Download Conservation of Energy

Conservation of Energy
Law of Conservation of Energy
• (p222) The law of conservation of energy
states that energy cannot be created nor
destroyed, but it can be changed from one
form to another
• Energy is changed from one form to another
when work is done on the object
• When work is done on an object to accelerate
the object, the kinetic energy of the object
changes & can be calculated by: W = Fd
or __E = Fnetd or __Ek = Ek2 - Ek1
• When work is done on an object to change
the vertical position the gravitational
potential energy of the object changes
and can be calculated by:
W = Fd
or __Ep = Fgh = magh
• When work is done on an object to
overcome friction the thermal energy
(heat energy) of the object increases and
can be calculated by:
Wf = Ffd or __TE = Ffd
• (p 223) The law of conservation of
mechanical energy states that in a
frictionless system mechanical energy is
conserved:
(add)
• If there is friction, then some mechanical
energy is converted to thermal energy:
(add)
Modify example problems p 223:
1.
2.
(add example 3 to bottom of p 224):
3. A 55 kg fat toddler slides down a slide that is
5.0m high. If his/her speed at the bottom is
2.9m/s how much heat was generated?
Power
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(p231) Power is the rate of doing work
Formulae: P = W/t or P = _E/t
Units: J/s or Watts (W)
“W” (Watts) should not be confused with “W”
(work)
• Power is scalar
• another useful equation: P = Fv from:
• See example problems 1 and 2 p 232
Efficiency
• Efficiency = work out
work in
• Efficiency = power out
power in
Thermal Energy
• (p 216) When work is done to accelerate an
object the kinetic energy increases
• When work is done on an object to change the
vertical position the gravitational potential
energy changes
• When work is done on an object to
overcome friction the thermal energy of the
object increases
• When the thermal energy of an object
changes the temperature of the object
increases
• Thermal energy is kinetic energy at the
molecular level
• Heat is the energy that is transferred from
a warm object to a cooler one
• Symbols: __Eh or
Q
Specific Heat Capacity
• (p 217) Specific heat capacity is defined
as the amount of heat that a unit mass of
substance can gain or lose in order to
change its temperature by one degree
• Symbol: c
• Units:J/kgoC
• Different substances have different
capacities to hold heat and therefore
different specific heat capacities
• Water:
• Copper:
• Aluminum
c = 4184 J/kgoC
c = 390 J/kgoC
c = 190 J/kgoC
• The heat gained or lost by a substance
depends on:
mass of the substance (m)
temperature of a substance (__t)
specific heat capacity (c)
Formula:
Specific Heat Capacity
Example problems (p217)
1.
2.
3.
• (add to margin of p 217) When a substance
reaches its boiling point or melting point,
heat energy must be added or removed for
a phase change to occur
• This energy does not change the
temperature of the substance but it does do
work to change the distance (potential
energy) between the particles:
solid
liquid
gas
• (add in the margin of p 218) During a phase
change there is no temperature change so a
new formula is required:
__E = __H m
•
•
•
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Vaporization involves liquid gas transitions
Fusion involves solid liquid transitions
See p 254 table 12.2
See example 1 p 254 of Merrill text
Multi-Step problems
• Sometimes an object undergoes both a
temperature change and a phase change:
(see example 2 p 255 of Merrill text)