Temperature in a Carnot heat engine

In summary, the problem involves a Carnot heat engine that lifts a 10 kg mass a height of 11 m using 14 J of heat per cycle to a cold reservoir at 0∘C. The task is to determine the temperature of the hot reservoir. Using the equation η=1-(Tc/Th)=W/Qh, where W is the work done, Qh is the heat absorbed by the hot reservoir, and Qc is the heat exhausted to the cold reservoir, the correct solution is Th=494 K=221°C.
  • #1
EightBells
11
1

Homework Statement


A Carnot heat engine takes 95 cycles to lift a 10 kg. mass a height of 11 m . The engine exhausts 14 J of heat per cycle to a cold reservoir at 0∘C.

What is the temperature of the hot reservoir?

Homework Equations


η=1-(Tc/Th)=W/Qh

The Attempt at a Solution


I've tried: (Energy to lift mass)/(number of cycles)=W, so (mgh)/95=((10kg)(9.8m/s^2)(11m))/95= W=11.35 J/cycle
∴ 1-(Tc/Th)=11.35/14, so Tc/Th=1-(11.35/14), Th=Tc/(1-(11.35/14))=273 K/(1-(11.35/14))=1440 K=1170°C

This is an incorrect answer, and logically it seems too high.

I also considered where the 14 J/cycle exhausted to the cold reservoir is the work out, but then I don't know how to calculate Qh so that I'd only have one variable in the equation listed under 'Relevant Equations'.
 
Physics news on Phys.org
  • #2
The problem statement says 14 J is the heat exhausted to the cold reservoir, so it's ##Q_\text{C}##, not ##Q_\text{H}##. You calculated ##W## correctly. How do you get ##Q_\text{H}## from ##Q_\text{C}## and ##W##?
 
  • Like
Likes EightBells
  • #3
vela said:
The problem statement says 14 J is the heat exhausted to the cold reservoir, so it's ##Q_\text{C}##, not ##Q_\text{H}##. You calculated ##W## correctly. How do you get ##Q_\text{H}## from ##Q_\text{C}## and ##W##?

Qh=W+Qc=11.35+14=25.35 J/cycle

Plug that into W/Qh=1-(Tc/Th) and Th=494 K=221°CThat's the correct answer, thanks so much!
 

1. What is a Carnot heat engine?

A Carnot heat engine is a hypothetical thermodynamic device that operates between two heat reservoirs at different temperatures and converts heat energy into work. It is considered to be the most efficient heat engine possible, as it follows a reversible thermodynamic cycle.

2. How does a Carnot heat engine work?

A Carnot heat engine works by taking in heat energy from a high temperature reservoir, using it to do work, and then releasing the remaining heat energy into a low temperature reservoir. This process is repeated in a cyclic manner, and the efficiency of the engine is determined by the temperature difference between the two reservoirs.

3. What is the Carnot efficiency?

The Carnot efficiency is the maximum possible efficiency of a heat engine operating between two temperature reservoirs. It is given by the formula: efficiency = (1 - Tlow/Thigh) x 100%, where Tlow is the temperature of the low temperature reservoir and Thigh is the temperature of the high temperature reservoir.

4. How does temperature affect the efficiency of a Carnot heat engine?

The efficiency of a Carnot heat engine is directly proportional to the temperature difference between the two reservoirs. This means that as the temperature difference increases, the efficiency also increases. However, the efficiency cannot exceed the Carnot efficiency, which is determined by the temperature ratio between the two reservoirs.

5. What is the significance of the Carnot heat engine?

The Carnot heat engine is significant because it provides a theoretical upper limit for the efficiency of all real heat engines. It also helps to understand the relationship between temperature and energy conversion, and serves as a benchmark for evaluating the performance of practical heat engines.

Similar threads

  • Introductory Physics Homework Help
Replies
3
Views
3K
Replies
1
Views
514
  • Introductory Physics Homework Help
Replies
2
Views
2K
  • Introductory Physics Homework Help
Replies
1
Views
769
  • Introductory Physics Homework Help
Replies
4
Views
5K
  • Introductory Physics Homework Help
Replies
20
Views
2K
Replies
1
Views
952
  • Introductory Physics Homework Help
Replies
4
Views
1K
Replies
2
Views
1K
Replies
1
Views
770
Back
Top