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Conservation of energy: 7.1 Examples of energy transfers analysed in terms of energy conserved - applying law of conservation of energy

(4 sections 7.1A to 7.1D)

Doc Brown's Physics exam study revision notes

INDEX for physics notes on conservation of energy, costs and wasted energy


7.1A. Energy conservation and a closed system

Reminders:

Law of conservation of energy - energy cannot be created or destroyed.

BUT, not all the energy in a system is useful, there is always some wasted or dissipated energy and their relative values can be quite different.

Know and understand that energy can be transferred usefully from one form to another, or stored, or dissipated, but energy cannot be created or destroyed.

This is the law of conservation of energy.

Another way of stating the law is to say the ...

'total energy of a system remains constant no matter what energy conversions take place'.

However, energy is only useful if it can be converted from one form to another.

Examples - from a suitable energy source ==> useful form (plus waste in most cases)

Energy stores have been described in Types of energy store - a comparison with examples explained

Here we are interested in how efficient are energy transfers to produce useful energy.

 

What do we mean by conservation of energy and a closed system?

A system is an object or objects involved in a particular situation which can be described in its own context without any energy being exchanged with its surroundings - the system does not absorb energy from, or lose energy too, the surroundings.

If energy is gained or lost by the system, it cannot be closed.

However, you can make an existing system into a closed system by including other things as part of it.

e.g. a car and its kinetic energy is limited because energy is being lost by friction (wheel-road) and air resistance (car body-air), but if you include the air around it and the road (the environment) you have 'constructed' a closed system.

Theoretically, irrespective of form, if the total energy at the start of a process doesn't equal the total energy at the end, that process can't happen.

You can use the work (E = Fd) and power (P = E/t) formulae to calculate an energy transfer-conversion from one energy store to another, but the formulae do not explain why the transfer takes place, nor does it tell you how much of the energy involves useful work or wasted energy. E can represent energy or work done - its all the same!

See Types of energy store - a comparison with examples explained, mechanical work done, power calculations

If there are no frictional forces operating, the work done on the object equals the work done = the energy is transferred to a useful energy store.

However, this is rarely the case since resistive forces are often present e.g. surfaces scraping against each other, air resistance etc.

Work must be done against these resistive forces which causes energy to be wasted - dissipated to the surroundings as heat due to friction - increasing the thermal energy store of the surroundings.

Under these circumstances, the energy transferred to the useful energy store as useful work, will be less than that transferred from the original energy store.

If work is done by the object itself, the work done does actually equal the energy transferred from the objects useful energy store.

See Types of energy store - a comparison with examples explained, mechanical work done, power calculations


7.1B. Types of energy transfer: What do energy transfers-conversions involve?

You need to be a bit more specific than just saying 'energy transferred' from x energy store to y energy store!

Energy is transferred electrically when a moving electrical charge (the current) is doing work against a resistance.

e.g. a battery providing electrical energy via the circuit to light a torch bulb.

Operating any electrical appliance or device from mains circuit electricity.

Energy is transferred by heating when a hotter object or material transfers heat energy to a colder object or material.

e.g. boiling water in an electric kettle - the hot element transfers heat to the water.

When you first put something to bake in an oven, heat is transferred from the hot air to the colder baking tin and contents.

Using a soldering iron to make an electric circuit link.

Energy is transferred mechanically when a force acts on an object to move it or change its shape e.g. pushing, pulling, stretching-expanding or squashing-compressing.

e.g. kicking a football - the force you exert from your leg accelerates the football and give it kinetic energy.

A car engine moving the wheels of a car.

Cutting a piece of cake!

Energy is transferred by radiation, usually the waves of electromagnetic radiation.

e.g. The Earth receives visible light, infrared and ultraviolet radiation from the Sun.

Infrared radiation from an electric fire.

Radio waves from a transmitter to a receiver.


7.1C. Some 'domestic' examples of energy transfers

Cooking 1: Our electricity supply is initially powered from either a fossil fuel chemical energy store, a nuclear energy store or a renewable energy store (hydroelectric, wind or solar etc.).

The initially energy store decreases and the kinetic energy store of the turbine blades and generator increases.

The generator converts the kinetic energy into electrical energy, which becomes thermal energy in a toasted sandwich maker the heat is transferred to the bread by conduction.

The electrical energy is converted into heat/thermal energy by the insulated electrical resistors in the iron cased toaster (iron is a good conductor of heat) - the thermal store of the heating elements is increased.

The thermal energy is transferred by conduction to the iron grill and the sandwich, increasing both their energy stores and cooking the sandwich.

See also The Usefulness of electricity, transferring electrical energy and cost calculations gcse physics revision notes

 

Gas fire appliance: The natural gas (methane) is a chemical energy store.

On combustion the fuel's chemical energy store decreases and the heat/thermal energy store of the waste gases increases.

The hot gases from a gas fire will always rise due to the immediate formation of a rising convection current which carries the heat around the room.

The decrease in the chemical energy store of the fuel gas therefore increases the thermal energy of the contents of the room, mainly by convective heat transfer.

The heat is also transferred by infrared radiation emitted from the hot flame - from the higher temperature flame to the lower temperature room.

 

Car headlamp

A car battery is a potential chemical energy store. When using the car, the chemical energy store of the battery decreases as electrical energy is produced to operate lights (visible EM radiation), ignition systems (heat), wipers (KE) etc.,

In the case of the headlamps the thermal energy store of the metal filament is increased (not LED lamp)

so some of the thermal energy becomes visible light and infrared radiation ...

... hence an early morning case of infrared radiation! Unlike 'modern' LED bulbs, 'old fashioned' filament bulbs emit quite a bit of IR heat radiation (from the increased thermal energy store of the filament). You can detect this with a frosty car where the central portion of the ice has melted on the transparent headlamp cover. Filament bulbs only convert ~10% of the electrical energy into visible light energy, most of the rest is converted into infrared EM radiation. The ice absorbs infrared equivalent to the latent heat of fusion (melting) and changes to liquid water. You can see this (in the above photographs) after the headlamps have been switched on for a few minutes on a frosty morning. The concentrated IR beam increases the thermal energy store of the plastic cover and the ice and the central part of the lamp cover warms up first melting the ice.

Energy store changes: The chemical energy store of the battery decreases as it is converted into electrical energy. The electrical energy increases the thermal energy store of the metal filament of the bulb. The thermal energy store of the filament decreases as it emits visible and infrared EM radiation. The absorbed EM radiation increases the thermal energy store of the headlamp cover and ice - causing the latter to melt. Eventually all the energy involved from the battery increases the thermal energy store of the surroundings.

A 'green' note: If there is, and its happening now in the UK and other countries, a change from very inefficient filament light bulbs to very efficient low energy LED light bulbs, there will be quite a reduction in the domestic demand for electricity. This reduced demand will help, on closure fossil fuel power stations, reduce CO2 emissions, reducing the greenhouse effect, and allow renewable energy resources to take over more of our electricity generation.

From the efficiency of light production old fashioned tungsten filament bulbs used in the home are very inefficient - ~5% of the electrical energy is transferred as visible light energy. The other ~95% of the energy ends up as thermal energy i.e. increase the thermal energy store of the room and its contents.

 

Cooking 2: The initial energy store to produce electricity has already been described with the toaster (and elsewhere on this page). The electrical current does work on the resistance convert electrical energy into heat energy - increasing its thermal energy store.

The electrical resistance elements of a cooker ring or a toaster become hot enough to emit a strong beam of infrared radiation to heat the contents of a pan or grill the toast.

Electrical energy is converted into heat/thermal energy which increases the thermal energy store of the heating elements and then increases the thermal energy store of the pan and contents or bread being toasted.

Eventually all the heat is wasted/dissipated to the surroundings, slightly heating up the kitchen.

See also The Usefulness of electricity, transferring electrical energy and cost calculations

 

Using a mobile phone

The battery is a chemical energy store (after charging).

The chemical energy is transferred as electrical energy.

The electrical energy is converted into light energy and sound energy.

There is also a little wasted thermal energy from the electrical circuits operating the phone.

 

A HiFi system

Electrical energy is converted into kinetic energy as the cones in the speakers are made to vibrate - their kinetic energy stores are increased.

The kinetic energy of the vibrating cones causes waves of sound energy to spread into the room.

Sound waves are vibrations in the air - a sort of kinetic energy.

Much of the sound energy is absorbed by objects in the room, so it dissipated-wasted as heat energy.

A small % of the sound energy vibrates your ear drums, increasing their kinetic energy store, and the ear 'system' sends electrical signals to the brain.


7.1D. More varied examples of conservation of energy transfers between energy stores

In all these examples you must treat them as far as possible as a closed system (so include the surroundings) and be able to account for all the energy transfers involved.

To put it as simply as possible: Total energy input = Useful energy output + Wasted (dissipated) energy

There is a separate section on EFFICIENCY - calculations and Sankey diagrams which deals with numerical calculations and details of useful input/output data.

1. When a gun fires, chemical energy is converted into heat energy, sound energy, light energy, but mainly into the kinetic energy store of the bullet by way off the rapidly expanding hot gases from the explosion.

When the bullet fires, the thermal energy store of the gases produced is increased.

When the bullet embeds itself into some material the kinetic energy of movement is converted into some sound energy, but mainly increases the thermal energy store of the material it hits.

Chemical energy store of bullet

thermal energy of hot gases ====>

mechanical conversion ====>

KE store of bullet (and recoiling gun)

mechanical conversion ====>

thermal energy - thermal store of material + sound, light, friction and heat losses to surroundings

 

2. Photovoltaic solar panels convert light energy directly into electrical energy which may be stored in a battery.

The Sun's nuclear energy and thermal energy stores are decreasing and the energy is transferred by radiation to the solar panel.

The chemical potential energy store of the battery is increased by the transfer of energy in the electrical charge of current.

 

3. TV and mobile phone: We use a large number of electrical devices in the home e.g.

A TV converts electrical energy into useful light and sound, but some waste heat from the electrical circuits is produced. Eventually all the electrical energy is dissipated in some way to the surroundings increasing its thermal energy store.

In charging a mobile phone battery you convert mains electrical energy into useful chemical potential energy. The electrical energy store decreases and the chemical potential energy store increases.

When using your mobile phone the useful chemical energy store decreases as it produces electrical energy, which in turn is converted into useful light and sound energy when using the phone. However, there is some wasted thermal energy added directly to the surroundings - you can detect this wasted heat as your phone warms up as you are using it.

 

4. Wind turbines convert kinetic energy into electrical energy. The kinetic energy store of the wind decreases, the kinetic energy store of the turbine blades increases.

This kinetic energy is converted into electrical energy by a generator.

The turbine does work in turning the generator and this kinetic energy is converted into electrical energy.

Some energy is wasted due to friction of moving parts - energy dissipated to increase the thermal energy store of the surroundings.

 

5. When you wind up a clock you are converting chemical energy from your body to mechanically create kinetic energy to increase the elastic potential energy store of the clock spring.

The potential energy store of the spring decreases as it moves the hands around giving them kinetic energy.

Suppose instead of a spring you have a falling weight?

In this case the winding up involves using kinetic energy to increase the gravitational potential energy of the clock weight.

As the weight falls the GPE store decreases.

Although using the term 'kinetic energy' seems ok, what you should appreciate that in winding up the clock and when the clock is freely working, a force is acting through a distance.

Since work = force x distance, your bodies chemical energy used = work done in winding up clock = work done in working the clock.

 

6. In some hydroelectric power schemes, excess 'off-peak' electricity is used to pump water back up into the reservoir.

This is mechanically using electrical energy to increase the kinetic energy of the pumped-water to move it upwards against the force of gravity.

The gravitational potential energy store is initially increased as the water builds up behind the dam and then decreased when the water flows down through the generators.

At peak demand times, extra water is released, accelerated by gravity, and so the dam's GPE energy store decreases and the kinetic energy (KE) store of the water increases.

The KE store of the water then decreases as it is mechanically converted into the kinetic energy of the generator which converts its KE into electrical energy (with some loss in sound and heat from friction).

The 'peak time' energy store changes can be expressed in a simple diagram

GPE store of water

gravitational acceleration - mechanical conversion ====>

KE store of the flowing water

KE of rotating generator - mechanical conversion ====>

electrical energy + friction and heat losses to surroundings

 

7. When you are cooking you are converting electrical energy (electric cooker) or chemical energy (gas cooker) into thermal energy to increase the thermal energy stores of the cooker ring and then the pan and its contents.

There will be heat energy losses due to convection, conduction or radiation to increase the thermal energy store of the surrounding air.

 

8. When you brake to slow down a moving car and bring it to a halt, the kinetic energy store of the car is decreased and energy is lost as thermal energy (heat), created by the friction between the brake pads and the discs on the wheels.

This is an example of a mechanical transfer between energy stores - resulting in initially increasing the thermal energy store of the brake pads.

Eventually this excess heat increases the thermal energy store of the surrounding air.

You are mechanically using your chemical energy (from food) to create the force of friction and the brakes mechanically convert the KE of the car into thermal energy - work is being done against the forces of friction.

A little sound energy is also involved - adding to the waste energy dissipated to the surroundings.

If you just take your foot of the accelerator, on a level road, the car will eventually come to a halt due to friction between the wheels and the road and the moving parts of the engine, and also, air resistance as the air brushes over the car body. BUT, the KE store of the car still decreases to the same amount as if you were braking and the thermal energy store of the environment increases the same amount too.

If a vehicles crashes into a stationary object, the contact force causes energy to be mechanically transferred from the vehicle's kinetic energy store to elastic potential energy store of the crushed vehicle parts, the thermal energy stores of the vehicle, object crashed into and the surrounding air (including some sound energy too - which also ends up as thermal energy!).

See also

Reaction times, stopping distances, safety aspects. calculations including F=ma

gcse physics notes

 

9. When a cricketer hits a cricket ball, there are all sorts of energy changes going on. The bodies chemical energy store is decreased as energy is used in the bodies muscles mechanical motion on swinging the bat.  Therefore the chemical energy is converted into increasing the kinetic energy store of the bat as it is swung at the ball. When the ball is hit the kinetic energy store of the bat decreases and the kinetic energy store of the moving ball is increased. However, some of the kinetic energy of the 'bat and ball' is converted (wasted) into sound and thermal energy - this eventually increases the thermal energy store of the environment.

However, the wasted energy is NOT wasted on the 3rd umpire in test cricket! The Umpire Decision Review System is a technology-based system used in cricket to help the match officials with their decision-making. The two on-field umpires or players can choose to consult with the third umpire to consider a decision of the on-field umpires. The technology used includes microphones to detect small sounds (due to friction, from KE of ball) made as the ball hits bat or pad (or neither), and infra-red thermal imaging to detect temperature changes as the ball hits bat or pad (heat from friction, from KE of ball). If there is no contact between bat/glove & ball or ball & pad, there is no increase above the background sound level and there is no 'hot spot' due to friction,

The sound effect is mainly used in the system these days?, so meet ...

... SNICKO the snickometer !!! The sound (if any) of the 'snick' is detected by a sensitive microphone in one of the cricket stumps.

The sound sequence of the movement of the batsman and bat is then portrayed electronically on an oscilloscope or a computer screen linked to a piece of music technology software.

The sound trace is also synchronised with a slow motion replay of the batsman's stroke.

 

10. Electrical machines that lift objects: When a crane lifts an object, the motor usefully, and mechanically, converts electrical energy into kinetic energy to lift the object. The lifted object has increased its gravitational potential energy store. There will be various losses due to - friction in the moving parts of the machine producing heat and sound, heat losses from the resistance of the electrical circuits (electrical store to thermal energy store).

input of electrical energy

electric motor - mechanical conversion ====>

useful KE of motor lifting object ====>

useful GPE store of object increased - mechanical conversion

+ heat and sound losses to surroundings

 

11. As a parachutist is dropped from an aircraft, the person has its maximum gravitational potential energy (GPE) store.

As the person falls, so does their GPE store as the GPE is mechanically converted into kinetic energy.

However, other energy conversions take place to. Some of the GPE is converted into heat and sound by air resistance. This happens both before and after the parachute is opened.

On landing on the ground the parachutist's GPE is reduced to zero and the rest of the kinetic energy is converted to heat and sound.

The energy conversions are:

The descent: GPE ====>  heat + sound + kinetic energy

On landing: kinetic energy ====>  heat + sound

12. A road vehicle

A non-electric car

Fuel (oil, diesel, hydrogen) is a store of chemical potential energy.

The chemical potential energy is converted to thermal energy when its burned in the engines cylinders.

The thermal energy store of the engine expands gas to drive the moving engine parts to increase their kinetic energy store - so the car moves!

There will be thermal energy losses from friction to the thermal energy store of the surrounding air (via air resistance) and moving part and road surface frictions.

Wasted or dissipated thermal energy which cannot be used or reclaimed.

An electric car

When the batteries are charged with electrical energy they become a chemical potential energy store.

When an electric car moves, the chemical energy store is converted into electrical energy.

The electrical energy drives a motor to move the car whose kinetic energy store is increased.

There will be thermal energy losses from friction to the thermal energy store of the surrounding air (via air resistance) and moving part and road surface frictions.

Wasted or dissipated thermal energy which cannot be used or reclaimed.

The electric car has less moving parts than a conventional fossil fuelled car, so the thermal energy loss due to friction should be less.

 

INDEX of notes on conservation of energy, costs and wasted energy


Keywords, phrases and learning objectives on energy conservation

Be able to describe, analyse and explain examples of energy transfers in terms of the law of conservation of energy.


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