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Perfect Energy Transfers
Name: Carol-Ann
Status: student
Age: N/A
Location: N/A
Country: N/A
Date: N/A
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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Update: June 2012
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