C-11 Energy Systems
• Energy
Transformation
and Power
•
Physics 2015
The workings of the universe can be
viewed as energy flowing from one
place to another and changing back
and forth from one form to another.
Law of Conservation of Energy
• As energy takes different forms and changes
things by doing work, nature keeps perfect
track of the total.
• No new energy is created and no existing
energy is destroyed.
Energy in a closed system
• The conservation of energy is most useful
when it is applied to a closed system.
• Because of the conservation of energy, the
total amount of matter and energy in your
system stays the same forever.
Energy in a closed system
• The total energy in the system is the potential
energy of the ball at the start.
• Later, the ball is at a lower height (h) moving
with speed (v) and has both potential and
kinetic energy.
11.1 Efficiency
• Efficiency is defined for a
process.
• A process is any activity
that changes things and
can be described in terms
of input and output.
• The efficiency of a
process is the ratio of
output to input.
EFFICIENCY
• Efficiency is defined as the ratio of output work to input
work.
• An ideal machine has equal output and input work
(Wo/Wi = 1) and its efficiency is 100%.
• Real machines have efficiencies less than 100% because
not all of the force is transferred into energy. (There is
always friction somewhere)
Calculate efficiency
1. A pulley system lifts a 1400 N rock a
distance of 1.2 m. A force of 365 N is
exerted and the rope is pulled 4.8 m.
a) What is the work input?
b) What is the work output?
c) What is the efficiency?
11.1 Efficiency
Efficiency can also mean the ratio of energy
output divided by energy input.
Efficiency
e = Eo
Ei
Energy output (J)
Energy input (J)
Example of Real Machine & Efficiency
• The work output is
reduced by the work that
is converted to heat,
resulting in lower
efficiency.
11.1 Efficiency
 According to the law
of conservation of
energy, energy cannot
ever be lost, so the
total efficiency of any
process is 100%.
• The work output is reduced by the work that is
converted to heat, resulting in lower efficiency.
11.1 Efficiency in natural systems
 Energy drives all the
processes in nature, from
winds in the atmosphere
to nuclear reactions
occurring in the cores of
stars.
 In the environment,
efficiency is interpreted as
the fraction of energy that
goes into a particular
process.
11.1 Efficiency in biological systems
 In terms of output work,
the energy efficiency of
living things is typically
very low.
 Almost all of the energy
in the food you eat
becomes heat and waste
products; very little
becomes physical work.
11.1 Estimating efficiency of a
human
• The overall energy
efficiency for a person is
less than eight percent.
• An average person uses
55–75 kilocalories per
hour when just sitting still.
• The rate at which your
body uses energy while at
rest is called your baseline
metabolic rate (BMR).
Efficiency in biological systems
 Since processes in the universe almost always
lose a little energy to friction, time cannot run
backward.
 If you study physics further, this idea
connecting energy and time has many other
implications.
Power
• It makes a difference how fast you do work.
POWER
• Power is the rate of doing work.
(Remember):
• Unit of measurement:
– Watt (W)= one joule of energy transferred in one
second (J/s)
– 1000 W = 1 kW
Power
 A unit of power is
called a watt.
 Another unit more
familiar to you is
horsepower.
 One horsepower (the
avg. power output of a
horse) is equal to 746
watts.
Power in human technology
• You probably use technology with a wide range of
power every day.
• Machines are designed to use the appropriate amount
of power to create enough force to do work they are
designed to do.
Power in natural systems
• Natural systems exhibit a much greater range of
power than human technology
• The sun has a total power output of 3.8 × 1026 W.
• The power received from
the sun is what drives the
weather on Earth.
Energy flow in systems
Energy flows almost always involve energy
conversions.
To understanding an energy flow:
1. Write down the forms that the energy takes.
2. Diagram the flow of energy from start to finish for
all the important processes that take place in the
system.
3. Try to estimate how much energy is involved and
what are the efficiencies of each energy conversion.
11.3 Energy flow in systems
• A pendulum is a system in which a mass swings back
and forth on a string.
• There are 3 chief forms of energy: potential energy,
kinetic energy, and heat loss from friction.
11.3 Energy flow in human
technology
The energy flow in technology can usually be
broken down into four types of processes:
1. Storage ex. batteries, springs, height, pressure
2. Conversion ex. a pump converting mechanical energy to
fluid energy
3. Transmission ex. through wires,
tubes, gears, levers
4. Output ex. heat, light, electricity
11.3 Energy flow
• The energy flow diagram
for a rechargeable electric
drill shows losses to heat
or friction at each step.
Energy flow in natural systems
• The energy flows in
technology tend to
start and stop.
• Many of the energy
flows in nature occur in
cycles.
• Water is a good
example.
Energy flow in natural systems
• A food chain is a series of processes through which
energy and nutrients are transferred between living
things.
• A food chain is like one strand in a food web.
• A food web connects all the producers and
consumers of energy in an ecosystem.
Energy flow in natural systems
• The energy pyramid
is a good way to
show how energy
moves through an
ecosystem.
The production of electricity is not very efficient because there
is so much energy lost as heat. When we use electricity, there is even
more energy lost.