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Old Sep 4th 2019, 01:08 AM   #1
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Question Temperatures of different substances in an oven

I need to sterilise some bricks and paving stones that became infected with a toxic, spore-forming bacterium that was sold as a means of ridding a pond of duckweed (lemna), but which kills all acquatic plants. The important thing is, I need to heat the bricks and stones above 100C.

I tried the sterilisation before, but it partly failed to kill the bacterium. My guess is that this was because the objects in the oven were moist and created a lot of steam. My physics question is, do different objects inside an oven heated, say, to 250C, achieve different internal temperatures?

Put another way, if I have two identical ovens and in one place a bowl of water, is the heat of the water necessarily limited to staying below 100C?

If in the other oven I place a substance into which I can stick a thermometer, let's say sand, will that achieve 250C internal temperature?

What is the relationship between substance and achievable internal temperature? Thanks!
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Old Sep 4th 2019, 01:55 AM   #2
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Originally Posted by theydonboy View Post
I tried the sterilisation before, but it partly failed to kill the bacterium. My guess is that this was because the objects in the oven were moist and created a lot of steam.
I don't know anything at all about microbiology, so I honestly don't know what would be required to make sure they're all killed off. I've been doing research into wastewater treatment plants, but most of the methods adopted at those facilities are chemical and biological, rather than physical.

My physics question is, do different objects inside an oven heated, say, to 250C, achieve different internal temperatures?
Once thermal equilibrium is reached, no; the temperatures of the bricks should be the same as the oven. Note that bricks might have a large thermal mass, which means it might take a long time to reach thermal equilibrium.

Put another way, if I have two identical ovens and in one place a bowl of water, is the heat of the water necessarily limited to staying below 100C?
If the the oven is set to 250 oC, the water will turn to vapour over time and the bowl will become empty. The steam and bowl will then eventually reach 250 oC (assuming the bowl doesn't melt!).

If in the other oven I place a substance into which I can stick a thermometer, let's say sand, will that achieve 250C internal temperature?
Yes, eventually.

What is the relationship between substance and achievable internal temperature? Thanks!
All substances can reach any temperature, but it may change phase depending on the temperature and pressure of the environment (typically gas, liquid or solid). The only exception to this is that there are exotic states of matter at extremely high temperatures and pressures or extremely low temperatures and pressures (e.g. plasma, superfluids, etc.). Things get complex with these states. Fortunately, these oddities are irrelevant to your problem since you're dealing with typical substances at fairly standard temperatures and pressures.

Bricks, stone, paving slabs and other heavy construction materials tend to have very large thermal masses. Thermal mass is an engineering term which describes a substance's ability to store heat, causing a degree of "inertia" in temperature profiles throughout the material (it is also sometimes called "thermal lag"; the idea that a material can take a long time to equilibriate to changes in ambient temperature). Materials with high thermal mass have:

i) high specific heat capacities (in J/(kg.K))
ii) high densities (in kg/m3)
iii) low thermal conductivities (in W/K)

More specifically, if you consider the equation that describes sensible heat transfer:

$\displaystyle Q = m c_p \Delta T$

then the thermal mass is the factor "$\displaystyle m c_p$"; the mass multiplied by the specific heat capacity.

Have you tried setting up a heat balance problem and tried to calculate the time taken for a brick to reach 250 oC?
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Old Sep 4th 2019, 02:14 AM   #3
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Some Bacterium can enter a highly resilient dormant state when placed in conditions that would otherwise kill them.
Then, when conditions improve, they return to normal.

Another problem is that bricks are porous
this allows the bacterium to get deep into the brick, where it is protected.

This means that surface treatments (e.g. washing with bleach) will not be successful.

You might have to bake for several hours to make sure that the heat gets all the way to the centre of the brick
and I would suggest higher temperatures than 100 deg.C.
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Old Sep 4th 2019, 02:27 AM   #4
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thanks Woody that's extremely helpful. Yes, I think that is exactly what happened. The bacterium, sold to counteract the duckweed, works by sequestering nutrients so that plants yellow and die. The stones I was using were extremely porous (limestone and brick), and the bacterium is definitely spore-forming because it was sold as a very fine white powder (which I imagine contained spores). Hence wanting to know how long it would take to generate high heat in the centres of bricks etc. It sounds as if, in a domestic oven, I might not be able to achieve enough heat to definitively destroy any spores that have penetrated and that the simplest solution is therefore to replace the edging stones for the pond with clean ones. thanks, Nigel
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Old Sep 4th 2019, 07:43 AM   #5
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Originally Posted by theydonboy View Post
Hence wanting to know how long it would take to generate high heat in the centres of bricks etc. It sounds as if, in a domestic oven, I might not be able to achieve enough heat to definitively destroy any spores that have penetrated and that the simplest solution is therefore to replace the edging stones for the pond with clean ones. thanks, Nigel
100C is a very low temperature for an oven ... the problem is the bricks are porous and wet so it will take many hours to evaporate all the water

You can either leave them somewhere dry and well ventilated for a week , by which time 90% of the water will have gone , then 2 hours in the oven ...

Or if you are in a rush put the wet bricks for 8 hrs in the oven ..

These times should cook the biggest and wettest of bricks
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Old Sep 4th 2019, 08:08 AM   #6
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Thanks that's extremely useful
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