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Section 3

Physics Unit

Section 3.0

3.1 Laws of Thermodynamics

3.2 The Development of Engine Technology

3.3 Useful Energy and Efficiency

Each section will have a number of different topics with practice problems to be completed.

Developed 2012

3.0 Principles of energy conservation and thermodynamics can be used to describe the efficiency of energy transformations.

This picture was drawn by the famous Rube Goldberg. Goldberg made fun of all the gadgets that were being created to make people’s lives easier. Although the above invention would actually work it involves a series of complex steps for a simple task: wiping the diner’s face with a napkin.

We can see the inefficiency of this contraption and can understand how the machine drawn exerts a lot of effort to produce a small result. In the real world, engineers do the opposite. They design efficient machines that exert minimum possible effort to produce largest possible results.

The industrial revolution was when engineers and scientists started focusing on increasing efficiency and as a result they developed some principles of how heat behaves.

3.1 Laws of ThermodynamicsPrior to no we have been focusing on energy transfers, which results in changes in motion or position of an object. In this section we will look at changes in temperature due to energy transformations or transfers.

SystemsScience 10: Physics Unit2

In order for us to be able to investigate energy transfers it is necessary to set boundaries for the objects that are involved. These boundaries are called systems, which we define as a set of interconnected parts. In energy transfers the system is the object or objects involved in the transfer and everything else is considered the surroundings or the environment.

Example: For a gas-powered lawnmower you could say the engine is the system and the other parts of the lawnmower, the ground, and the air could all be the surroundings. The boundaries that you set are arbitrary however and can be changed. You could say that the entire lawnmower is a system and the ground and air are the surroundings.

There are different types of systems that you need to state when studying energy transfers.

Open system –

Example: Earth is an open system since it can exchange both energy and matter with its surroundings.

Closed systems –

Example: A closed soup can is able to have energy move into the can but matter cannot move into the can.

Isolated system –

Draw a diagram to describe each system.

The First Law of Thermodynamics and the Law of Conservation of Energy

Science 10: Physics Unit3

It is essential that we recognize that heat is different than work. Work involves the movement of matter from one location to another, whereas heat is a transfer of thermal energy from one location to another. Both heat and work can affect systems.

Energy of a system can be increased in two ways:

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Energy of a system can be decreased in two ways:

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In the previous section we were introduced to the law of conservation of energy. We state the law of conservation of energy in more general terms: energy cannot be created or destroyed; it can only be transformed from one form to another. This means that the total amount of energy never changes.

The first law of thermodynamics is very similar to the law of conservation of energy except with special attention heat. The first law of thermodynamics states that ________________________________________________________________________________________________________________________________________________________________________

Whenever heat is added to a system it transforms into an equal amount of some other form of energy.

Example: If heat is added to a system the amount of energy in the system has no changed instead some energy is converted to another form. If 20N of heat energy is added to a system then that 20N of energy is not created or destroyed but rather converted to some other form of energy, say mechanical energy.

The amount of energy that is added to the system is the same amount of energy that leaves the system in another form. This leaves the same amount of energy in the system at the beginning as at the end.The Perfect Machine Cannot Be AchievedIdeally once energy is added to start a machine, the machine should convert 100% of the energy input directly into mechanical energy output without any energy loss. If no energy is converted to other energy forms this would mean that the


machine would operate indefinitely. These types of machines are called ___________________________ or perpetual motion machines. It is impossible to create a truly perfect machine.

In order for a machine to be classified as a perfect or perpetual motion machine, all the mechanical energy in the system must be completely conserved as ______________ _______________________________

Example: If no energy is converted to the surroundings when we dropped a bouncy ball to the ground it would mean that the amount of gravitational potential energy that the ball starts with is equal to the amount of kinetic energy that the ball has when it hits the floor. If no energy is converted to the surroundings then the ball should bounce to the same height as initially dropped. This is not possible because some energy is converted to sound, heat, and deformation when the ball hits the floor.

Diagram

The Second Law of ThermodynamicsIf you place a hot-water bottle in your bed the water bottle will eventually cool down and your bed will warm up. Eventually the bed and the water bottle will reach the same temperature. The heat transfers from the water bottle to the bed so the total amount of energy remains constant. This is the first law of thermodynamics. This example also illustrates the second law of thermodynamics, ________________________ __________________________________________________________________________________. Never in a natural system does heat move from a cold object to a hot object.

Diagram

Lets relate this law to an engine. In an internal combustion engine, the fuel in the combustion chamber burns at a high temperature, causing the piston to move and gain mechanical energy. The remaining energy is expelled as heat through the exhaust. The exhaust heat has a lower temperature than the input heat. An


internal combustion engine is an example of a heat engine, which is _________________________ _________________________________________________________

It is stated in the definition of the second law of thermodynamics that in a natural system heat moves from a hot object to a cold object. However, heat can be made to move from a cold object to a hot object. In order to do so, work must be done!

In order to have heat to be transferred from a cold object to a hot object we need what is called a heat pump. A heat pump is a device _______________________________ _______________________________________________________________

A common example of a heat pump is a refrigerator. A refrigerator pumps heat from the cold interior of the fridge to the outside where the air is warmer than the inside (transfer of heat from cold to hot). This process is not natural and thus work needs to be done to accomplish this. This is done by the refrigerator using electric energy to pump a refrigerant through copper piping. The refrigerant has a very low boiling point and changes from a liquid to a gas. This process begins at a compressor where the refrigerant emerges as a cool liquid. It is pumped through the copper piping where it absorbs heat from the interior of the fridge. As the refrigerant absorbs heat the temperature rises and then the refrigerant vaporizes into a gas and flows to a compressor where the gas is compressed causing the temperature to rise. When this happens the gas gives off thermal energy, which is transfer to the outside air. The refrigerant is then pumped into the condenser where it is cooled and liquefied and the process repeats.

The Development of Engine TechnologyEvery day we rely on machines to make tasks easier. Early scientist and engineers were no different. The machines that we use now-a-days are much more complex than the machines that were first used. Early machines were limited to simple mechanisms such as the lever, pulley, wheel and axle, and the screw. Initially the sources of energy to operate these machines were humans and animals. Later wind and flowing water were also used. These energy forms were things that people could see or touch and the “hidden” sources of energy were not yet discovered.

The first machine to use hidden energy sources was Hero’s steam engine. This machine was only a novelty device and did nothing useful so people still did not recognize heat as a useful source of energy for machines.

For most of recorded history there were humans and animals to do all the necessary tasks, and humans used wood to fuel their fires so there was no need to invented sophisticated machines. This changed in the 1600s when people switched


to burning coal instead of wood. This caused the demand for coal to increase and therefore deeper mines were dug to extract it.

A major problem in coal mining was pumping water from the deep mines. The Archimedes screw and the Persian wheel were existing machines that they used but these simple machines could not lift the water very far upwards out of the mine. A reciprocating pump was developed but it relied on atmospheric pressure which could only push the water up to a height if 9m so it was also not effective for deep mines. This left miners with the need for a machine with an engine that could be continuously operated and had a powerful energy source.

Archimedes Screw Persian Wheel Reciprocating Pump

Developing TechnologyWe all know that technology does not suddenly appear. Your iPhone did not just magically appear on a table one day. Developing technology involves a step-by-step process. The first step usually involves an understanding of a particular scientific concept. It is these concepts that are built on to create the technology. The new technology that is built almost always has flaws or drawbacks, which others try to improve. As they try to improve this technology their understanding of scientific concepts increases and therefore new scientific discoveries are made and new technologies.

Gunpowder Engine In 1680 Christian Huygens saw that a reciprocating pump needed a force to dive the piston forwards and a force to pull it back. We wondered if an internal source of energy could create this force and experimented with gunpowder. He found that gases generated by an explosion inside the engine drove a piston forward into a cylinder. The drawbacks were that this technology was not developed because of the hazard of the explosions and because there was not powerful enough mechanism to pull the piston back so the engine could operate continuously.

Heat EngineIn 1654 Otto van Guericke demonstrated tremendous forces of vacuums. He fitted two hollow hemispheres together and created a vacuum inside by extracting the sir through a valve. Two teams of


eight horses pulling in opposite directions could not pull the hemispheres apart. His other discovery was that water increases it volume to 1300 times when heated to form steam. Using these discoveries Denis Papin designed the first heat engine, which used heat to create steam to do work. The drawback to this machine was that Papin had a difficult time making a large enough drum for the water to be heated.

Example: The steam-powered engine relied on heat warming the water in the bottom of the cylinder and heating hot enough to produce steam. The steam then pushed a piston up.

Savery EngineIn 1698 Thomas Savery invented the first successful steam-powered pump, which was used to lift water out of mines. The drawback to this engine was that the pump could only lift water to a height of 6m so it wasn’t any improvement on the animal powered pumps from before.

Newcomen EngineIn 1712 Thomas Newcomen patented a heat engine that had a boiler produce steam that forced the piston up a cylinder. When cold water was sprayed on the outside of the cylinder the steam would condense and the piston would move back down. The drawback to this was that the cycle of heating a cooling the cylinder was very inefficient.

Watt EngineIn 1763 James Watt was asked to repair a Newcomen engine and was shocked by the poor performance. He saw the tremendous waste of heat when water was heated and cooled in the same cylinder. Watt designed a new, more efficient steam engine that had a separate condenser to cool the steam so that boiler cylinder always remained hot. This reduced the amount of heat required to operate


and made this engine over three times more efficient than Newcomen’s engine. Steam engines at this point in time were used more than just hauling water out of mine; they drove the huge machinery in mills as well as in trains and ships. The drawback to Watt’s engine was that they were very large and needed big boilers. Because of this steam engines could not be made small enough to replace horse-drawn carriages. These engines were also hot, dirty and very inefficient.

Internal Combustion EngineIn 1801 Philippe Lebon invented an engine that used coal gas ignited by an electrical spark. This was an internal combustion engine meaning energy was released by burning fuel, ignited by an electrical spark inside the engine. The problem with this engine was that there still was not enough force necessary to operate a machine.

In 1867 Otto and Eugen Langen improved the efficiency of the engine by compressing the coal gas-air mixture before ignition. Under pressure, the explosion of the mixture produces more force. They developed the four-stroke internal combustion engine, which is still used in many modern automobiles. This engine works by moving a cylinder up a cylinder, compressing the gas-air mixture. The firing of the spark plug ignites the mixture creating high temperature and pressure in the cylinder. The high pressure moves the piston down the cylinder and the movement of the piston turns the crankshaft, which turns the wheels. Almost every engine in the early 1900s could produce the same power as one horse (1hp). The drawback to this engine was coal was being used as the fuel. Coal does not burn very hot so the engine was not very powerful.

The next innovation came in the 1880s after Gottlieb Daimler designed an engine, which used gasoline instead of coal gas. Petroleum (gas) burns much hotter than coal gas making the engine more practical and thus led to the mass production of engines for automobiles.


Developing Future TechnologiesDeveloping technologies can lead to new scientific discoveries, which we saw in the development of the engine. Engineers have predicted that using solar wind (discovered in the early 1950s) will be more efficient that rocket fuel in propelling human-operated space vessels across the vast expanse of space. They have designed spacecraft with wind sails based on concepts of wind energy and interplanetary magnetic field theory.

New aspects of the science behind the technology are being unraveled all the time. Scientists have revealed yet again that anything is possible. The development of future technologies is limited only by the limits of our existing scientific and technological knowledge.

3.3 Useful Energy and EfficiencyAfter machines had been developed to harness energy transformations to do work, the focus shifted to how efficient these machines could do the work.

If machines or engines are to produce mechanical energy, then they must have moving parts. These moving parts rub against each other and produce frictions, which in turn produces heat. Heat from friction is unwanted in most machines.


Engineers try to reduce the amount of friction between moving parts to reduce waste heart. Magnetic technology is allowing for some near-zero friction situations by minimizing contact between moving parts. Remember at the start of this physics unit that the MAGLEV train was designed to overcome friction by eliminating wheels and tracks. Instead high-powered magnets force the train to levitate (rise) above the guideway and move forward.

MAGLEV train: http://www.youtube.com/watch?v=iaElPV0FWJ0

Useful EnergyThe purpose of a machine is to convert the initial energy added to the machine into the type of energy needed to do work that you want done. All other types of energy produced are considered wasted energy or wasted work. The initial energy source is called energy input. The desired energy needed to do the work is called useful energy output. The work the machine is suppose to do is the useful work output.

Example: The purpose of a light bulb is to provide light by converting electrical input energy to light output energy. However a light bulb also produces heat in the process. The light is useful energy output and the heat is wasted energy.

Systems with moving parts always lose some energy as heat, which is consistent with the first and second laws of thermodynamics. In the first law, the energy that is supplied to a system must equal all the energy that is gained by the system.

Example: The energy supplied to your body (the system) by food energy must equal the energy of all the useful work done plus all the wasted energy, which includes heat, and mechanical energy of the moving parts of your body.

According to the second law of thermodynamics, heat flows from hot to cold objects and in the process can be made to do work. However during the thermal energy transfer some energy is always lost to the surroundings. Thus the efficiency of a system can never be 100%. This means that you can never come close to getting the energy out of a system that you put into it.

EfficiencyEfficiency is a measurement of how effectively a machine converts energy input into useful energy output. It is expressed as a ratio.

efficiency=useful work outputtotalwork output

Efficiency is usually expressed as a percent and this tells you the percent efficiency of a machine.


If you have a hard time remembering which number goes on top and which number goes on the bottom think of a test score. The total that the test was out of goes on the bottom and the amount of questions you got right or did well on goes on the top. Likewise the total work output goes on the top and the good stuff or the useful work output goes on the top.

Converting Total Mechanical Energy to Useful Mechanical EnergySome machines or systems involve the conversion of mechanical energy to another form of mechanical energy. To calculate this we use the equation below.

Example: A crane uses mechanical energy to do the work of lifting a load vertically to the top of a building. The load gains mechanical energy in the form of gravitational potential energy.

Percent efficiency=usefulmechanical energy∨workoutputtotal mechanical energy∨work input

X100%

¿Em (usefuloutput )

Em (total input )X 100%∨¿

W(usefuloutput )

W (total input ) X 100%

Example: A crane lifts a load of construction materials from the ground to the second floor of a building. In the process, the crane does 2.30 X 104J of work or mechanical energy input while doing 8.00 X 103J of useful work or mechanical energy output in lifting the load. What is the mechanical percent efficiency of he crane?

¿Em (useful output )

Em (total input )X 100%

¿ 8.00 X 103J

2.30 X104J X 100%

= 34.8% The crane is 34.8% efficient.Example: An internal combustion engine with an efficiency of 15% is used to

do 3.20 X104J of useful work, or mechanical energy output. Calculate the mechanical energy input that had to be supplied by the combustion of fuel in the engine.

¿Em (useful output )

Em (total input )X 100%


15.0=8.00 X 103 J

Em( total input ) X 100%

0.15=8.00 X103 J

Em(total input )

Em (totalinput )=8.00X 103J0.15

= 2.13 X 105J

Practice Problems: page 216 #1-2

Transfer of Total Thermal Energy to Useful Thermal Energy

If a pot of water is sitting on a hot stove element, the heat from the element will be transferred to the pot and the water. The efficiency of heat transfer in a system may be determined using the following formula.

Percent efficiency=heat usefuloutputheatuseful input

X100%

Example: In heating a pot of water, 2.00 X 103J of heat was supplied by the stove element. If only 5.00 X 102J of heat was actually gained by the water (heat output), what was the percent efficiency of the stove element?

Percent efficiency=heat usefuloutputheat total input

X100%

¿ 5.00 X102J

2.00 X103JX100%

= 25.0%


The stove element has an efficiency of 25.0%

Practice Problems: page 217 #3

Conversion of Total Thermal Energy to Useful Mechanical Energy or Vice Versa

In many systems, the energy conversions involve thermal energy and mechanical energy. For example, the thermo-electric converter, or thermo-couple, converts thermal energy into mechanical energy to turn a fan. Not all the heat input is converted to mechanical energy in the process. Some of the heat is lost to the surroundings. To calculate the percent efficiency of this type of device that converts heat to mechanical energy, use the following equation:

Percent efficiency=Em (useful out put)

heat totalinputX 100%

Percent efficiency=heat usefuloutputEm(total input )

X100%


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