Transcript P3a_Revision_lesson

OCR 21st Century Science
Unit P3a Revision
Sustainable energy
Energy Consumption
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The demand for energy is predicted to rise by a large amount
in the next few decades:
What issues will this rise in demand cause?
Fuels
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A “fuel” is something that can be burned to release heat and
light energy. The main examples are:
Coal, oil and gas are called “fossil fuels”. In
other words, they were made from fossils.
Some definitions…
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A renewable energy source is clearly one that can be _______
(“renew = make again”), e.g. _____, solar power, biogas etc.
A ___________ energy source is one that when it has been
used it is gone forever. The main examples are ____, oil and
gas (which are called ______ ____, as they are made from
fossils), and nuclear fuel, which is non-renewable but NOT a
fossil fuel.
Electricity is called a “________ source” because it is
converted from other forms – what would these forms be in
batteries, wind turbines and solar panels?
Words – non-renewable, coal, fossil
fuels, wood, renewed, secondary
Pollution
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When a fuel is burned the two main waste products are _____
dioxide and ________ dioxide.
Carbon dioxide is a _________ ___ and helps cause _______
_________. This is produced when any fossil fuels are
burned.
Sulphur dioxide, when dissolved in ________, causes ______
_____. This is mainly a problem for ___ power stations.
Nuclear power stations do not produce these pollutants
because they don’t ____ fossil fuels.
Words – sulphur, coal, global warming, carbon,
acid rain, greenhouse gas, rainwater, burn
Using Electricity
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Basically, electrical devices are used to transfer electrical
energy to the environment:
+
-
This light bulb will transfer
light and heat to the
surroundings.
Energy and Power
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The POWER RATING of an appliance is simply how much
energy it uses every second.
In other words, 1 Watt = 1 Joule per second
E
E = Energy (in joules)
P = Power (in watts)
T = Time (in seconds)
P
T
Some example questions
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1) What is the power rating of a light bulb that transfers
120 joules of energy in 2 seconds?
60W
2) What is the power of an electric fire that transfers
10,000J of energy in 5 seconds?
2KW
3) Rob runs up the stairs in 5 seconds. If he transfers
1,000,000J of energy in this time what is his power
rating?
0.2MW
4) How much energy does a 150W light bulb transfer in a)
one second, b) one minute?
150J,
9KJ
5) Jonny’s brain needs energy supplied to it at a rate of
40W. How much energy does it need during a 50 minute
physics lesson?
120KJ
6) Lloyd’s brain, being more intelligent, only needs energy at
a rate of about 20W. How much energy would his brain
use in a normal day?
630MJ
Power
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Power is “the rate of doing work”.
The amount of power being used in
an electrical circuit is given by:
Power = voltage x current
in W
in V
in A
P
V
I
We can use this equation to analyse power stations:
1) A transformer gives out 10A at a voltage of 50V. What is
its power output?
500W
2) An electric fire has a power rating of 2KW. If it runs on a
voltage of 230V what is the current?
8.7A
3) Electricity is transmitted along some lines in the National
Grid at 400KV. If the current is 1KA what would be the
power through the wire?
400MW
The Cost of Electricity
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Electricity is measured in units called “kilowatt hours” (kWh).
The kilowatt hour is a unit of energy but the Joule is too small
to count so we use the KWh instead. For example…
A 3kW fire left on for 1 hour uses 3kWh of energy
A 1kW toaster left on for 2 hours uses 2kWh
A 0.5kW hoover left on for 4 hours uses __kWh
A 200W TV left on for 5 hours uses __kWh
A 2kW kettle left on for 15 minutes uses __kWh
The Cost of Electricity
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To work out how much a device costs we do the following:
Cost of electricity = Power (kW) x time (h) x cost per kWh (p)
For example, if electricity costs 8p per unit calculate the cost
of the following…
1) A 2kW fire left on for 3 hours
48p
2) A 0.2kW TV left on for 5 hours
8p
3) A 0.1kW light bulb left on for 10 hours
8p
4) A 0.5kW hoover left on for 1 hour
4p
Reading Electricity Meters
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1) How many units of electricity
have been used?
2) If 1 unit costs 10p how much
has this electricity cost?
The 9 types of energy
Type
Heat
Kinetic (movement)
Nuclear
Sound
Light
Chemical
Electrical
Gravitational potential
Elastic potential
3 example sources
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The Laws of Physics
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There are many laws of physics, but one of the most important
ones is:
Energy cannot be created or
destroyed, it can only be converted
from one form to another
Energy changes
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To describe an energy change for a
light bulb we need to do 3 steps:
1) Write down the
starting energy:
2) Draw an arrow
Electricity
3) Write down
what energy types
are given out:
Light + heat
What are the energy changes for the following…?
1) An electric fire
2) A rock about to drop
3) An arrow about to be fired
Conservation of Energy
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In any energy change there is ALWAYS some “waste” energy:
e.g. a light bulb:
Electricity
Light
+
heat
In this example HEAT is wasted and it is transferred to
the surroundings, becoming very difficult to use.
Describe the following energy changes and state the “waste”
energy or energies:
1) A vacuum cleaner
2) A TV
3) A dynamo/generator
Efficiency
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Efficiency is a measure of how much USEFUL energy you
get out of an object from the energy you put INTO it.
For example, consider a TV:
Electrical
Energy (200J)
Sound (40J)
Efficiency = Useful energy out
Energy in
x100%
Some examples of efficiency…
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1) 5000J of electrical energy are put into a
motor. The motor converts this into 100J of
movement energy. How efficient is it?
2) A laptop can convert 400J of electrical energy
into 240J of light and sound. What is its
efficiency? Where does the rest of the
energy go?
3) A steam engine is 50% efficient. If it delivers
20,000J of movement energy how much
chemical energy was put into it?
0.2 or
20%
0.6 or
60%
40KJ
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Energy Transfer (“Sankey”) diagrams
Consider a light bulb. Let’s say that the bulb runs on 100
watts (100 joules per second) and transfers 20 joules per
second into light and the rest into heat. Draw this as a
diagram:
“Input” energy
100 J/s
electrical
energy
“Output” energy
20 J/s
light energy
80 J/s heat
energy (given to
the surroundings)
Example questions
Consider a kettle:
2000 J/s
electrical
energy
Sound
energy
Wasted
heat
Heat to
water
1) Work out each energy value.
2) What is the kettle’s
efficiency?
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Consider a computer:
150 J/s
electrical
energy
10 J/s
wasted
sound
20 J/s
wasted
heat
Useful
light and
sound
1) How much energy is converted
into useful energy?
2) What is the computer’s
efficiency?
Reducing Energy Usage
How can we reduce
energy usage?
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