B What is meant by "upper limit of work done on Earth"?

[Lotto]

TL;DR Summary

I have this problem and I don't undestand what am I to do. What is meant by that work?

I think that the work is meant to be work done for instance in power stations. Or is it similar to work I do on a body when I lift it for example? But how can we then do that work on our Earth? I just need to understand the task, otherwise I want to solve it myself.

The problem involves thermodynamics.

 

[kuruman]

This looks like a Fermi problem to me. Is that all that is given to you or there is more than you posted? How do you know that it involves thermodynamics?

 

[Lotto]

This looks like a Fermi problem to me. Is that all that is given to you or there is more than you posted? How do you know that it involves thermodynamics?

Because I was said it involved thermodynamics. And yes, this is the whole task. I am just confused, I don't know what am I supposed to estimate.

 

[Lotto]

This looks like a Fermi problem to me. Is that all that is given to you or there is more than you posted? How do you know that it involves thermodynamics?

And do you think the work is meant to be, for example, (physical) work done on Earth by humans in power stations , or is it to be work done on Earth that is similar to work I do on a body when a push it and transfer it? This is the problem, I don't know how to understand the work. What do you think?

 

[Steve4Physics]

And do you think the work is meant to be, for example, (physical) work done on Earth by humans in power stations

Probably the output of nuclear and fossil fuel power stattions (ones not using renewables).

, or is it to be work done on Earth that is similar to work I do on a body when a push it and transfer it?

Unlikely.

It would make (slightly) more sense if the question asked for power rather than work, because the time-period of interest is unspecified.

Hope I’m not giving too much help, but if you can’t get further clarification from your teacher, you could proceed as follows:

Treat the outer part of the earth as a black body. Consider:
- the power from the sun that gets absorbed;
- the power radiated by the earth as a function of its temperature.

Guess that ‘work done’ (W), is essentially the total output of power stations using non-reewable fuels. Suppose this corresponds to a power P. This 'work' ultimately ends up as heat, causing a planetary temperature-rise.

Can you construct an equation which contains the various powers and the earth's (surface) temperature? Then decide what’s an acceptable temperature and use your equation to find P.

If you want an actual ‘work’ value over 1000 years for example, you can calculate it from P.

 

[Lotto]

Guess that ‘work done’ (W), is essentially the total output of power stations using non-reewable fuels. Suppose this corresponds to a power P. This 'work' ultimately ends up as heat, causing a planetary temperature-rise.

What do you mean? That these power stations generate energy that we eventually use for cooking etc., and therefore it ends up as heat? Beacuse this part confuses me.

 

[Steve4Physics]

What do you mean? That these power stations generate energy that we eventually use for cooking etc., and therefore it ends up as heat? Beacuse this part confuses me.

It’s important to realise that all energy-transformations ultimately end up with energy converted to thermal energy (‘heat’).

It may not always be obvious, because many steps can be involved. For example, when you walk, you are converting solar energy (stored as chemical energy from plants in your food) to kinetic energy and ‘heat’ in muscles. The kinetic energy then gets converted heat in the ground and in the soles of your shoes due to friction and contact deformations. Even the energy you transfer to the air when you move through it gets converted to ‘heat’, warming the air a tiny amount. So the 'useful' energy from the (very hot) sun has been converted to (less useful) 'heat' at a lower temperature.

I shouldn't have singled-out power stations. I interpret 'work' in your question to mean all non-solar energy used on earth.

For your question, just consider the only 2 sources of heat to be:

- the energy absorbed from the sun;

- the energy due to all activities on earth where the energy source is not the sun; it really doesn't matter what these are, but could include nuclear and geothermal for example (and fossil fuels depending on what timescales you are considering); collectively, they give the 'work' you are interested in; you don't need to know exactly what they are.

The only way the earth can lose energy is by emitting black body radiation.

 

[256bits]

And do you think the work is meant to be, for example, (physical) work done on Earth by humans in power stations , or is it to be work done on Earth that is similar to work I do on a body when a push it and transfer it? This is the problem, I don't know how to understand the work. What do you think?

doesn't really matter who what does the work - a running jumping horse, a tree growing higher, a human made mechanical machine, ..
Make or post some assumptions and conditions. See post #2, @kuruman is one way of going about it.

condition - planet remains habitable
assumption - question --> would the earth be habitable during an ice? would the earth remain habitable if it grows hotter and the polar ice caps melt? How hot/how cold? From where should we start - ice age/hotter earth/ now? Does it make a difference?

condition - same climate for future generations.
assumption - a steady state earth where the temperature if the earth remains constant, implying

condtion - energy in = energy out [ plus energy storage ]

You might want to adjust conditions/assumptions better - I just wrote these down perhaps too quickly. Think about them before accepting as fact.

Assumption -if you would like to concentrate on how much work humans can extract, then
Scenario - Cave man - no mechanical systems
Does the cave man lifting a rock higher to make a 'rock wall home' change the temperature of the earth?

Scenario - mechanical systems - cave man has progressed into a highly technological society.
A machine lifts the rock. Does the lifting of the rock change the temperature of the earth? Does it matter where the machine received its energy from - ie direct solar, hydro-electric, fission power plant, fusion power plant, hydrocarbon power plant( ie wood, coal, vegetable oil )

Scenario - sun is blocked off
One simplification is to use only fussion power plants to supply earth's energy needs, with the sun's energy reflected away from the earth.
Another is to block off the direct sunlight to earth, but then capture the sun's energy with some kind of solar array and beam that down to solar energy plants.

PS
Do we really have to go through all that mumbo-jumbo, or is there a more direct approach.