How long does it take to get heat? (thermodynamics )

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    Heat Thermodynamics
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Discussion Overview

The discussion revolves around calculating the time required to heat water using a specific amount of fuel, particularly coal, in a thermodynamic context. Participants explore various formulas and considerations related to heat transfer, energy content, and the properties of materials involved.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant requests a formula to determine how long it takes to boil 1 kg of water starting at 20°C using coal.
  • Another participant proposes a formula involving specific heat and temperature change, suggesting it relates to the energy provided by the fuel.
  • A different viewpoint emphasizes that the burn rate of coal and heat transfer properties of the container will significantly affect the heating time, noting that the heating process may not be uniform.
  • One participant introduces Joule's law, presenting an alternative formula that relates power to mass and temperature change, but acknowledges the need for additional details about the coal burning process.
  • There is a critique of the initial formula proposed, pointing out inconsistencies in units and suggesting that caloric power of the fuel should be considered instead of specific heat.
  • Another participant expresses understanding of Joule's law and acknowledges the discussion.

Areas of Agreement / Disagreement

Participants express differing views on the appropriate formulas and considerations for the problem, indicating that there is no consensus on a single approach or formula to use for the heating time calculation.

Contextual Notes

Participants highlight the importance of assumptions regarding the burn rate of coal, heat transfer efficiency, and the uniformity of heating, which remain unresolved in the discussion.

lasha1
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Can anyone tell me the formula.
For example: How long does it take to boil 1kg 20°C water in steel container. With 1 kg of coal (or something). I want to understand the formula of time attitude to heat.:smile:
 
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I think it's the following formula:
cmwaterΔT=qmfuelt where t is time q is specific heat of fuel.
 
That would depend on the burn rate of the coal and the heat transfer properties of the steel container. Don't forget that the transfer rate will change as the water warms. I haven't looked it up, but you should also make sure that the coal contains enough energy to bring the water to a boil. Then you need to assume that the heating is so even that there is no boiling till all the water gets hot enough. Uneven heating changes everything.
 
Ok FastChecker. But with Joule lenz's law we can get the following formula: Pt=mcΔt where P is power of kettle and t is time that needed to get heat.
 
Your second post came as I was writing my reply and I never saw it. So you have equations that you are satisfied with. Good.
 
lasha1 said:
I think it's the following formula:
cmwaterΔT=qmfuelt where t is time q is specific heat of fuel.
This equation does not make sense. The units are not consistent. If q is indeed specific heat of fuel you need some temperature.
But is more likely you mean the caloric power of the fuel. In any case, time is not supposed to be there

lasha1 said:
Ok FastChecker. But with Joule lenz's law we can get the following formula: Pt=mcΔt where P is power of kettle and t is time that needed to get heat.
This equation is OK but you changed the OP problem. Originally you said that you use some coal to heat up that water. Now you seem to switch to some kettle. You cannot apply this to OP question without knowing all kind of details about the burning of coal and heat transfer.
 
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Yes. But I checked lenz's law:biggrin:. I understand it. Thank you.
 

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