually experiment that but my physics notes say so). And the temperature after the heater was turned off increases for a while and then starts to decrease. So I suppose it is because heaters heating rod may be at a high temperature than the block and hence it radiates heat to the block even after the heater is turned off.

 

[russ_watters]

I am assuming this is likely so that the block reaches thermal equilibrium with itself (not it's surroundings). In other words, so any temperature gradients throughout the material diffuse and the temperature becomes roughly constant throughout as that is what you're really hoping to measure.

Agreed. And I would go further to say that I don't like picking the peak in the test procedure because the peak is not necessarily when uniformity is reached -- that may (probably does) happen later. That can be identified by when the temperature curve establishes its expected shape.

 

[sophiecentaur]

When a heating experiment for example when measuring the specific heat capacity of an aluminum block, why is it practical to turn the heater off and wait for the highest temperature reading on the thermometer and take that reading.
Why does the reading even rise when the heater is turned off? Shouldnt it directly start falling as soon as the heater is turned off?

You have chosen a very suitable metal for measuring specific heat capacity. It is a very good conductor, which means that temperatures will tend to be 'fairly even' throughout it.
Heat experiments have gone a bit out of fashion since I did my A levels You guys get told about fundamental particles and Cosmology so we had more time for the basics. (I assume you are not as old as me!) You will not have had endless weeks of Callender and Barnes apparatus, Searle's Bar and Lee's Disc. Those, and other apparatus, were designed to measure conductivity and SHC in such a way that the errors are minimised.

Picking the peak in temperature (in the right place) is clearly one way to avoid the basic problem of conductivity. Using two thermometers - one near the heater and one near the outside, would at least tell you something about how good your answer will be. With an electric heater mounted inside, the inside thermometer would go to a higher temperature than its final - just as the outside one will take time to 'get there. More measurement locations would be even better. Also, of course, you need very good insulation and a to measure the energy input accurately (measuring the right V and I, for instance).