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Energy Conservation & Dissipation (Energy & Power (E = Pt :star:…
Energy Conservation
& Dissipation
Energy store
changes
Energy Stores
Chemical
:
fuels, food batteries (lithium)
Kinetic
:
Energy obj has from
moving
GPE
:
Obj energy from
position
(above ground)
Elastic Potential
:
Spring energy from
squash/ stretch
Thermal
:
Energy obj has from
temperature
Energy can be trans from one store
to another
Falling
GPE
decreases
transferred to
KE
KE
increases
falls faster, hits ground
KE trans to
sound
,
heat
Battery
Chemical trans to
electric
Electric current in wires
Bulb trans
electric
into
light
and
heat
energy
Conservation
of Energy
Energy can't be created or destroyed
:star:
System
(Group of) objects
Closed System
No net change
to energy stored in the system
Energy loss
Heat loss; air resistance
friction
Bungee Jumping
GPE trans to
KE
Falls faster
KE trans to
Elastic
Rope tightens until stop
and
reverse
,and
repeat
Energy
& work
W = Fs
:star:
Energy transferred
= work done
(Nm)
Friction
Main transfer to heat
Rub hands, warm up
Brake pads get hot
Atmosphere enter burn
GPE
stores
Ep = mgh
:star:
Lifting
Chemical in muscle
Trans to
KE
, to
GPE
in object
Lifting = GPE increase
Force
needed to lift
Equal, Opposite to weight
GPE change = Work done
Weight = mg
Elastic, KE
energy stores
Ek = ½mv2
:star:
Height proportional
to speed
squared
Ee = ½ke2
:star:
F = ke
k =
spring constant
Energy
dissipation
Useful energy
:
Energy trans
where it's wanted, way it's wanted
Wasted energy
:
Energy
not usefully transferred
dissipated
Jet
Chemical trans to:
GPE, KE - useful
Heat, Sound - waste
Friction
In machines always causes waste
Brake, Exercise
Gym machine, Drill
Spreading out
Waste
dissipates
Useful dissipates
eventually
More spread out = less
useful
Energy &
Efficiency
Efficiency
Useful output
trans by device
÷
Total input
supplied to device
No
device is
>100%
you would create energy
Improvement
Friction =
Lubricate
Resistance = diff
wires
Air resist =
Streamline
Sound = cut noise (
tighten
)
Electrical appliances
Home energy most supplied by Gas, Oil,
Electricity
Electricity most dominant
Appliance
design purpose
;
Waste
little energy as possible
Light bulb
:
Light \ Heat (filament)
Electrical Heater
:
Heat \ Light (glowing)
Electric Toaster
Heat \ Excess heat (case)
Hairdryer
KE, Heat \ Sound, Excess heat
Electric motor
KE, GPE \ Heat, Sound
Clockwork radio
No electricity need
GB;
Trevor Baylis
Early
1990s
Wind spring
(store)
Unwinds =
KE
; turns
small electric
generator
Energy
& Power
E = Pt
:star:
More power
= faster rate of energy transfer
Orders of Magnitude:
Estimate (~) to nearest power of 10
-10,000 MW = ~ 10^4 MW
300N lift in 0.5s
300N = 300J
P = 300 / 0.5
P = 600J/s
P = W (J/s)
Waste P = Total P - Useful P