Natural Sciences

Task 21

Grade: 7

Presented By: Mrs Young

Pitlochry Primary School Educator

Key concepts

Conduction – heat transferred within or between solid objectsthat are in direct contact with each other.

Convection – liquids and gases are heated by convectionRadiation – radiation can transfer heat energy even if there

is no solid object, liquid or gas to carry the energy.Heat – is the transfer of thermal energy from a system to its

surroundings or from one object to another as a result of a difference in temperature. Heat is measured in joules(J). This is because heat is a transfer of energy.

Temperature – is the measure of how hot or cold a substance feels and it is measured in degrees Celsius. Temperature is a measure of the average kinetic energy of the particles in an object or system. We use a thermometer to measure the temperature of an object or substance.

Summary – Energy • Potential energy is energy which is stored.• Kinetic energy is energy which an object has because it is moving.• Energy cannot be created or destroyed. It can only be transferred from

one part of a system to another.• Energy is transferred within systems. The input energy is transferred

through the system and energy is conserved.• There are various energy systems, such as:ØMechanical systemsØ Thermal systemsØ Electrical systemsØ Biological systems

• Energy is also transferred between different systems.

• Heat is energy that is transferred from a hotter to a cooler part.• Temperature is a measure of how hot or cold a substance is.• Heat (energy transfer) occurs in 3 ways; conduction, convection and

radiation.• During conduction, the objects must be touching each other for

energy transfer to take place.• Some materials such as metals conduct heat well.• Some materials, such as plastics and wood, slow down or prevent

conduction. They are called insulators.• Convection is the transfer of energy within liquids or gases.• Convection current refers to the movement of a liquid or gas during

energy transfer. The liquid or gas moves upwards from the heat source (as it expands) and then downward as the liquid or gas cools.

Summary – Energy Transfer• Heat is energy that is transferred from a hotter to a cooler object.• Temperature is a measure of how hot or cold a substance feels.• Heat(energy transfer) occurs in three ways: conduction, convection and

radiation.• During conduction the objects must be touching each other for energy transfer

to take place. • Conductors conduct heat well while insulators conduct heat poorly.• Convection is the transfer of energy through liquids or gases. • A convection current refers to the movement of a liquid or a gas during energy

transfer.• Radiation is the transfer of energy where objects do not have to be physically

touching. It does not need a medium and can take place in space.• Dark matt surfaces are good absorbers of radiant heat. Light, shiny substances

reflect more heat than they absorb.

Examples of convection

An eagle that uses updrafts of warm air. It glides higher and higher using very little movement of its wings. It is using convection currents.

Heaters are best placed on the floor because warm air moves upwards. Air conditioners are best placed near the ceiling because cold moves downward.

Summary – Heat Transfer• Heat is transferred by conduction, convection and radiation.• In some cases heat transfer is advantageous, but in others heat transfer

needs to be minimised or prevented.• Insulating materials are used to minimise heat loss and heat gain in systems.• Metals are good conductors of heat. Non-metals are good insulators of heat.

Non-metals are used as insulating materials.• We use insulators to keep our homes warm in winter and cool in summer.

This helps to conserve energy and electricity.• Indigenous homes in Southern Africa make use of insulating materials to be

energy efficient in our climate.

Summary – Energy transfer to surroundings 1

• Energy entering the system is called the input energy. • The energy is transferred in a system to provide a useful output energy.• Not all the input energy is transferred to a useful output. Some of the energy is wasted

or lost. The useful output is therefore less than the input energy as some of the output energy is wasted.

• The efficiency of a system is determined by how much of the input energy is transferred to useful output energy. The greater the wasted output energy, the less efficient the system.

• A Sankey diagram is used to show energy transfer in a system. • In a Sankey diagram the arrows represent the portion of the input energy which is

transferred to useful energy output and the portion which is transferred to the surroundings and wasted.

From coal to electricity1. The large chunks of coal are first crushed into a fine powder.

This is called pulverisation.2. The coal is transported to a furnace where it is burnt.3. The thermal energy from the burning coal is used to boil water

and generate steam.4. The steam pushes the blades of the turbine and the turbine

spins.5. The turbine is connected to the shaft of the generator which

then turns large magnets within wire coils, which generates electricity.

6. The electrical current is sent through the power lines to businesses and homes.

Summary – Energy transfer to surroundings 2

• The national electricity grid is a system in which the energy is conserved. It makes a complete circuit.

• In the power station, the coal is burned and steam is produced. The steam turns a turbine. The turbine turns a generator which produces electricity. This is transferred to the power lines in the national grid.

• Dynamos are a type of generator that can be used to produce small amounts of electricity, such as for a bicycle lamp.

• Electricity is expensive and we need to conserve energy to reduce our household costs.

• When fossil fuels are burnt they release greenhouse gases into the atmosphere. We need to reduce our electricity consumption to reduce pollution.

The Earths spins on its own axis each day.

The axis of the Earth is an imaginary line through the centre of the Earth running from the North Pole to the South Pole. The Earth spins or rotates on the axis like a spinning top. It takes 24 hours to go round once. This spinning causes day and night because one half of Earth is facing towards the Sun and is bathed in its radiation while the other half is in darkness.The Earth’s axis is not vertical. It is tilted at an angle of 23.5°. When the Earth has the South Pole pointing towards the Sun, it is summer in the south

Intensity of Solar radiation

The angle of tilt of the Earth does not change as the Earth moves in its yearly orbit around the Sun. In the southern summer the South Pole is tilted towards the Earth, the Sun appears higher in the sky in the southern hemisphere. When the South Pole is tilted away from the Sun, the Sun appears lower in the sky. When the southern hemisphere is tilted towards the sun the daylight hours are longer and the night time hours shorter.

Direct and oblique raysThe tilt of the Earth affects the amount of radiant energy that falls on one unit area of the Earth. Look at the diagram below. If the surface is facing towards the Sun the Sun’s radiation falls on a small area. If the surface is facing away from the sun the same amount of radiation falls on a much larger area. The direct rays have to pass through a thin layer of air while the oblique rays pass through a much thicker layer of air. This also affects the temperatures as the air absorbs some of the radiation.

It is this change that causes the four seasons.

The Sun’s energy sustains all life on Earth.

The Sun releases a huge amount of energy. While the Earth is orbiting the Sun , some of its radiant energy falls on it. The Sun’s energy does three important things.Heat energy from the Sun heats up the Earth so that it is suitable for us to live in.Radiant energy from the sun is used by plants during the process of photosynthesis. Energy is stored in the plants.Warmth from the Sun causes evaporation and start the water cycle which is essential to support life on Earth. Life could not exist on Earth without the radiant energy from the Sun.

Fossil fuelsOver millions of years dead bodies, which still contain their energy, are covered by mud and soil. As the layers of mud and soil thicken they press on the dead remains. More and more layers bury the layers further and the pressure increases. The dead plants and animals get squeezed, heated and crushed. They are called fossil fuels because they are rich in energy.Coal is usually made from the remains of large tree trunks and leaves that have fallen into a swamp millions of years ago. It is found in layers of rock. Oil is usually made from tiny water creatures that sank to the bottom of a swamp where they did not decompose. The process is similar to that for coal. It is usually found as a thick liquid trapped by rocks. Sometimes the remains are found as a gas trapped by rocks. The process is similar to that of oil except that the temperatures and pressures are greater while it is being formed.

Summary – Earth and beyond• The Earth revolves around the Sun completing one orbit every 365 and ¼ days. As

the Earth revolves around the Sun it also spins on its own axis completing one rotation in 24 hours.

• The Earth’s rotation axis is tilted in space. The axis is offset from the vertical by 23.5°.

• The tilt of the Earth’s axis is responsible for the seasons on Earth• Areas near the equator are warmer than areas near the poles because they

receive more direct light.• The Sun’s energy is captured and used by plants to produce carbohydrates which

the plant uses and stores. Plants form the basis of food chains.• The energy stored by plants millions of years ago is available to us today in fossil

fuels• Coal, crude oil and natural gas where formed millions of years ago from the

remains of dead animals and plants.• Life on Earth depends on the Sun’s stored energy in fossil fuels.

Key conceptsGravity – the pulling force exerted between two objects.

The gravitational force depends on the mass of the objects and the distance separating them.The strength of gravity on the moon is one-sixth that of the Earth.

Mass – the amount of matter in the object. Mass is measured in kilograms and is independent of where you measure it.

Weight – weight can change as it depends on the mass of the object as well as the strength of gravity acting on it.

Tides – the predictable, repeated rise and fall of sea-level on Earth.Spring tides – when the Earth, Sun and moon are in line, their combined

gravitational pull produces extreme high tides and extreme low tides which we call spring tides.

Intertidal zones – the zone found between the high and low shorelines.Shoreline eco-system – where the ocean or sea meets the land.

Gravity is the tendency of objects to attract each other. The Sun, the Earth and the Moon are kept in place by a force called gravity. The Earth has mass. It therefore attracts other masses and pulls on them by gravity. Gravity pulls us back down to Earth. The greater the mass of an object, the more gravity it causes. Earth is much larger than the Moon; it has more mass and so exerts a greater pull.Astronauts on the Moon found that they could jump much higher than they could on Earth. This is because the mass of Earth is greater than the mass of the Moon and therefore exerts a greater pull.The nearer two masses are the greater the gravity they have while the further away they are the lower the gravity they have on each other.

TidesTides are the predictable, repeated rise and fall of sea-levels on Earth caused by the gravitational pull of the Moon.Tides can be predicted and low and high tide tides are published in tide tables.

The difference between high and low tide is at its maximum at spring tide, when the Sun, Moon, and Earth are lined up in a straight line. When the Sun and Mon are at right angles to each other, the Sun’s gravitational pull partially cancels out the Moon’s gravitational pull and produces less extreme tides when the difference between high and low tide is at its minimum. These are called neap tides.

Spring tide Neap tide

The effects of tides on shoreline ecosystemsThe region of the beach between high tide and low tide levels is called the intertidal zone. The intertidal zone is a harsh environment for the marine animals to live. During storms the surf can be very rough and plants and animals must be able to withstand the battering from big waves and not get washed away. Plants and animals that live here are underwater at high tide but are exposed to air during low tide. They face hot temperatures in summer and low temperatures in winter so they must be able to adapt to different temperatures.

Animals exposed to air at low tide may be soaked in fresh water when it rains and yet be soaked in salty sea water at high tide. They must also be able to adapt to different concentrations of salt as the tides go in and out.

Summary of historical developments• People have watched the stars for thousands of years. They have

created stories about the stars and the constellations which have been passed on from generation to generation.

• Early scientists believed that the Earth was at the centre of the Solar system.

• Copernicus found that the observations of the planetary motion could be more easily explained if the Sun were at the centre of the solar system.

• Galileo was the first astronomer to use a telescope and found that Jupiter has moons orbiting around it.

• Newton discovered gravity and explained that planets and moons are held in orbit by the force of gravity.

• New discoveries are continuously made using telescopes.

Summary• The Moon orbits the earth once every 27.3 days. The Moon also spins on its

own axis once every 27.3 days. Due to both these time periods being the same, we only ever see one side of the Moon from Earth.

• Gravity is the force that acts between all objects with mass. The size of the force acting on the objects is proportional to their masses and inversely proportional to their distance from each other.

• The Earths gravity is responsible for holding the Moon in orbit around the Earth.

• Neap tides occur when the Sun and Moon are at 90 degrees to each other.• Spring tides occur when the Sun and Moon are in line with each other.• The rise and fall of the tides affects marine life living along shorelines. They

have adapted to this harsh environment in many ways to prevent themselves from drying out and from being washed away by strong waves.

What to study for the exam – a guide• Pay attention to the key concepts and summaries in this presentation.• Heat transfer – conduction, convection and radiation. Remember to think of

examples of each.• Energy systems including examples• Insulation • Absorbent and reflective surfaces• Law of conservation of energy and ‘wasted’ energy• Dynamos• Gravity• Tides• Shoreline eco-systems• Historical developmentsRemember to use your textbook and Exam Fever!