Energy / Energy Changes & HeatLearning

Objectives

For ACHIEVEMENT students must be able to:

1. describe what energy is and give some examples2. distinguish between ‘active’ and ‘potential’ forms of energy3. describe different types of energy4. provide examples of energy changes5. define the joule as the unit of energy6. state the Law of Conservation of Energy7. describe how heating can cause convection currents8. describe how thermal energy affects particles of matter

For MERIT students must be able to:

1. identify energy transformations occurring around you2. trace back energy stories and draw energy chains3. calculate energy efficiency and wastage of appliances4. survey other people’s ideas about energy5. explain what temperature measures6. illustrate how a thermometer works7. explain why heating causes expansion8. ensure that experimental results are reliable9. explain how thermal energy is conducted through solids10. explain how thermal energy can be transferred by radiation11. locate information using a Science encyclopaedia

For EXCELLENCE students must be able to:

1. relate the use of materials to their thermal conductivity2. discuss everyday situations in terms of energy stories, energy transformations3. correctly write up an investigation including a valid method, reliable results, a concise

conclusion, and a discussion with correct science ideas

Energy is the ability to do work. Everything around us depends on energy. Cars depend on the energy stored in the fuel they use. Energy is used in homes, offices and industry to run all sorts of machines. It is used for lighting and heating our homes, and for cooking and storing food.

The more energy something has, the more work it can do. Anything that does work must have a supply of energy. A motorbike will not keep running unless it is supplied with petrol. Petrol provides energy that the engine uses to do work,

Energy has a unit called the joule(J). Because a joule is only a small amount of energy, it is common to use kilojoules (kJ).

Activity

Name the form of active energy present in each of the following situations.

(a) Energy in a thrown paper dart

(b) Energy when a dog barks

(c) Energy that is able to be detected by our eyes

(d) Energy that is given off from hot objects

(e) Energy that allows a light bulb to glow

Activity

Name the main type of potential energy associated with each of the following examples.

(a) Battery in a radio controlled car

(b) An inflated balloon

(c) Fireworks in a packet

(d) A dam of full water

(e) A bungy rope at full stretch

(f) A ball at the top of a hill

Law of Conservation of EnergyEnergy cannot be created or

destroyed.It can only be transferred or

transformed.

Energy transferEnergy transfer is the movement of one type of energy from one object to another. The form of energy doesn’t change.

Eg. Heat energy in cooking element is transferred to metal sauce-pan, then transferred to the water in the sauce-pan, heating the water.Summery

Energy transfer is when __________________________________________________________________________________________________

An example of energy transfer is

_________________________________________________

Energy in the form of heat, electricity, motion, or sound can change forms and move from one system to another. This can be demonstrated by making a cup of tea. For example, electricity, one form of energy, changes form to thermal energy (heat) through the stove and it transferred by conduction to the water for tea. The cup warms the hands, and the tea warms the insides of the person drinking the tea. This is energy transfer.

Energy transformation

Is when one type of energy is converted into one or more different forms of energy.

Hand-out: Notes

u

Energy Transformation

Energy exists in many forms in our everyday lives; among these forms are mechanical (energy of motion of large objects), electrical, chemical, thermal, sound, and light. Energy in one form, kinetic or potential, can be converted into any other form.

One of the things that make energy an important quantity in our lives is the many forms it can take. It can exist in the form of motion. This is known as kinetic energy. The motion can be of different things. If the motion is of a large object, the kinetic energy is said to be mechanical. If the moving objects are electrically charged, they are said to form an electric current. If the moving objects are individual molecules, there are two possibilities. If their motion is organized into waves, their kinetic energy is associated with sound. If their motion is completely disorganized, their kinetic energy is associated with what we call heat (physicists call it “thermal energy”). Another form of kinetic energy is light (and other forms of electromagnetic radiation, like radio waves and microwaves).

Other forms of energy do not have the form of motion, but they can cause an increase in motion at a later time. Water at the top of a dam can spill over the dam. A battery can produce an electric current when it is connected into a circuit. Fuels can be burned to produce heat. All of these are examples of potential energy.

Exercise: Copy and complete each of these sentences.

A moving object has ___________ energy. Energy which is stored is called ____________ energy. A boulder rolling down hill is losing ____________ ______________ energy, but gaining ______________ energy. Burning piece of coal changes ______________ potential energy into ____________ and ____________ energy. Spring can _____________ energy which can be released later.

Chemical potential Kinetic

Radiant Chemical potential

Electrical Radiant heat

Chemical potential Kinetic, sound

Exercise

What energy changes are being described in each of the following? (use arrows)

(a) The wind blew hard, turning the wind-mill noisily as it pumped the water from under the ground into the trough.

(b) At the flick of a switch the washing machine started turning and churning the clothes.

(c) ‘… two, one, zero’ The rocket belched fire and smoke, the ground shook and, with a deafening roar, the rocket left the launch pad.

(d) The lightning flashed, and the thunder crashed. The rimu tree was split right down the middle.

Using the Law of conservation, contrast and compare energy transfer and energy transformation.

Energy transformations occur when one type of energy changes to a different type of energy in the same object.

Energy transfers occur when the same type of energy moves from one object to a different object.

Regardless of what happens within a system, the total amount of energy in the system remains the

same unless energy is added to or released from the system.

Exercise

What is the source of energy for a solar-powered car? What energy change occurs when the car is moving? How would such cars operate at night or on cloudy days?

Law of Energy Conservation (Pg41) 1st part of the law 2nd part of the law 3rd part of the law

Energy stories worksheetFor each of the pictures on the sheet – write an energy chain.

1.

2.

3.

4.

5.

6.

7.

8.

9.

10

11.

12.

Energy efficiency (%)

To calculate % efficiency

When energy is being transformed by a machine, it is rarely 100% efficient in the transformation. Most energy wasted or loss is lost as heat due to friction.

A car uses chemical potential energy (petrol) transforms it into kinetic energy (car moves). For every kilojoule of energy in petrol that a car uses, only 250J of energy goes into kinetic energy.

% efficiency of car running on petrol for every 1000J of petrol = (250J / 1000J) X 100 = 25% efficiency

How are the particles arranged in a solid, a

liquid and a gas?

Pg 44-45 (Year9 Science)

Thermal energy has to do with the movement of the particles that make up an object or substance. The particles of solid objects move by vibrating. Those of liquids and gases are free to move about as well. Heating an object will make the particles vibrate or move faster. Nerves in your skin can sense these increased movements of the particles.

Temperature indicates how much thermal energy an object or substance has. Temperature is related to the average kinetic energy of the particles (atoms or molecules) making up a substance or object. If gaining thermal energy causes a change of state, the energy is used to free up the particles. When a loss of thermal energy causes a change of state, the particles move less freely.

Mass and Thermal energy

The amount of thermal energy an object has depends on how fast its particles are moving or vibrating and its mass.

A red-hot needle has much less thermal energy than a red hot poker. Different substances also respond differently to gaining the same amount of thermal energy.

Material and Thermal energy

Heating equal masses of oil and water for an equal time results in the oil being much hotter than the water. Water is able to absorb the thermal energy without its temperature rising as much as the oil’s temperature.

Heating and expansion

When substances and objects gain thermal energy, their particles move faster and push each other further apart. So gaining thermal energy causes solids, liquids and gases to expand in volume.

Solid and Heat

Solids expand the least because there are strong forces of attraction between the particles. On extremely hot days railway lines can buckle owing to expansion. Liquids expand more because the forces are weaker.

Gas and Heat

Gases expand the most because only weak forces hold the particles together. In hot air balloons, the hot gases expand and the balloon becomes less dense than the cold air outside, so the balloon rises.

Gases expand the most because only weak forces hold the particles together. In hot air balloons, the hot gases expand and the balloon becomes less dense than the cold air outside, so the balloon rises. When a substance loses thermal energy, its particles move more slowly and do not push apart so much. So, as they cool, solids, liquids and gases contract in volume.

Heat transfer

Heat is a type of energy called heat energy. Heat can be transferred (moved) by three main processes:

1. conduction2. convection3. radiation

During heat transfer, thermal energy always moves in the same direction.

HOT COLD

Heat energy only flows when there is a temperature difference from a warmer area to a cooler area.

Give 4 examples of situations where heat moves from a hot to a cold place?

1. Conduction

Colder end

Particles that are very close together can transfer heat energy. One way they can do this is as they vibrate. This type of heat transfer is called conduction.

Conduction is the method of heat transfer in solids but not liquids and gases. Why do you think this is?

Heat flow

A material that allows vibrations to be passed on easily from particles to particle is a good conductor of heat. A poor conductor is also called in insulator. Metals are good conductors. Glass, air, wood and plastic are poor conductors and therefore are good insulators.

2. Convection

Thermal energy can be transferred from one location to another by convection. This can occur in liquids and gases where the particles are free to move, but not in solids. Convection is the bulk transfer of thermal energy by currents.

Cooler liquids (or gases) sink to take the place of the heated liquid (or gas). This rising and sinking causes convection currents to occur.

Experiment: Potassium Permanganate

Method:

1. Fill one beaker with cold water and another with hot water.2. Place a finger over the top of a drinking straw and lower into one

of the beakers.3. Remove finger from the straw and carefully drop one crystal of

potassium permanganate down the straw.4. Slowly lift the straw away and write down any observations.5. Repeat the above process in the beaker of hot water and write

down any observations.

Results

Cold water: ___________________________________________

Hot water: ___________________________________________

Conclusions: Explain the difference in your observation using your understanding of particle theory.

__________________________________________________________________________________________________________

Results

The particles of water are moving more rapidly in the beaker of hot water than the beaker of cold water.

Potassium permanganate did not dissolve fast when dropped into cold water. As soon as heat was turned on, it started to dissolve very rapidly in the water.

Conclusions:

The potassium permanganate is soluble in water and the dissolving process happens fast in hot water than in cold water. This is because ………….

Convection currents can’t occur in solids because the particles are held in fixed positions – but can they occur in liquids and gases?

1. Convection in a lava lamp

The properties of liquids can be used to explain how this lava lamp works. There are two liquids in a lava lamp – a clear colourless one and a coloured one. When heated the coloured liquid moves through the colourless liquid.

A lava lamp consists of oil, and wax in a glass, and a heat source (a light bulb) placed underneath the glass. When the lamp is turned on the bulb gets hot. As the bulb heats up some of the heat from the bulb is transferred to the glass by radiation. Radiation occurs when heat is transferred between two

objects that are not touching each other. (The bulb is not touching the glass.) As the glass heats up, some of its heat is transferred to the wax by conduction.

As the wax heats up it begins to melt and become less dense than the surrounding oil. This change in density causes the melted wax to rise. As the wax rises, some of its heat is lost. After it has been at the top for a while it loses most of its heat and becomes denser so it sinks.

The movement of wax in a lava lamp is similar to convection currents that occur in the earth's mantle, and in the air.

A heat source can heat up air or liquid, causing it to become less dense and rise. As the air or liquid rises, cooler air or liquid above it sinks because it is denser, and in turn is heated when it gets close to the heat source.

2. Sea Breeze: Convection current in Air

When a gas is heated the particles that make it up gain more kinetic energy. As the kinetic energy of these particles increases they collide more and being to spread out. This gas is now less dense and it will then rise.

3. Thunderstorms

The land warms up more quickly than the water.During the day, a sea breeze can happen: • The warm air above the land rises.• The cooler air above the water sinks.• The cooler air moves toward the land.• The moving air forms a sea breeze.

Water cools down more slowly than land. At night, a land breeze can happen:• Cooler air above the land sinks.• The cooler air moves out over the ocean.• This pushes the warmer air upward.• The cooler air forms a land breeze.

This movement of air and heat is called CONVECTION.The winds are said to be part of a CONVECTION CURRENT.

Thunderstorms produce lightning and thunder. They often produce strong winds and heavy rain, as well. Thunderstorms often form by convection on hot, humid days.

1)The Sun warms the ground.2)The air near the ground is warmed by conduction.3)The warm air rises, carrying water vapour with it. This is

convection current.4)As it rises, the warm air slowly cools. Water vapour in the air

condenses to form puffy clouds.5)As warmer, moist air rises, the remaining water vapour

condenses. The puffy clouds grow into large, dark thunderheads. Soon, a thunderstorm will occur.

Because the water particles are spread out more they are said to be less dense than the cooler water at the top. The cooler water at the

top sinks and the warmer, less dense water rise to the top. As the cooler water sinks it heats up and moves back to the top. The circular motion continues for as long as the water is being heated up.

Convection is the transfer of heat by the actual movement of the warmed matter. The heat moves with the fluid. Convection occurs in liquids and gases.

Explain transfer of heat in (a) and (b).

(a) (b)

3. RadiationHow do heat travel through space?

There are no particles between the Sun and the Earth, so the heat cannot travel by conduction or by convection. The heat travels to Earth by infrared

waves. These are similar to light waves and are able to travel through empty space.

These waves, like light, do not need particles to travel.

This means that infrared waves can :

Travel through a vacuum Travel at the same speed as light Be reflected and absorbed.

Infrared waves heat object that absorb them.

Absorbing thermal radiationRadiation involves energy travelling as waves from one place to another without any matter being involved. Certain surfaces are better at absorbing thermal radiation than others.

Matte black surfaces are the best absorbers of radiation. Shiny surfaces are the worst absorbers because they reflect most of the radiation away.

Matte black white silver

Best absorber Worst absorber

(good reflector)

Based on your understanding of heat absorption, what colour t-shirt would you wear on a hot summers day to keep cool? Explain you answer.

Exp : BLACK AND SILVER

1. Fill two cans with equal amount of water.2. Place the thermometers into each silver and black can.3. Place the cans at a same location(outside).4. Measure the temperature every 2 min.

Time(s)0 2 4 6 8 10 12

Temp (C)

BlackSilver

Draw a line graph of your results.

Use the graph to describe any trends or patterns in your results.

Which type of kettle would cool down faster – a black kettle or a shiny metallic kettle?

Based on your understanding of radiation explain your choice.

Absorption and reflection are opposite processes. Therefore a good absorber of radiant energy reflects very little radiant energy and appears dark. A perfect absorber reflects no radiant energy and appears perfectly black. Good reflectors of radiant energy are poor absorbers. Light colored objects reflect more light and heat than darker colored objects.

This is why we where lighter colored clothing in summer to stay cool.

Experiment: Burning Almond

Aim: To find energy content of an almond.

Result: Joules = volume of the water x temperature increase x 4.2 J/(gC)

Conclusion:

Heat energy of one almond = _________ kilojoules.

almond