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Power, Efficiency, and Machines
PhysicsIn this lesson, we will discuss the
following:
Power
Efficiency
Machines
Definition of Power
Power is defined as how quickly work is done.
Abbreviation for Power
Power is abbreviated P.
P
Formula for Power
Power is equal to work divided by time.
t
WP
And, work is equal to change in energy.
t
EP
SI Units for Power
The SI unit for power is the Watt (W).
s
JW
1
11
1 Joule
1 second
Power is a Scalar Quantity
Power, like work and energy, is a scalar quantity. In other words, power has a magnitude and a unit. Power does NOT have a direction.
Using Power, Work, and Time in Calculations
Problem: What is the power of a light bulb which does 216,000 Joules of work in one hour?
Solution:
WP
s
JP
hour
JP
t
WP
60
3600
000,2161
000,216
Using Power, Work, and Time in Calculations
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the page at
the right does not
open.
Definition of Efficiency
Efficiency is defined as the percentage of useful energy you get out of a machine.
Abbreviation for Efficiency
Efficiency is abbreviated ε.
Lower
-case
epsil
on.
Formula for Efficiency
The efficiency of a machine is equal to useful work you get out of the machine divided by the work put into the machine times 100%.
%100in
out
W
W
Formula for Efficiency
Or, the efficiency of a machine is equal to useful energy you get out of the machine divided by the energy put into the machine times 100%.
%100in
out
E
E
Efficiency is Dimensionless
Efficiency is a dimensionless quantity. In other words, efficiency has no unit(s), unless you call percent a unit. I don’t.
Efficiency is a Scalar Quantity
Efficiency is a scalar quantity. In other words, it has no direction.
Using Efficiency in Calculations
Problem: What is the efficiency of a car which puts out 300 J of energy for every 1000 J of energy it uses?
Solution:
%30
%1001000
300
%100
J
J
E
E
in
out
Using Efficiency in Calculations
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open.
Machines and the Law of Conservation of Energy
All machines must follow the Law of Conservation of Energy. In other words, the energy that is put into a machine is always at least as large as the useful energy that is taken out of the machine.
In addition, the work that is put into any machine is always at least as large as the useful work that is taken out of the machine.
Machines and the Law of Conservation of Energy
If a machine is 100% efficient, the work put into a machine is exactly equal to the useful work that is taken out of a machine.
Workin = Workout
(Forcein) x (Distancein) = (Forceout) x (Distanceout)
If yo
u w
ant t
o pu
t a
smal
l for
ce in
to a
m
achi
ne…
Then
you
mus
t
exer
t tha
t sm
all
forc
e ov
er a
long
dist
ance
…And if you w
ant to
get out a larger
force…
And only get a small
distance out.
Machines and the Law of Conservation of Energy
Workin = Workout
(Forcein) x (Distancein) = (Forceout) x (Distanceout)
A car jack follows the Law of Conservation of
Energy. A small force put into the jack exerted over a long distance results in a force large enough to pick up a car but over a
very small distance.
Six Examples of Simple Machines
Below you will find six examples of simple machines:
1. Lever2. Inclined Plane3. Wheel and Axle4. Screw5. Wedge6. Pulley
All of these machines follow the
Law of Conservation of Energy!
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Six Examples of Simple Machines
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page at the right does not
open.
Six Examples of Simple Machines
Click HERE if the web
page at the right does not
open.