1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Heating the inside of a box thanks to a lamp

  1. Mar 22, 2017 #1
    1. The problem statement, all variables and given/known data
    The box's measurements are 277*120*195mm.
    The lamp's power is 15W.
    Trying to figure out how long it will take to heat the box so that it reaches a given temperature.
    According to Wikipedia ρair=1,2kg/m3 and cp air=1004J*K-1*kg-1

    2. Relevant equations
    ΔU=m*cp*ΔTemperature
    E=p*Δt

    3. The attempt at a solution
    If we suppose that the inside of the box is only air, we will have
    vair=0.277*0.120*0.195=0.0064818m3
    So, mair=vairair
    mair=0.00777816kg
    And,
    ΔU=mair*cp air*ΔTemperature
    We'll have,
    ΔU≈7.8ΔTemperature
    And if we assume there is no energy loss,
    ΔU=E
    So
    7.8ΔTemperature=15Δt
    Thus
    Δt=0.52ΔTemperature.

    The time I get seems to be too short.
    Thanks in advance :wink:
     
  2. jcsd
  3. Mar 22, 2017 #2
    I think your answer is correct. The volume is very small - a cube around 20 cm in each dimension.
     
  4. Mar 22, 2017 #3

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Is the box airtight?
     
  5. Mar 23, 2017 #4
    Thank you for replying :)

    Actually, there are five little holes on the box. They allow to put temperature sensor inside the box.

    Is it possible to calculate the loss of energy due to these holes?

    ps: When we do the experience, it takes around 30 seconds to gain 3°C.
     
  6. Mar 23, 2017 #5

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    If it's an incandescent lamp then most of the power is emitted as heat in the lamp itself. So you might need to allow for the heat capacity of the lamp and the fitting or wires?.

    If the lamp is an LED then most (more?) of the power is emitted as light and that will be absorbed by the walls of the box. So you may need to allow for the heat capacity of the box walls. In addition some of the heat will be conducted and radiated to the outside.

    Is the air in the box stirred so that the temperature recorded is representative?
     
  7. Mar 23, 2017 #6

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    With difficulty. You would need to know the air flow rate into and out of the box. Easier to just seal them up.
     
  8. Mar 23, 2017 #7

    kuruman

    User Avatar
    Homework Helper
    Gold Member

    Are you measuring the temperature of the air inside the box or of the box itself? What about the heat capacity of the box? Your original post asks about the box reaching "a given temperature."
     
  9. Mar 24, 2017 #8
    It is an incadescent lamp, but I don't know whether I should consider its heat capacity to be E=P*Δt or use the heat capacity of tungsten.

    The air inside the box is not stirred.
    So I guess that the majority of the heat transfert is done by convection.

    We are measuring the temperature of the air inside of the box (sorry for the ambiguity).
     
  10. Mar 24, 2017 #9

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    Pretty sure convection wont even start until the glass of the bulb is hot and convection can be quite a slow process. I would use the specific heat capacity of the glass part of the bulb. The filament gets white hot pretty quick so it's heat capacity will be very low.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Heating the inside of a box thanks to a lamp
Loading...