Transcript Conservation of Energy

Conservation of Energy
Energy is the ability to do work. When work is done energy is
transferred . Energy can only be turned from one form to another.
Energy cannot be created or destroyed.
Consider a car running down a slope :
At the top of the slope the
car has potential energy. As
it runs down the slope the
potential energy ( Ep ) gets
transformed into kinetic
energy. (Ek)
Conservation of energy 2
If there are NO frictional forces to overcome then
Ep at top = Ek at bottom of slope
( Expts on this Loop the Loop, Car running down ramps)
In the world outside NAT 5 Physics frictional forces act and so work
must be done to overcome friction and there is always some
‘waste’ heat and sound energy produced
Coal Power Stations( Revision )
Heat exchanger
Burning coal
In boiler
Turbine
generator
Coal Power Station
Chemical Energy
Heat Energy
( burning coal )
Heat Energy
Kinetic Energy
( steam moving through pipes )
Kinetic Energy
Electrical Energy
( generator turning )
Hydro - Electric Power Station
Water has
potential
energy
Water flows
through
pipes
simulation
Hydro - Electric Power Station
• Potential Energy
Kinetic Energy
( water flowing through pipes)
• Kinetic Energy
Electrical Energy
( turbine turns generator )
Pumped Storage schemes
Energy
Pumped Storage schemes
During periods of peak demand they operate as conventional
hydro electric stations.
During early hours of morning the excess electrical energy
generated by the coal and nuclear stations is used to pump water
back behind the dam. The pumps are inefficient. This allows the
coal and nuclear stations to be run at peak efficiency 24 / 7.
Expt to determine efficiency of pump storage
schemes.
Energy
Generation of electrical energy
There must be a changing magnetic field
about a conductor to induce a voltage
across it:
Move a magnet towards and away from a
wire Or move the wire so that the
magnetic field changes
Energy
The size of the induced voltage can be increased
by :
1. Increasing the number of coils of wire
2. Increasing the magnetic field strength
3. Increasing the rate at which the field is cut
North pole
wire
South pole
As the wire is moved up / down an
a.c. current / voltage is induced
Model generator
Voltage is induced across stator coils.
Working Generator
The rotating coil is an electromagnet unlike the permanent
magnet in a model generator. There is a voltage induced
across the fixed coils ( stator coils ).
To increase the size of the induced current the current
flowing through the electromagnet is increased. If the coil
spins faster then a bigger voltage is induced but the
frequency also increases.
The stator coils are fixed. The
voltage is induced across these.
The rotor is a spinning
electromagnet.
Heat Energy
Temperature of a substance is a measure of the mean
kinetic energy of its particles.
Units : Degrees Celcius, oC
Heat is a form of energy. The heat stored in a substance is
the total kinetic energy of all its particles.
Units are Joules, J.
100cm3 of water at 1000C contains more heat energy than
10cm3 water at the same temperature.
Heat energy flows from areas of high temperature to low
temperature.
Heat Energy
Specific Heat Capacity
The energy required to raise the temperature of
1 kg of a substance by 1 0C.
The same quantity of energy is released if 1 kg cools down
by 10C.
Units are J kg-1 0C-1
Water has the highest specific heat capacity :
4180 J kg-1 0C-1.
i.e. 4 180 J of energy are required to raise the temperature
of 1 kg of water by 10C.
Eexpts
on c of water and metal blocks.
Heat Energy
Eh  c. m. T
Eh = heat energy : J
C = specific heat capacity : J kg-1 0C-1
M = mass : kg
 T = change in temperature : 0C
Heat Energy
Calculate the final temperature if 20 kJ of heat
energy are absorbed by 10 kg of water at an
initial temperature of 20 0C.
Eh = 20 kJ
m = 10 kg
= 20 000 J
c = 4180 J kg-1oC-1
Eh  c. m. T
Eh
 T 
c.m
Heat Energy
Working
20000
T 
4180 x 100
T  0.05 C
0
Change of State
When a material changes state there is NO
temperature change.
Temp
(oC )
Liquid to gas
Solid to liquid
Time ( s )
http://www.bbc.co.uk/learningzone/clips/latent-heat-and-changes-ofstate/4551.html
Expts on cooling by evaporation and measure latent heat of fusion /
vapourisation of water
Change of State
1. When we sweat the liquid water evaporates,
the energy for this comes from our skin and
we cool down.
2. Ice packs in cool boxes melt. The energy
for this comes from the air and food in the
cool box and it cools down.
3. Coffee makers inject steam into cold milk.
The steam turns into liquid giving out
energy which is absorbed by the milk,
raising its temperature
http://www.bbc.co.uk/learningzone/clips/what-energy-changes-happenduring-freezing/8759.html
Change of State
Fridges :A refrigerant is pumped
around the ice box. It turns from
liquid into gas. The energy required
for this is taken from the food / air
and it cools down.
The refrigerant is pumped to the
back of the fridge and compressed.
When it turns from gas into liquid it
gives out energy.
The fridge is a heat pump. Energy is
transferred from inside the fridge to
the outside.
Change of State
Specific latent heat of fusion ,lf ,
The energy required to turn 1 kg of a solid at
its melting point into 1 kg of liquid at the same
temperature.
Units are joules per kilogram , J kg –1
Eh = m x lf
heat energy
fusion
mass
latent heat of
Change of State
Example
Calculate how much ice cream is melted at
0oC if it absorbs 400 kJ of heat energy.
Lf = 3.34 x 105 J kg-1 Eh = 400 kJ m =?
Eh  m x l f
400000  m x 3.34 x10
m  1.2 kg
5
Change of State
Specific latent heat of vapourisation , lv,
The energy required to turn 1 kg of a liquid at its
boiling point into 1 kg of gas at the same
temperature.
Units :Joules per kilogram, Jkg-1
Eh = lv x m
Heat energy
(J)
latent heat of vapourisation
( J kg -1)
mass
( kg )
Change of State
Example
An 12 V electric heater draws a current of
2.5A. It is immersed into a beaker of boiling
water. Calculate the mass of water turned into
gas if it is switched on for 5 minutes.
Lv = 2.26 x 106 J kg-1.
V = 12 V
I = 2.5 A
T = 5 mins = 300 s
Calculate power of heater then energy supplied
P = V x I = 12 x 2.5 = 30 Watts
E = P x t = 30 x 300 = 9000 Joules
This energy is used to turn the liquid into gas
Use Eh = m x lv
Eh = 9000 J
9000 = m x 2.26 x106
M=?
Lv = 2.26 x
106
J
kg-1
m = 4.0x10-3 kg or 4 g