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Name: Carol-Ann
Status: student
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Question:
What energy transfers are 100% efficient?


Replies:
Carol-Ann,

Whether an energy transfer is 100% efficient depends on what kind of energy you want to have after the transfer. A 100% efficient transfer is one in which the energy is just as usable after the event as before. Any transfer of energy that is completely reversible is considered to be 100% efficient. An interaction that involves only gravity would be 100% efficient. This could be possible on the moon, or anywhere else that doesn't have air resistance. An event is usually less than 100% efficient because of heat. Heat involves random motion. Heat is difficult to control. To make use of the randomness of heat energy, such as powering a steam engine, requires extra energy to organize it.

Forces such as friction and air resistance turn kinetic energy into heat energy. These are the two major forces that reduce efficiency. The radiation, usually infrared and visible, emitted by a heated wire reduces the efficiency of an electric circuit. Electric force can be 100% efficient. Gravity can be 100% efficient. Magnetic force can be 100% efficient. It all depends on the environment, on keeping heat and radiation from being emitted.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


Hi Carol-Ann,

I would appeal to the 2nd law of thermodynamics to answer this one. It can be expressed using derivative calculus like this:

dS = dq / T

S = entropy
q = heat
T = temperature
d -> derivative operator, basically meaning "change in"

So, "change in entropy = change in heat divided by temperature"

The mark of an 100% efficient energy transfer is that it is completely reversible (you can get all that energy back if you could reverse the process). That also means that you have created no extra entropy in doing so. So, according to the equation, that means that there was no energy transfer from the system in the form of heat. However, in the real world all energy conversion processes suffer from some heat loss. Here are some numbers on energy conversion processes you will be familiar with:

Jet engine - 40% efficient
Solar cell - 43% efficient
Muscle tissue - 27% efficient
Incandescent light bulb - 10% efficient
Fluorescent bulb - 28% efficient
LED light - 35% efficient

Look at those incandescent light bulbs, only 10% efficient! No wonder they get so hot. :)

Regards,

John Strong


Excellent question - and not an easy one to answer.

My first impulse was to say NONE. But then on consideration I changed my answer to ALL.

What exactly do you mean by energy transfer, and what do you mean by efficient?

Since the Law of conservation says that energy can neither be created nor destroyed, but remains constant within any closed system (and believe me they are really hard to find!) that means that we can say that ALL energy transfers are 100% efficient, because 100% of the energy is converted into some other form of energy. The problem is that not all the forms of energy resulting from the conversion may be energy in a form which is useful to us.

Example 1: I put a kettle on the gas stove. After a while the kettle boils - I can make my coffee. I have converted chemical energy from the gas into heat energy in the water. Since gas burns quite efficiently (there's that word again) then 100% of the chemical energy gets converted into something. Heat is good, but not all of it ends up in the water - some heats the kettle itself, the gas stove top gets hot. The air above gets hot. A lot of heat energy is wasted, but it is all there somewhere. But then again - heat is not the only energy form that comes from a flame - what about light. Not much good for heating water, but it is still a form of energy produced from the gas. The products produced by burning the gas also have energy - so not ALL the energy of the gas gets converted into heat and light - only the DIFFERENCE in energy between the gas you start with (propane perhaps) and the gases which are produced when you burn it - such as carbon dioxide and water. They have a good deal less energy that propane, but they still have quite a bit locked up in their molecules.

Example 2: Much simpler - A billiard ball rolls into another ball. The first ball stops and the second starts to move. A classic physics demonstration of a collision imparting energy from one body to another. Why does the first ball stop? Because its kinetic (movement) energy is transferred to the other ball. In most cases the speed of the second ball after the collision seems to match the speed of the first ball before. Is the transfer 100% efficient? ALL the energy from the first ball went SOMEWHERE - but did it all go to the second ball? Some energy is lost as sound energy (The 'click' as the balls hit) and a small amount goes to creating damage to both balls (Microscopic, but examine some 10 year old billiard balls and you can start to see the damage - the balls are no longer smooth and shiny.) Some is lost as heat - again tiny but measurable with infra-red sensitive cameras and slow motion. A significant amount of energy is always being lost to friction with the pool table - but the since the amount lost at the moment of impact cannot be measured - we could pretty much ignore it. The ball traveling away however immediately starts to slow down due to friction. A collision between two billiard balls is a pretty efficient transfer - in that ALMOST all the energy you start with gets transferred to the second ball - only a small amount goes to other (dare I say - less useful) forms of energy.

Example 3 - A light bulb produce heat - which is wasted when you want light( That goes for any kind of light bulb - incandescent are the worst, CFLs are much better, but still lose about 10 - 15% in heat, and even LEDs lost 2 - 5% or so in heat)

Example 4 - A car typically only turns about 15 - 20% of the energy from its fuel into forward movement of the car. A huge amount is lost just chucking the engine itself around (The internal combustion engine is inherently inefficient) so we get energy lost as vibrations, noise, friction including that damage thing I mentioned before about the billiard balls. Then there is the wind created by the vehicle and the drag that results from that. There is the deformation of the tyres (especially if they are under inflated) The noise the tyres create on the road, and the damage to the road itself. Energy for all of those things has to come from the fuel - whether it is gasoline (petrol in Australia) or hydrogen or electricity from the grid, or solar panels. Cars are incredible inefficient (and unfortunately US cars are amongst the worst!)

So you see that no energy transfer goes without paying a penalty somewhere in energy LOST as forms we don't want, No energy ever disappears - it just goes where we don't want it - and that is why there can never be a transfer of energy which is 100% efficient in achieving what WE want the universe to do.

Nigel Skelton
Barkly College
Tennant Creek AUSTRALIA


This question has several "definition of terms" that need to be properly defined. First, any process that converts thermal energy into "work", the answer is "easy". None. This is a consequence of the second law of thermodynamics. Second, if the "conversion" is the conversion of heat from a higher temperature to a colder temperature with no other processes involved, the "conversion" can be 100% (provided there are no loss of thermal energy due to poor insulation, for example). In practice this may not be easy to achieve. I am guessing your question relates to the conversion of thermal energy to work. In that case, the efficiency is a result of the second law of thermodynamics. In that case the efficiency E is given by the formula: E = (T hot - T cold) / T hot. In this case the efficiency, E, is always less than 100%.

Vince Calder



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