Heat Transfer Problem Confused

In summary, the conversation discusses a problem involving an electrical resistor connected to a battery and the equation Estored=Ein-Eout+Thermal_Egenerated is applied to determine the values of Ein, Eout, Eg, and Est. The participants discuss the terms and come to the conclusion that Ein and Est are both 0, while Eout and Eg are non-zero. They also mention that the problem is not clearly worded and may assume certain things without stating them explicitly.
  • #1
aisu1
4
0
I'm super confused about this problem. I don't even know where to start. It seems very simple, but I just can't figure it out..

An electrical resistor is connected to a battery. After a brief transient, the resistor assumes a nearly uniform, steady-state temperature of 95 degrees C, while the battery and lead wires remain at the ambient temperature of 25 degrees C. Neglect the electrical resistance of the lead wires.

Consider the resistor as a system about which a control surface is placed and the equation Estored=Ein-Eout+Thermal_Egenerated is applied. Determine the corresponding values of Ein (W), Eout (W), Eg (W), and Est (W).

I have a feeling that a lot of these terms cancel out.. but I just don't even know where to start.
 
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  • #2
Hi aisu1, welcome to PF. Start by reasoning through each term individually. What are your thoughts?
 
  • #3
Thanks :)

Well, I think that there's in Ein maybe because it's heat and heat goes from higher to lower temperature. So, Eout is q=q''A.. But I don't know if the heat gets transferred via convection, conduction to the outside air, radiation, or through the wires.. And either way, I feel like I'm not given enough information to figure out the thermal conductivity of the resistor.

I think the thermal energy generated is the power generated by the battery.. But that could be the Ein (if Ein isn't 0).
 
  • #4
aisu1 said:
Well, I think that there's in Ein maybe because it's heat and heat goes from higher to lower temperature.

The resistor is hotter than the surrounding environment. What does that tell you about [itex]E_\mathrm{in}[/itex]?

(Personally, I don't like the way the problem is constructed where [itex]E_\mathrm{in}[/itex] and [itex]E_\mathrm{out}[/itex] are separate terms. There should be one term; if it's positive (negative), then energy is entering (leaving).)

aisu1 said:
And either way, I feel like I'm not given enough information to figure out the thermal conductivity of the resistor.

The thermal conductivity governs temperature changes within an object. The system boundary encloses the entire resistor, so its thermal conductivity is irrelevant.

aisu1 said:
I think the thermal energy generated is the power generated by the battery.. But that could be the Ein (if Ein isn't 0).

Another problem with the wording of this problem is that it seems to be assumed (but not stated) that [itex]E_\mathrm{in}[/itex] and [itex]E_\mathrm{out}[/itex] represent transfers of thermal energy only, not electrical energy. But anyway, I agree that the thermal energy generation term is meant to include the power delivered by the battery.

And what can you say about the stored energy term?
 
  • #5
The resistor is hotter than the surrounding environment. What does that tell you about Ein?

So, Ein would equal 0, because there is no heat transfer to the resistor, just away from it.

And what can you say about the stored energy term?

I think the stored energy would be 0 also. You only have the generated thermal energy and the energy out terms.
 
  • #6
Right on!
 
  • #7
Thanks :) I think it makes much more sense now.
 

1. What is "heat transfer" and why is it important in science?

Heat transfer is the movement of thermal energy from one object or substance to another. It is an important concept in science because it helps us understand how energy is transferred and distributed in various systems, such as in the Earth's atmosphere or in the human body.

2. What are the different types of heat transfer?

The three main types of heat transfer are conduction, convection, and radiation. Conduction is the transfer of heat through direct contact between two objects. Convection is the transfer of heat through the movement of fluids, such as air or water. Radiation is the transfer of heat through electromagnetic waves.

3. How can I calculate heat transfer?

The formula for calculating heat transfer is Q = mcΔT, where Q is the amount of heat transferred, m is the mass of the object, c is the specific heat capacity of the material, and ΔT is the change in temperature. This formula applies to situations where there is no change in phase.

4. What factors affect heat transfer?

The rate of heat transfer is affected by several factors, including the temperature difference between two objects, the distance between them, the surface area of the objects, and the type of material through which heat is transferred. Other factors such as air flow and insulation can also affect heat transfer.

5. How can I solve a heat transfer problem?

To solve a heat transfer problem, you will need to identify the type of heat transfer involved and gather all the necessary information, such as the temperatures of the objects and their specific heat capacities. Then, you can use the appropriate formula to calculate the amount of heat transferred. It is important to double check your calculations and consider any additional factors that may affect the problem.

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