GRADE 7
NATURAL SCIENCES
ENERGY AND CHANGE
MEMO
WORKBOOK
CONTENTS
Sources of energy
Introduction
Where does energy come from? How is electricity actually produced? Renewable and non-renewable energy sources
Non-renewable energy
Renewable energy Hydropower
Wind energy
Biomass energy
Geothermal energy
Nuclear energy
Solar energy
The sun is the ultimate source of energy
Advantages and disadvantage of renewable and non-renewable energy
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Potential and kinetic energy
Introduction
Potential energy
Elastic potential energy
Gravitational potential energy
Water reservoirs
Chemical potential energy
Energy is measured in joules
Homework 13
Kinetic energy
Potential and kinetic energy in systems
Mechanical systems
Thermal systems
Electrical systems
Biological systems
Homework 14
The law of conservation of energy
Energy cannot be created or destroyed
Energy can be transferred
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Heat transfer
Introduction
Heat as transfer of energy
Homework 15
Conduction
Homework 16
Experiment: Heat energy transfer in different materials
Convection Let’s label the diagram:
Radiation
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GRADE 7
TERM 3
NATURAL SCIENCES
ENERGY AND CHANGE - MEMO
MODULE 3:
ENERGY AND CHANGE
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Sources of energy
Introduction
Why do we need energy? We need energy to light our homes, power our cars, cook our food and to be
able to walk, think and write. Everything we do is connected to energy.
Energy is defined as the ability to do work.
We use energy to do WORK, for example when we lift something, or warm it up, or light something. As
you can see, energy is an essential part of our daily lives.
Where does energy come from?
There are seven forms of energy. Forms of energy are types of energy. These are:
•
Electrical energy, e.g. from lightning or a light bulb
•
Light energy, e.g. from the sun
•
Heat energy, e.g. when we warm up food in the oven
•
Chemical energy, e.g. from batteries
•
Sound energy, e.g. the sound of a buzzing bee
•
Mechanical energy, e.g. when the fan turns
•
Nuclear energy from unstable atoms that split.
Forms of energy can be converted from one form to another.
There are many sources of energy. Sources of energy is where the energy comes from. Energy can come
from millions of different places. We are only going to learn about the following sources of energy that
produce electrical energy:
• Fossil fuels (coal, oil, natural gas)
• Hydropower
• Wind energy
• Biomass energy
• Geothermal energy
• Nuclear energy
• Solar energy
In all these instances the aim is to make electricity. So we need to convert a form of energy into
electrical energy.
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How is electricity actually produced?
Regardless of the source of energy, electricity is produced in the same way. Electricity is produced by
turning turbines.
The way a turbine looks will vary depending on the source of the energy.
Here are some examples of turbines:
Turbine in coal-fired power station
Turbine for wind power
Turbine for hydropower
Turbine for nuclear power plant
It all works on the same concept: Energy is required to turn the turbines to produce electricity.
Most power plants today burn coal, which is used to heat water and produce steam. The steam is the
energy that turns the turbine.
In the case of wind energy, the wind provides the energy that turns a turbine.
In the case of hydropower, flowing water provides the energy that turns a turbine.
The spinning turbine causes large magnets to turn and release electrons, pushing them into electrical
wires. This is known as electricity.
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Renewable and non-renewable energy sources
All energy sources can be categorised as either renewable or non-renewable.
Non-renewable energy
What is non-renewable energy?
Energy that gets used up and cannot be replaced
Fossil fuels are non-renewable.
Fossil fuels include coal, oil and natural gas.
Fossil fuels are formed from the remains of living things, including plants and animals.
How are fossil fuels created?
When plants and animals die, they are gradually buried by layers of rock. These buried remains are put
under pressure and heat. Certain chemical reactions take place. Over time, they turn into fossil fuels,
where chemical energy is stored.
Plants and animals die and get buried all the time. So why do we consider fossil fuels a non-renewable
energy source?
Reserves are being depleted much faster than new ones are being made
When we burn fossil fuels, chemical energy is transferred into heat energy and light energy. The light
energy is not used, but the heat energy is used to boil water. The boiling water forms steam. The steam
then turns the turbines in the form of mechanical energy. This generates electricity in the form of
electrical energy.
Most of the electricity that we use today comes from fossil fuels. Why is this not a viable source of energy
for the future?
It takes very long to form, making it non-renewable.
The reserves will run out – scientists believe in 50 to 100 years.
With time, it will get more and more expensive because it will become harder to tap into the deeper
reserves of coal, oil and gas.
Renewable energy
What is renewable energy?
Energy that does not run out and can be replaced
Here are the most common types of renewable energy and how they work:
Hydropower
Moving water has mechanical energy. Therefore, mechanical energy in the moving water is converted
into mechanical energy in the turning turbine, which is converted into electrical energy.
In order to generate electricity from the mechanical energy in moving water, the water has to be moving
with sufficient speed and volume to turn a turbine.
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1. Wave machines use the
up-and-down movement
of waves to turn turbines.
Wave hitting a wave machine that contains a turbine
2. Tidal barrages use the
energy of the water as
the tide turns to turn
turbines.
Turbines that turn as the tide turns
3. Hydroelectric power
schemes store water high
up in dams. When the
water falls, turbines are
turned.
Hydroelectric power plant
Wind energy
Wind is caused by currents in the earth’s atmosphere. This moving air has huge amounts of mechanical
energy. Therefore, mechanical energy in the wind is converted into
mechanical energy in the turning turbine, which is converted into electrical
energy.
Wind turbine
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Biomass energy
Biomass fuels come from living things. Crops and wood are biomass fuels.
For example, what do we get when we burn wood? Heat and light
We can therefore turn heat and light energy into electrical energy. This works in much the same way as
when burning fossil fuels:
The chemical energy is transferred into heat energy and light energy. The light energy is not used, but
the heat energy is used to boil water. The boiling water forms steam. The steam then turns the turbines
in the form of mechanical energy. This generates electricity in the form of electrical energy.
This is the same process as when coal is burnt. So what is the difference, in other words, why is biomass
energy renewable while fossil fuels are non-renewable?
As long as we continue to plant crops and trees and to replace those we cut down, we will always have
biomass fuel.
Geothermal energy
In some places, rocks underground are hot. Think of natural hot
springs. These are warmed up by hot underground rocks. These
hot rocks are called magma. Magma feeds volcanoes.
Deep wells are drilled and cold water is pumped to the magma.
The water runs through fractures in the rocks and heats up. It
returns to the surface as hot water and steam. The steam is then
used to turn turbines.
Therefore, heat energy in the magma and steam is transferred to
mechanical energy in the turning turbine, which is transferred into
electrical energy.
Hot rocks/magma
Nuclear energy
Certain atoms are radioactive, which means that they have
an unstable nucleus. This means that the atom splits in two.
When it splits, it releases a tremendous amount of energy.
This energy is then used to heat water. The boiling water
forms steam. The steam then turns the turbines to generate
electricity.
Therefore, nuclear energy is transferred into heat energy in
steam, which is transferred into mechanical energy in the
turning turbine, which is transferred into electrical energy.
Nuclear power plant
Solar energy
There are two types of solar energy: solar cells and solar panels.
Solar cells: Solar cells convert light energy from the sun directly into
electrical energy. This is used for example in a solar calculator. It is
also used in larger applications in industry and in homes to provide
electricity.
Solar cells
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Solar panels: Solar panels are different to solar cells because they
convert light energy into heat energy. It is often found on roofs of
buildings where the most sunlight is received. They then heat water
in the geyser or the water of a swimming pool. This results in less
electricity being used.
How is solar energy different to the other types of energy that we
have learnt about?
No turbine is turned to generate electricity.
Solar panels
The sun is the ultimate source of energy
We are not talking about solar energy here!
We use many different forms of energy, and almost all of them originate from the sun.
The sun does not only provide light and heat energy. Energy from the sun may change into many
different forms. Let’s discuss a few.
How do fossil fuels originate from the sun?
Fossil fuels come from dead plants and animals. Without the sun, plants would not exist and in turn
animals would not exist.
How does wind power originate from the sun?
Heat energy from the sun causes changing weather patterns that produce wind.
How does hydropower originate from the sun?
Water flows in rivers because heat energy from the sun causes evaporation and rain that keeps water
moving through the water cycle.
Advantages and disadvantage of renewable and non-renewable energy
What are the main DISADVANTAGES of non-renewable energy?
1. Non-renewable energy will eventually run out.
2. Non-renewable energy emits many pollutants into the atmosphere and hence is associated with
negative environmental impacts.
What are the main ADVANTAGES of non-renewable energy?
1. Non-renewable energy is a reliable source of energy.
2. Large amounts of electricity can be generated from non-renewable energy.
3. Non-renewable energy is relatively cheap.
What are the main ADVANTAGES of renewable energy?
1. Renewable energy will never run out.
2. Renewable energy is less detrimental to the environment than non-renewable energy.
What are the main DISADVANTAGES of renewable energy?
1. It is difficult to generate the quantities of electricity that we need using renewable energy.
2. Renewable energy is relatively expensive to start up.
3. Renewable energy often relies on the weather, making is less reliable than non-renewable energy.
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Potential and kinetic energy
Introduction
We need energy for everything that we do. What is energy?
Energy is the ability to do work.
Energy cannot be created from nothing, and energy cannot disappear or be destroyed. Energy can
change from one form to another.
Potential energy and kinetic energy are different types of energy that can be transferred (changed).
Potential energy
What is potential energy?
Potential energy is energy that is STORED in an object.
It means that the energy is not used right now, but it can be used to do work later.
There are different types of potential energy:
• Elastic potential energy
• Gravitational potential energy
• Chemical potential energy
Elastic potential energy
What is elastic potential energy?
Energy that is stored in elastic material when we stretch it or compress it.
The more we stretch a spring, the higher the elastic potential energy.
If you compress a spring, it also has elastic potential energy because when you let go, the spring sections
will separate. Therefore, the more we compress a spring, the higher the elastic potential energy.
If you stretch a rubber band and then release it, it will fly across the classroom. This tells us that potential
energy was stored in the elastic when it was stretched.
If you stretch it even more, it will fly even further when let go, meaning that the potential energy that
was stored in it was stored in it was higher..
Gravitational potential energy
What is gravitational potential energy?
Energy that is stored in an object because of its position above the earth.
The earth attracts all objects towards its centre with the force of gravity. If a book lies on a table, it has
gravitational potential energy because of its position above the ground. If the book falls off the table and
hits a glass standing on the floor, the glass would fall over and maybe even break. This shows us that the
book had stored energy when it was lying on the table.
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Water reservoirs
Water reservoirs are built to provide water to a certain
area. They are built uphill from the houses in an area.
In this way, the stored water has enough gravitational
potential energy to flow downwards to the houses that
they supply.
Depending on how much higher the reservoir is from
the houses, the water pressure will vary from high flow
to just a trickle.
Water reservoir in Tennessee, USA
If the reservoir is much higher than the position of the
houses, there will be high water pressure. If the reservoir is only a little higher than the position of the
houses, there will be low water pressure.
When a reservoir is built lower than the houses or when water is taken out of a borehole, the water has to
be pumped up to the houses. This is because the water below the level of the houses does not have
gravitational potential energy.
Chemical potential energy
What is chemical potential energy?
Energy that is stored n particles of substances such as food and fuel.
In order to release this energy, a chemical reaction needs to take place.
An energy drink has chemical potential energy. When athletes drink it, their bodies digest it (chemical
reactions occur) and they get more energy to run their race.
You can find the amount of chemical potential energy in any food by looking at the food label.
A battery also has chemical potential energy stored in it. When a battery is inserted into a torch, the
torch can be switched on. This is because a chemical reaction has taken place inside the battery that has
allowed the light bulb to light up.
Another example is wood or coal. When wood or coal are burnt, they release their stored chemical
potential energy in the form of heat and light energy.
Energy is measured in joules
All energy is measured in units called
joules (J). The energy content in food is labelled
on the food packaging. Food packaging shows the
amount of energy stored in the food in kilojoules
(kJ) per 100 g (1 kJ = 1 000 J). In the case of drinks,
it shows kJ per 100 ml. Therefore, we can see how
much chemical potential energy the food contains.
Based on the diagram below, how many kJ of
energy are there in 100 ml of fruit juice?
224 kJ.
Nutritional information on a bottle of fruit juice
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Homework 13
1. Look at the following pictures and decide what kind of potential energy it is.
Elastic potential energy
Chemical potential energy
Elastic potential energy
Gravitational potential energy
Chemical potential energy
Elastic potential energy
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2. Look at the following food labels and identify the amount of energy that each food contains
per 100 g. List the food and energy value in ascending order.
White bread
Milo cereal
Low-fat yogurt
Biscuits
Low-fat yogurt 256 kJ
White bread 985 kJ
Milo cereal 1 620 kJ
Biscuits 2 159 kJ
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Kinetic energy
An object has potential energy when it is at rest.
What is kinetic energy?
Kinetic energy is energy that an object has because it is moving
or being used.
When a car moves forward, it gains kinetic energy. The air in
blowing wind has kinetic energy. Water that falls in a waterfall has
kinetic energy.
If you let go of a stretched rubber band, it flies through the room.
The elastic potential energy changed into kinetic energy.
In the last section we learnt about seven forms of energy. What were they?
Electrical energy, light energy, heat energy, chemical energy, sound energy, mechanical energy,
nuclear energy
What form of kinetic energy specifically does the flying elastic band have?
Mechanical
Potential and kinetic energy in systems
What is an energy system?
An energy system consists of different parts that work together to store, use or give out energy so that
a specific task can be done.
When different parts of the system interact, energy is transferred to another form. The energy transfer
causes a change, and a specific task can be done.
Again, we learnt about seven different forms of energy, but here were are going to focus on four energy
SYSTEMS:
• Mechanical systems
• Thermal systems
• Electrical systems
• Biological systems
Mechanical systems
A mechanical system uses force to do a specific task. We use machines or tools to do work for us. We can
lift a heavy box with a crowbar or hit a cricket ball with a bat. If the box or the cricket ball move, they have
mechanical kinetic energy.
Potential energy can be changed to kinetic energy in a mechanical system. Take, for example, a plastic
ruler that is bent. The ruler has elastic energy when it is bent. As soon as you let go of the ruler, it moves
back into place. It has kinetic energy.
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Experiment:
1.
2.
3.
4.
Place a bottle top on a desk.
Bend a ruler a little bit.
Release the ruler so that it hits the bottle top and then moves across the desk.
Repeat, but this time bend the ruler a lot.
What did you observe in the two instances?
When we bent the ruler a lot, the bottle top moved much further than when we bent the ruler a little
bit.
Explain your observation.
When we bent the ruler a little bit, there was only a small amount of elastic potential energy in the ruler
and in turn there was only a small amount of mechanical kinetic energy in the bottle top.
When we bent the ruler a lot, there was a large amount of elastic potential energy in the ruler and in
turn there was a large amount of mechanical kinetic energy in the bottle top.
Thermal systems
Thermal energy is produced by heat. Heat energy is transferred between different parts of a thermal
system. When a substance it heated, the particles move faster. The movement causes the temperature of
the substance to rise.
Hot water in a cup that is placed on a table eventually becomes cold. Where does the heat go and why?
It goes to the surroundings because the surroundings are cooler than the water in the cup.
Experiment:
1. Pour hot water into one cup and cold water into another cup.
2. Add two drops of food colouring to each.
3. Observe the movement of the food colouring in both cups.
What did you observe in the two instances?
The colour spread faster through the hot water than the cold water.
Explain your observation.
The mechanical kinetic energy of the particles increases with heat. Because the particles move faster
in the hot water, the colour gets shifted around faster.
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Electrical systems
If a battery is connected to a closed circuit, electric current will flow. The battery stores
chemical energy. Chemical reactions inside the battery supply charges that can move
through electrical wires. Electric current is the movement of charges inside electrical
wires. Because the charges move, they have kinetic mechanical energy. We cannot see
the charges move, but we can observe the effect of an electric current.
Biological systems
All living organisms require energy for their vital processes, e.g. respiration and
digestion..
Where does biological energy originate? The sun
Where is biological energy stored? In plants and animals
In what form of energy is biological energy stored? Chemical potential energy
Energy that comes from the sun is converted into chemical energy by plants in the process of
photosynthesis. Other animals eat the plants or they eat animals that have eaten the plants.
A horse eats grass. What energy is stored in the grass that the horse ate?
Chemical potential energy
What type of energy is stored in an energy drink? Chemical potential energy.
What happens to this energy when it reaches the body of the athlete?
It gives energy to the athlete so he/she can run. This means that the chemical potential energy is
converted into mechanical kinetic energy.
Homework 14
Complete the table below. The first row is an example:
Example of an energy system
A man running
Is it a mechanical, thermal,
Identify the energy transferred
electrical or biological system? in the system
Chemical potential energy inside
Biological
the body of the man to mechanical
kinetic energy in his legs
A cat drinking milk
A pair of scissors cutting a
piece of paper
An electric fan that’s turning
A hot stone placed in cold
water
A windmill that’s turning
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The law of conservation of energy
What does the law of conservation of energy state?
Energy cannot be created or destroyed. It can only be transferred from one system to another or
converted from one form to another.
We learnt that energy is measured in joules. The total number of joules at the beginning and at the end is
the same. This is demonstrated in the diagram below. What does the diagram show?
3 600 J of electricity is supplied to the bulb. Electrical energy is converted to light energy (360 J) and
heat energy (3 240 J). 3 240 J + 360 J = 3 600 J.
You can see from the diagram above that no energy was created or destroyed. The total number of joules
at the beginning and at the end is the same.
Energy cannot be created or destroyed
Think of money as an analogy. When you use money, it doesn’t disappear. It is only transferred to another
person, who can then use it. If you don’t have any money, you cannot create it from nothing. In the same
way, energy does not disappear when we use it. It goes somewhere else where it is used in a different
form. It also cannot be created from nothing.
If you lift up an object, it gains gravitational potential energy. The higher you lift it, the the more
gravitational potential energy it gains. When you release it and it drops, the mechanical kinetic energy
will equal the gravitational potential energy that the object had before it was dropped.
Energy can be transferred
Consider an electric doorbell. When you press the button, electrical energy is transferred to the bell and
the bell makes sound energy.
When the electric windows of a car open and close, what type of energy is transferred into what type of
energy?
Electrical energy into mechanical energy
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Heat transfer
Introduction
Everything is made up of matter. Matter is made up of tiny particles called atoms. The particles are always
moving, meaning that they have mechanical kinetic energy. This movement generates heat energy. Even
when something is very cold, it generates a small amount of heat. Why?
Because of the particles that are always moving
Heat energy can be transferred from one place to another by:
•
Conduction in solids, liquids and gases.
•
Convection in liquids and gases.
•
Radiation through anything that will allow radiation to pass.
Heat as transfer of energy
What is energy transfer?
When energy moves from one place to another place.
What is another name for heat energy? Thermal energy
Thermal energy is energy that is produced by heat. There is heat transfer when there is a temperature
difference in two places.
Does heat energy transfer occur from a hotter place to a cooler place or from a cooler place to a hotter
place?
Hotter place to a cooler place
When will the heat energy transfer be complete?
When both places have the same temperature
So heat transfer is the movement of heat from a hotter place to a cooler place until the entire system is at
the same temperature.
When you touch something hot, such as hot water, you feel the heat burning you. This is because the
hot water transfers heat energy to your hand. Your hand is cooler than the hot water, so heat energy is
transferred from water to your hand..
If you put a frying pan on a stove, the pan will be hot in a few minutes. Why?
Because the heat is transferred from the stove to the pan until they are at the same temperature.
If you now put a block of butter into the hot pan, the butter will melt because the heat energy is now
getting transferred from the hot pan to the cold butter.
When it is a cold day, we feel cold because our body temperature is hotter than the outside temperature.
Why doesn’t a lizard feel cold on a cold day?
Because a lizard is cold blooded, so its body temperature is the same as the outside temperature.
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Homework 15
How is HEAT energy transferred in the each of the following pictures? Explain in full. The first one is an
example:
Sharpening a pencil
When you sharpen a pencil, the friction between the sharpener and the pencil produces heat. The heat is
transferred to both the pencil and the sharpener.
Using a hairdryer
A hair dryer produces hot air that is transferred to your wet hair in order to dry it quickly.
Using an iron
An iron has a hot base that let off steam. This hot steam moistens and heats up the clothes, which
allows them to straighten when the pressure of the iron is applied.
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Drinking a cup of hot coffee
For just a few seconds, a hot cup of coffee transfers heat to your hands, lips, mouth, oesophagus and
stomach. (Our body maintains a stable body temperature so the effect does not last long.)
Burning wood in a fireplace
Burning wood produces heat, which then transfers the heat to the entire room.
Conduction
What is conduction?
It is the transfer of heat within an object or between objects that are in direct physical contact with
each other.
In conduction, the heat energy travels from the source of heat through the object, or from the warmer
object to the cooler object until they have the same temperature.
In the diagram below, one part of the object is hot, and the other part is cold. The heat energy will
transfer from:
the hot side to the cold side until the entire object is at the same temperature.
The diagram below is an example of conduction between two solid objects – the hot plate of the stove
and the pan. The heat energy will transfer from from the hot plate to the pan until both objects are at
the same temperature.
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You experience heat transfer by conduction whenever you touch something that is hotter or colder than
your skin, e.g. when you wash your hands in hot water or in cold water.
When you wash your hands in hot water, the heat energy from the water gets transferred to your hands.
When you wash your hands in cold water, the heat energy from your hands gets transferred to the water.
The form of heat energy is always from hot to cold..
The diagram below is an example of conduction. The heat travelled from the hot plate of the stove, to
the pot and the pot’s handle..
Then when you touch handle, some of the heat is transferred to your hand..
We usually refer to conduction as heat transfer between solids. However, conduction may also take place
between a solid and a liquid, a solid and a gas, a liquid and a gas, two gases or two liquids. For example,
when you make a hot cup of coffee, heat energy is transferred from the hot water to the mug and the
spoon..
This is a good example of conduction between a solid and a liquid.
On a cold day, we don’t want the heat energy to be transferred from our body to the surroundings. We
therefore wear clothes that are thick and made of materials that slow down the transfer of energy..
When you remove food from the oven, you use oven gloves in order to prevent the heat of the oven dish
from transferring to your hands.
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Homework 16
In the following pictures, use an arrow to show the direction of the heat transfer through conduction.
Walking on hot sand
Using an iron
Holding a hot mug of tea
1. Name three examples of heat energy transfer through conduction in the home that have not been
discussed in this handout.
When you have a bath, the heat transfers from the water to your body.
When you use the oven, heat is transferred from the oven to the oven dish to the food that is being
cooked.
When you work on the computer, heat is transferred from the computer to the surroundings.
2. Name three examples of heat energy transfer through conduction in industry that have not been
discussed in this handout.
Gold is melted to make gold bars.
To make steel, different metals are melted and mixed together.
In coal-fired power plants, cooling towers are used to condense steam back into water.
Experiment: Heat energy transfer in different materials
Materials: ceramic cup, glass cup, polystyrene cup, plastic cup, metal cup
Methods:
1. Fill each cup halfway with boiling water.
2. Wait 2 minutes.
3. Touch each cup.
What did you observe?
The metal cup feels the hottest. The polystyrene cup feels the least hot.
As you can see from the experiment above, some materials transfer heat energy faster and more
effectively than others. We say that such materials are good conductors of heat.
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GRADE 7
TERM 3
NATURAL SCIENCES
ENERGY AND CHANGE - MEMO
For example, metals are good conductors of heat. However, some metals are better conductors of heat than
other metals. Aluminium is an excellent heat conductor, which is why we use it to make pots and pans.
Some substances, such as plastic, are poor conductors of heat. We call such materials heat insulators.
Insulators are used in situations where we don’t want the heat energy to be conducted. For example, the
pot’s handle should not conduct heat, or else we will burn our hands.
Heat insulators are also important to prevent heat energy from being wasted by being lost to the
surroundings. For example, people have their geyser covered in a geyser blanket, which is a heat
insulator and slows down the heat loss from the geyser. This in turn helps to save electricity.
Convection
We already learnt that all matter is made of tiny particles, and that those particles are always moving.
Look at the diagram below. The particles that make up a solid are really close together with hardly any
spaces between them. The particles that make up a liquid are further apart. The particles that make up a
gas are much further apart.
Where do particles move around most freely – in a solid, a liquid or a gas?
They move around most freely in a gas, and less freely in a liquid. There is the least movement in a
solid.
We refer to gases and liquids as fluids.
What happens to the particles of the fluid as the fluid is warmed up?
The particles start to move around faster, they expand (meaning that they take up more space) and
they rise.
Consider the diagram on the next page. The particles of water at the bottom of the pot, closer to the heat
source, heat up first. Those particles now rise and take over the space of the colder particles, which are
forced to sink. This is how heat energy moves around in a liquid or gas.
Let’s label the diagram:
The hotter liquid rises
The colder liquid sinks
What is convection?
It is the transfer of heat through liquids or gases
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GRADE 7
TERM 3
NATURAL SCIENCES
ENERGY AND CHANGE - MEMO
This upward movement of the hot fluid and downward movement of the cold fluid forms a convection
current.
A heater is switched on in a small enclosed room. Explain how the room gets heated.
Hot air is generated by the heater. This hot air from the heater rises and forces the cold air down.
Convection currents forms, circulating the air, until it is at the same temperature.
Now you open the window, allowing cold air in. Look at the diagram below:
As you can see, the hot air from the heater rises. The cold air coming from the outside immediately rises.
The hot air escapes from the upper portion of the window. When the window is open, no convection
currents form in the room, so the room cannot heat up. The cold air accumulates at the bottom of the
room, while the hot air accumulates at the top of the room and escapes through the
window. This is a waste of electricity.
Radiation
What is radiation?
The transfer of the sun’s energy to the earth via electromagnetic waves.
This is what electromagnetic waves look like. They travel from
the sun to
earth at the speed of light in about 8 minutes.
Electromagnetic radiation is able to move through a vacuum.
What is a vacuum?
Empty space where there is no matter, not even gas
This is different to conduction and convection, which transfer
heat energy through matter.
An object will absorb the electromagnetic waves and become hot. This
means that electromagnetic radiation gets transferred into heat energy.
A dark object absorbs the electromagnetic radiation better than a light object.
That is why darker objects get hotter than lighter objects. Matt objects also
absorb radiation better than shiny objects. The diagram on the next page
shows that a matt black object will get hotter than a shiny white object.
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