Energy and Efficiency

Sec 3.3 Energy and
P. 335 - 342
Law of Conservation
 Energy cannot be created or destroyed.
 It does not appear or disappear, can only be
converted from one form to another.
 However, we usually find that the output
energy of a device or system is smaller
than the input energy, sometimes much
 Most often the energy is lost to heat.
 Example:
when heating a pot, not all the heat
goes into the pot, some is lost to the
 Mechanical systems also lose a lot of energy. They
lose some energy to sound, heat from friction.
These are non-usable forms of energy.
 In fact, all mechanical systems dissipate or lose
some energy, so their usable output energy is
always less than their input energy.
Understanding Efficiency
 The efficiency of a device is the ration of useful
energy that comes out of a device to the total
energy that went in.
The more input energy that a device converts to
useable output energy, the more efficient the
device is.
 Efficiency is usually calculated as a
% Efficiency = Joules of useful output
Joules of input energy
x 100%
5 J of light
100 J electric
energy going in
95 J lost to heat
 The efficiency of this light bulb is 5 J/100 J x 100% =
5 %. In other words only 5% of the energy used is
converted to light, and the rest is lost to heat.
 A diesel truck produces 47.5kJ of useful energy from
125 kJ of diesel fuel. What is the trucks efficiency?
% Efficiency = Joules of useful output
Joules of input energy
% Efficiency = 47.5 kJ x 100%
125 kJ
% Efficiency = 0.38 x 100%
= 38% efficient
x 100%
 A lawn mower consumed 8 MJ of chemical energy.
The efficiency of this mower is 35% efficient. How
much useful output energy did the lawn mower
The lawn mower used 35% of 8 MJ.
0.35 x 8 = 2.8 MJ
% Efficiency = Joules of useful output x 100%
Joules of input energy
35% = Joules of useful output x 100%
8 MJ
8 MJ x 0.35 = 2.8 MJ
Section 3.4 Reducing the Energy Wasted
by Devices
 Read pgs 339 - 342
 Page 343

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