Some general science questions.

[foges]

1) Energy efficience of humans:
I have come to realize how much energy food has (whats written on the pacakge) for example wheat cookies have 2000 kJ / 100g. Say i can bench 50 kg, that equals a force of ~500 N, and say my arms are 1m long (they arnt) that equals an energy of 500 J = 0.5 kJ. That in theory should mean out of 100 g of cookies i should have enough energy to bench 50 kg 4000 times!!! and im damn sure that i cant do that. So where does that energy go? does the body create enormous amounts of heat (ive heared the head releases most of the body's heat, is that because most of those 2000 kJ go to "running" the brain?)? Does the excrete still contain most of that energy?

2) Energy loss in cars:
We put enormous amounts of fuel in our cars, yet we rarely actually gain potential energy from it? Sounds prety inefficient to me. So where is this energy lost? Yes everything is turned into heat, but what parts of the car is it that are so inefficent and create all that heat? (if someone had a percentage breakdown of the different parts, eg: engine 50%, breaks 10 %, etc...). And why dont we use the energy that is given off, like attaching a stirling motor to the car engine and generating electricity that way?

3) Energy losses from the earth:
Seeing as we get tons of energy from the sun, do we radiate it most away or what?? (else the earth would have to become super hot)

4) Stirling engine with or without separate displacer:
What stirling engine is more efficient, the one with or without a separate displacer? I would think it would have to be the one with a separate displacer. See what im refering to here at the bottom. To me it seems that there would be quite a big energy loss between the cooling and the contraction steps seeing as you are partially compressing warm air and expanding cold air. This would have to impact the momentum of the flywheel.

5) Not exactly technical: the Cuban Unit Convertible (currency in cuba):
I just got back from cuba and the whole time i was there i tried understanding how much money people actually have. I was told by different locals that a doctor earns 40 CUC and others usually earn around 15 CUC. Yet these prices are nothing like what you (turists) pay. For example a 2 hour horse ride cost 40 CUC. If the horse ride guy can get even just one ride per day he should earn 30 x 40 = 1200 CUC per month. Since all these people "work for the government" they are supposed to pay that money to the goverment and wait for their 15 CUC monthly check from the government. This doesnt make a lot of sense, prety much 1 % of what tourists pay them they actually get to keep, youd have to be stupid to tell the governement that you earn that much and actually give the governement that much money. So say people keep even only a bit of the money (75 CUC = 5 times what you should be getting) you would be very rich. This could however only apply for people working in the tourism industry because only those people deal with that much money.

My question is however what happens to the other people, doctors for example, why would you become a doctor if you could earn more by offering horse rides. And those people who dont work directly with tourists yet dont work directly with tourists like painters or something. Or what about people not living in the few touristy cities, are they dirt poor in comparison to everyone living in the larger cities?

6) Where can i get a small piston and cylinder (~7mm diameter). Ive been looking at ebay but can only find expensive ones for cars. Im sure there must exist some model ones that cost a fraction? (I want to build a sterling engine (not using a baloon)

Thanks guys, sorry for the long questions

 

[Born2bwire]

2) Cake yumminess to humans:

 

[foges]

lol sorry bout that i hit the post button by accident without writing anything (useful)

 

[CycloWizard]

1. Work is not a state function. Sure, you can calculate how much potential energy you're imparting on the bench bar to lift it 1 m, but that is NOT how much energy your body consumes to actually lift it. You also use energy to lower it. Energy must also be transported to the muscles doing the work, which takes time that is relatively long compared to how long it takes you to bench one time (or even a whole set). This, among other things, causes fatigue in the muscles doing the work. The reason you can't bench 50 kg 4000 times isn't because you don't have enough energy in your body - it's that your muscles simply aren't designed to do that. And yes, all of the energy input into your body is eventually transformed into work or heat, with the majority being heat. Your body has to burn enough calories to keep your internal temperature at about 99°F, which is substantial.

2. In a car, you're converting chemical potential energy in the fuel to kinetic energy. If you drive up a hill, some of this kinetic energy is transformed into potential energy. All of the energy you put into your gas tank will end up as heat. The inefficiency comes from the engine (thermal and frictional inefficiencies), drag, and rolling friction where the tires meet the pavement. I'll just guess and say that 50% of the loss is due to thermal inefficiencies in the engine, 20% due to drag, 30% due to friction between you and the road (including braking). It's not cost effective to recover the heat lost since it is not lost over one specific area but over your entire engine.

edit: made my percentages sum to 100.

 

[emfiend]

1. A more fundamental explanation is that a lot of the energy consumed through food goes into keeping the body alive such as breaking down of sugars, transporting ions etc, all of which consumes a large amount of energy. The energy is lost through heat mostly (I would guess) through phonons that are either convected or conducted away. Radiation, by its strict physical definition, occurs on a very small scale in the body.

So the energy conversion is never 1-to-1 to muscle strength. In other words, just because you consume X Joules of energy does not mean you will have X Joules of energy to exert through your muscles. A great part of that goes into electrical pulses for synaptic function (in brain), lactose generation from muscle exertion, Ca++ signalling to get one muscle to talk to the next, recombination energy lost through sweat/piss/dumps (LOL), etc. Add all of that up and maybe you get back the X Joules.

2. Internal combustion is inherently inefficient (not sure on exact percentages) and not all is lost to heat. Most of it leaves in the recombined by products of the chemical reaction, ie O2, CO2, H2O, etc.

 

[Nathelion]

http://en.wikipedia.org/wiki/I...gine#Engine_Efficiency your average car has a mechanical efficiency of around 20%.

 

[bendixG15]

6) Where can i get a small piston and cylinder (~7mm diameter). Ive been looking at ebay but can only find expensive ones for cars. Im sure there must exist some model ones that cost a fraction? (I want to build a sterling engine (not using a baloon)

Try the hobby world. Maybe a model airplane engine would work for you.
(Saturday night blahssss... )

 

[jagec]

Originally posted by: Nathelion
http://en.wikipedia.org/wiki/I...gine#Engine_Efficiency your average car has a mechanical efficiency of around 20%.

Yes, and when you accelerate, that turns into kinetic energy. When you're cruising, you're just producing enough to counteract wind resistance and frictional forces. When you brake, all that kinetic energy goes into heat at the brakes.

Why we don't capture it? A low-temperature (100C is considered low temperature when you're talking about running a heat engine), inconsistent heat source is a very low-quality form of energy, and you'd barely be able to capture a few percent of that energy if you ran a heat engine from it.