Heat Engines Heat Pumps Physics Montwood High School R. Casao

Heat Heat EnginesEngines

Heat Heat PumpsPumpsPhysicsPhysics

Montwood High SchoolMontwood High School

R. CasaoR. Casao

Heat Engine CycleHeat Engine Cycle A heat engine typically uses energy A heat engine typically uses energy

provided in the form of heat to do work provided in the form of heat to do work and then exhausts the heat which cannot and then exhausts the heat which cannot be used to do work.be used to do work.

The first law and second law of The first law and second law of thermodynamics constrain the operation thermodynamics constrain the operation of a heat engine. of a heat engine. The first law is the application of conservation The first law is the application of conservation

of energy to the system, and of energy to the system, and the second sets limits on the possible the second sets limits on the possible

efficiency of the machine and determines the efficiency of the machine and determines the direction of energy flow. direction of energy flow.

First Law of First Law of ThermodynamicsThermodynamics

The first law of thermodynamics is The first law of thermodynamics is the application of the conservation of the application of the conservation of energy principle to heat and energy principle to heat and thermodynamic processes: the thermodynamic processes: the change in internal energy (change in internal energy (U) of a U) of a system is equal to the heat (Q) system is equal to the heat (Q) added to the system minus the work added to the system minus the work (W) done by the system.(W) done by the system.

Mathematically: Mathematically: U = Q - WU = Q - W

Internal EnergyInternal Energy Internal energy is defined as the energy Internal energy is defined as the energy

associated with the random, disordered motion associated with the random, disordered motion of molecules.of molecules.

It is separated in scale from the macroscopic It is separated in scale from the macroscopic ordered energy associated with moving objects; ordered energy associated with moving objects; it refers to the invisible microscopic energy on it refers to the invisible microscopic energy on the atomic and molecular scale. For example, a the atomic and molecular scale. For example, a room temperature glass of water sitting on a room temperature glass of water sitting on a table has no apparent energy, either potential table has no apparent energy, either potential or kinetic . But on the microscopic scale it is a or kinetic . But on the microscopic scale it is a seething mass of high speed molecules seething mass of high speed molecules traveling at hundreds of meters per second.traveling at hundreds of meters per second.

Internal EnergyInternal Energy In the context of physics, the In the context of physics, the

common scenario is one of adding common scenario is one of adding heat to a volume of gas and using heat to a volume of gas and using the expansion of that gas to do work, the expansion of that gas to do work, as in the pushing down of a piston in as in the pushing down of a piston in an internal combustion engine. an internal combustion engine.

First Law of First Law of ThermodynamicsThermodynamics

Heat engines such Heat engines such as automobile as automobile engines operate in a engines operate in a cyclic manner, cyclic manner, adding energy in adding energy in the form of heat in the form of heat in one part of the cycle one part of the cycle and using that and using that energy to do useful energy to do useful work in another part work in another part of the cycle. of the cycle.

PV DiagramsPV Diagrams Pressure-Volume (PV) diagrams are a Pressure-Volume (PV) diagrams are a

primary visualization tool for the study primary visualization tool for the study of heat engines. Since the engines of heat engines. Since the engines usually involve a gas as a working usually involve a gas as a working substance, the ideal gas law relates substance, the ideal gas law relates the PV diagram to the temperature so the PV diagram to the temperature so that the three essential state variables that the three essential state variables for the gas can be tracked through the for the gas can be tracked through the engine cycle. engine cycle.

PV DiagramsPV Diagrams For a cyclic heat engine For a cyclic heat engine

process, the PV diagram process, the PV diagram will be closed loop. The will be closed loop. The area inside the loop is a area inside the loop is a representation of the representation of the amount of work done amount of work done during a cycle. Some during a cycle. Some idea of the relative idea of the relative efficiency of an engine efficiency of an engine cycle can be obtained by cycle can be obtained by comparing its PV comparing its PV diagram with that of a diagram with that of a Carnot cycle, the most Carnot cycle, the most efficient kind of heat efficient kind of heat engine cycle. engine cycle.

Heat EnginesHeat Engines A heat engine typically uses energy A heat engine typically uses energy

provided in the form of heat to do work provided in the form of heat to do work and then exhausts the heat which cannot and then exhausts the heat which cannot be used to do work. Thermodynamics is be used to do work. Thermodynamics is the study of the relationships between the study of the relationships between heat and work.heat and work.

The first law is the application of The first law is the application of conservation of energy to the system, conservation of energy to the system, and the second sets limits on the possible and the second sets limits on the possible efficiency of the machine and determines efficiency of the machine and determines the direction of energy flow. the direction of energy flow.

Energy Reservoir ModelEnergy Reservoir Model One of the general ways to illustrate a heat One of the general ways to illustrate a heat

engine is the energy reservoir model. The engine engine is the energy reservoir model. The engine takes energy from a hot reservoir and uses part takes energy from a hot reservoir and uses part of it to do work, but is constrained by the second of it to do work, but is constrained by the second law of thermodynamics to exhaust part of the law of thermodynamics to exhaust part of the energy to a cold reservoir. In the case of the energy to a cold reservoir. In the case of the automobile engine, the hot reservoir is the automobile engine, the hot reservoir is the burning fuel and the cold reservoir is the burning fuel and the cold reservoir is the environment to which the combustion products environment to which the combustion products

are exhausted.are exhausted.

Second Law of Second Law of ThermodynamicsThermodynamics

Second Law of Thermodynamics: It Second Law of Thermodynamics: It is impossible to extract an amount of is impossible to extract an amount of heat Qheat QHH from a hot reservoir and use from a hot reservoir and use it all to do work W . Some amount of it all to do work W . Some amount of heat Qheat QCC must be exhausted to a cold must be exhausted to a cold reservoir.reservoir.

The maximum efficiency which can The maximum efficiency which can be achieved is the Carnot efficiency. be achieved is the Carnot efficiency.

Second Law of Second Law of ThermodynamicsThermodynamics

Carnot CycleCarnot Cycle The most efficient The most efficient

heat engine cycle is heat engine cycle is the Carnot cycle, the Carnot cycle, consisting of two consisting of two isothermal processes isothermal processes and two adiabatic and two adiabatic processes.processes.

The Carnot cycle can The Carnot cycle can be thought of as the be thought of as the most efficient heat most efficient heat engine cycle allowed engine cycle allowed by physical laws.by physical laws.

Carnot CycleCarnot Cycle In order to approach the Carnot In order to approach the Carnot

efficiency, the processes involved in efficiency, the processes involved in the heat engine cycle must be the heat engine cycle must be reversible and involve no change in reversible and involve no change in entropy. This means that the Carnot entropy. This means that the Carnot cycle is an idealization, since no real cycle is an idealization, since no real engine processes are reversible and engine processes are reversible and all real physical processes involve all real physical processes involve some increase in entropy. some increase in entropy.

Carnot CycleCarnot Cycle The conceptual The conceptual

value of the Carnot value of the Carnot cycle is that it cycle is that it establishes the establishes the maximum possible maximum possible efficiency for an efficiency for an engine cycle engine cycle operating between operating between TTHH and T and TC C ..

hot

coldhotmax T

TTe

Combustion EnginesCombustion Engines

Combustion engines: burn fuel to produce Combustion engines: burn fuel to produce the heat input for a thermodynamic cycle.the heat input for a thermodynamic cycle. Burning fuel turns chemical energy into heat Burning fuel turns chemical energy into heat

energy.energy. By-products of combustion have a very high By-products of combustion have a very high

temperature and produce a very high pressure.temperature and produce a very high pressure. Results: piston pushed downward and a Results: piston pushed downward and a

fraction of the heat energy is converted to fraction of the heat energy is converted to mechanical work.mechanical work.

Some heat energy is carried away by the high Some heat energy is carried away by the high temperature exhaust gases, and some is lost temperature exhaust gases, and some is lost to the cylinder walls.to the cylinder walls.

First law of First law of thermodynamics for thermodynamics for combustion engine:combustion engine:

Mathematically: QMathematically: QHH = Q = QCC + W + W

QQHH = heat input due to fuel = heat input due to fuel combustioncombustion

QQCC = heat energy lost = heat energy lost W = workW = work Net heat absorbed per cycle: Net heat absorbed per cycle:

QQTT = = QQHH + Q + QCC


Work output for combustion engine: Work output for combustion engine:

W = QW = QHH - Q - QCC

Efficiency for combustion engine: Efficiency for combustion engine:

H

CH

H QQQ

QW

e


Gasoline EngineGasoline Engine Five successive processes occur in Five successive processes occur in

each cycle within a conventional each cycle within a conventional four-stroke gasoline engine.four-stroke gasoline engine.

During the During the intake strokeintake stroke of the piston, of the piston, air that has been mixed with gasoline air that has been mixed with gasoline vapor in the carburetor is drawn into vapor in the carburetor is drawn into the cylinder.the cylinder.

During the During the compression strokecompression stroke, the , the intake valve is closed and the air-fuel intake valve is closed and the air-fuel mixture is compressed approximately mixture is compressed approximately adiabatically.adiabatically.

Gasoline EngineGasoline Engine At this point, the At this point, the sparkspark plug ignites the air-fuel plug ignites the air-fuel

mixture, causing a rapid increase in pressure and mixture, causing a rapid increase in pressure and temperature at nearly constant volume.temperature at nearly constant volume.

The burning gases expand and force the piston The burning gases expand and force the piston back, which produces the back, which produces the power strokepower stroke..

During the During the exhaust strokeexhaust stroke, the exhaust valve is , the exhaust valve is opened and the rising piston forces most of the opened and the rising piston forces most of the remaining gas out of the cylinder.remaining gas out of the cylinder.

The cycle is repeated after the exhaust valve is The cycle is repeated after the exhaust valve is closed and the intake valve is opened.closed and the intake valve is opened.

How Stuff Works Gasoline Engine Animation How Stuff Works Gasoline Engine Animation

Otto CycleOtto Cycle






Diesel EnginesDiesel Engines

The main differences between the gasoline The main differences between the gasoline engine and the diesel engine are: engine and the diesel engine are: A gasoline engine intakes a mixture of gas and A gasoline engine intakes a mixture of gas and

air, compresses it and ignites the mixture with air, compresses it and ignites the mixture with a spark. A diesel engine takes in just air, a spark. A diesel engine takes in just air, compresses it and then injects fuel into the compresses it and then injects fuel into the compressed air. The heat of the compressed compressed air. The heat of the compressed air lights the fuel spontaneously.air lights the fuel spontaneously.

A gasoline engine compresses at a ratio of 8:1 A gasoline engine compresses at a ratio of 8:1 to 12:1, while a diesel engine compresses at a to 12:1, while a diesel engine compresses at a ratio of 14:1 to as high as 25:1. The higher ratio of 14:1 to as high as 25:1. The higher compression ratio of the diesel engine leads to compression ratio of the diesel engine leads to better efficiency. better efficiency.


Gasoline engines generally use either carburetion, in which the air and fuel is mixed long before the air enters the cylinder, or port fuel injection, in which the fuel is injected just prior to the intake stroke (outside the cylinder). Diesel engines use direct fuel injection -- the diesel fuel is injected directly into the cylinder.

How Stuff Work Diesel Animation


Note that the diesel engine has no Note that the diesel engine has no spark plug, that it intakes air and spark plug, that it intakes air and compresses it, and that it then compresses it, and that it then injects the fuel directly into the injects the fuel directly into the combustion chamber (direct combustion chamber (direct injection). It is the heat of the injection). It is the heat of the compressed air that lights the fuel in compressed air that lights the fuel in a diesel engine.a diesel engine.

Dodge HemiDodge Hemi Hemi: (HEM -e) adj. Mopar in type, V8, hot Hemi: (HEM -e) adj. Mopar in type, V8, hot

tempered, native to the United States, tempered, native to the United States, carnivorous, eats primarily Mustangs, carnivorous, eats primarily Mustangs, Camaros, and Corvettes. Also enjoys Camaros, and Corvettes. Also enjoys smoking a good import now and then to smoking a good import now and then to relax.relax.

The hemispherically shaped combustion The hemispherically shaped combustion chamber is designed to accommodate chamber is designed to accommodate large valves and put the spark plugs close large valves and put the spark plugs close to the center of the combustion chamber.to the center of the combustion chamber.

In a HEMI engine, the In a HEMI engine, the top of the combustion top of the combustion chamber is chamber is hemi-hemi-sphericalspherical, as seen in , as seen in the image. The the image. The combustion area in the combustion area in the head is shaped like head is shaped like half of a sphere. An half of a sphere. An engine like this is said engine like this is said to have "hemi-to have "hemi-spherical heads." spherical heads."

In a HEMI head, the In a HEMI head, the spark plug is normally spark plug is normally located at the top of located at the top of the combustion the combustion chamber, and the chamber, and the valves open on valves open on opposite sides of the opposite sides of the combustion chamber. combustion chamber.

Advantage: Advantage: HorsepowerHorsepower

The engine produces 345 horsepower, and compares very favorably with other gasoline engines in its class. For example Dodge 5.7 liter V-8 - 345 hp @ 5400 rpm Ford 5.4 liter V-8 - 260 hp @ 4500 rpm GMC 6.0 liter V-8 - 300 hp @ 4400 rpm GMC 8.1 liter V-8 - 340 hp @ 4200 rpm Dodge 8.0 liter V-10 - 305 hp @4000 rpm Ford 6.8 liter V-10 - 310 hp @ 4250 rpm

The HEMI Magnum engine has two valves per cylinder as well as two spark plugs per cylinder. The two spark plugs help to solve the emission problems that plagued Chrysler's earlier HEMI engines. The two plugs initiate two flame fronts and guarantee complete combustion.

Disadvantage:Disadvantage: If HEMI engines have all these advantages, why If HEMI engines have all these advantages, why

aren't all engines using hemispherical heads? It's aren't all engines using hemispherical heads? It's because there because there are even better configurations even better configurations available today. available today.

One thing that a hemispherical head will never have One thing that a hemispherical head will never have is is four valves per cylinderfour valves per cylinder. The valve angles would . The valve angles would be so crazy that the head would be nearly impossible be so crazy that the head would be nearly impossible to design. Having only two valves per cylinder is not to design. Having only two valves per cylinder is not an issue in drag racing or NASCAR because racing an issue in drag racing or NASCAR because racing engines are limited to two valves per cylinder in engines are limited to two valves per cylinder in these categories. But on the street, four slightly these categories. But on the street, four slightly smaller valves let an engine breath easier than two smaller valves let an engine breath easier than two large valves. Modern engines use a large valves. Modern engines use a pentroofpentroof design design to accommodate four valves.to accommodate four valves.

Disadvantage:Disadvantage: Another reason most Another reason most

high-performance high-performance engines no longer engines no longer use a HEMI design is use a HEMI design is the desire to create a the desire to create a smaller combustion smaller combustion chamber. Small chamber. Small chambers further chambers further reduce the heat lost reduce the heat lost during combustion, during combustion, and also shorten the and also shorten the distance the flame distance the flame front must travel front must travel during combustion. during combustion. The compact The compact pentroof design is pentroof design is helpful here, as well.helpful here, as well.

Gas Turbine EnginesGas Turbine Engines

In a gas turbine, a pressurized gas spins a In a gas turbine, a pressurized gas spins a turbine. turbine.

In all modern gas turbine engines, the In all modern gas turbine engines, the engine produces its own pressurized gas, engine produces its own pressurized gas, and it does this by burning something like and it does this by burning something like propane, natural gas, kerosene or jet fuel.propane, natural gas, kerosene or jet fuel.

The heat that comes from burning the fuel The heat that comes from burning the fuel expands air, and the high-speed rush of expands air, and the high-speed rush of this hot air spins the turbine. this hot air spins the turbine.


Two big advantages of the turbine Two big advantages of the turbine over the diesel: over the diesel: Gas turbine engines have a great Gas turbine engines have a great

power-to-weight ratiopower-to-weight ratio compared to compared to gasoline or diesel engines. That is, the gasoline or diesel engines. That is, the amount of power you get out of the amount of power you get out of the engine compared to the weight of the engine compared to the weight of the engine itself is very good. engine itself is very good.

Gas turbine engines are Gas turbine engines are smallersmaller than than their reciprocating counterparts of the their reciprocating counterparts of the same power.same power.

Gas Turbine EnginesGas Turbine Engines The main disadvantage of gas turbines is The main disadvantage of gas turbines is

that, compared to gasoline and diesel that, compared to gasoline and diesel engines of the same size, they are engines of the same size, they are expensiveexpensive. . Because they spin at such high speeds and Because they spin at such high speeds and

because of the high operating temperatures, because of the high operating temperatures, designing and manufacturing gas turbines is a designing and manufacturing gas turbines is a tough problem from both the engineering and tough problem from both the engineering and materials standpoint. materials standpoint.

Gas turbines also tend to use more fuel when Gas turbines also tend to use more fuel when they are idling, and they prefer a constant they are idling, and they prefer a constant rather than a fluctuating load. That makes gas rather than a fluctuating load. That makes gas turbines great for things like transcontinental turbines great for things like transcontinental jet aircraft and power plants, but explains why jet aircraft and power plants, but explains why you don't have one under the hood of your car. you don't have one under the hood of your car.


Three parts of the gas turbine engine: Three parts of the gas turbine engine: CompressorCompressor - Compresses the incoming - Compresses the incoming

air to high pressure air to high pressure Combustion areaCombustion area - Burns the fuel and - Burns the fuel and

produces high-pressure, high-velocity gas produces high-pressure, high-velocity gas TurbineTurbine - Extracts the energy from the - Extracts the energy from the

high-pressure, high-velocity gas flowing high-pressure, high-velocity gas flowing from the combustion chamber.from the combustion chamber.

Gas Turbine Operation Animation

Heat PumpsHeat Pumps Heat pumps: a mechanical device Heat pumps: a mechanical device

that moves energy from a region at a that moves energy from a region at a lower temperature to a region at lower temperature to a region at higher temperature. higher temperature.

Heat pump can be described by a Heat pump can be described by a thermodynamic cycle just like that of thermodynamic cycle just like that of an engine. System absorbs heat at a an engine. System absorbs heat at a low temperature and rejects it at a low temperature and rejects it at a higher temperature. higher temperature.

Heat PumpsHeat Pumps Heat pumps have long been used to cool Heat pumps have long been used to cool

homes and buildings, and are now homes and buildings, and are now becoming increasingly popular for heating becoming increasingly popular for heating them as well.them as well.

Heat pump contains two sets of metal coils Heat pump contains two sets of metal coils that can exchange energy by heat with the that can exchange energy by heat with the surroundings: one set is on the outside of surroundings: one set is on the outside of the building in contact with the air or the the building in contact with the air or the ground; and the other set in the interior of ground; and the other set in the interior of the building.the building.

Heat PumpsHeat Pumps

Heat PumpsHeat Pumps In the heating mode, a circulating fluid In the heating mode, a circulating fluid

flowing through the coils absorbsflowing through the coils absorbs energy energy from the outside and releases it to the from the outside and releases it to the interior of the building frominterior of the building from the interior the interior coils.coils. The fluid is cold and at low pressure when it is The fluid is cold and at low pressure when it is

in the external coils, where it absorbs energy in the external coils, where it absorbs energy by heat from either the air or the ground.by heat from either the air or the ground.

The resulting warm fluid is then compressed The resulting warm fluid is then compressed and enters the interior coils as a hot, high-and enters the interior coils as a hot, high-pressure fluid, where it releases its stored pressure fluid, where it releases its stored energy to the interior air.energy to the interior air.

Heat PumpsHeat Pumps First law of thermodynamics for First law of thermodynamics for

heat pump:heat pump: Q QHH = Q = QCC + W + Winin

QQCC = heat removed from low = heat removed from low temperature reservoirtemperature reservoir

QQHH = heat pumped into high = heat pumped into high temperature reservoirtemperature reservoir

Win = work inputWin = work input

Coefficient of Coefficient of PerformancePerformance

Effectiveness of a heat pump is Effectiveness of a heat pump is described in terms of a ratio called described in terms of a ratio called the coefficient of performance (COP). the coefficient of performance (COP). In the heating mode, the COP is In the heating mode, the COP is defined as the ratio of the heat Qdefined as the ratio of the heat QHH moved to a higher temperature moved to a higher temperature region divided by the work input region divided by the work input required to transfer that energy.required to transfer that energy.

COP (heating mode) = COP (heating mode) =

in

H

WQ


The COP is similar to the thermal efficiency The COP is similar to the thermal efficiency for a heat pump in that it is a ratio of what for a heat pump in that it is a ratio of what you get (energy delivered to the interior of you get (energy delivered to the interior of the building) to what you give (work input).the building) to what you give (work input).

Because QBecause QHH is generally greater than W is generally greater than Winin, , typical values for the COP are greater than 1. typical values for the COP are greater than 1. It is desirable for the COP to be as high as It is desirable for the COP to be as high as

possible.possible. Example: if the COP for a heat pump is 4, the Example: if the COP for a heat pump is 4, the

amount of energy transferred to the building is 4 amount of energy transferred to the building is 4 times greater than the work done by the motor in times greater than the work done by the motor in the heat pump. the heat pump.


Maximum possible COP is called the Maximum possible COP is called the Carnot COP and is never achieved by Carnot COP and is never achieved by a real heat pump and depends on a real heat pump and depends on the high and low temperature the high and low temperature between which the pump operates.between which the pump operates.

Carnot COP (heating mode) = Carnot COP (heating mode) =

ch

h

ch

h

in

h

TTT

QQQ

WQ

Heat PumpsHeat Pumps Heat pumps can also operate in the Heat pumps can also operate in the

cooling mode. Air conditioners and cooling mode. Air conditioners and refrigerators are examples of heat refrigerators are examples of heat pumps operating in the cooling pumps operating in the cooling mode.mode.

Energy is absorbed into the Energy is absorbed into the circulating fluid in the interior coils; circulating fluid in the interior coils; then, after the fluid is compressed, then, after the fluid is compressed, energy leaves the fluid through the energy leaves the fluid through the external coils.external coils.

Heat PumpsHeat Pumps The heat pump must have a way to release The heat pump must have a way to release

energy to the outside. Refrigerator as an energy to the outside. Refrigerator as an example:example: A refrigerator cannot cool the kitchen if the A refrigerator cannot cool the kitchen if the

refrigerator door is left open.refrigerator door is left open. The among of energy leaving the external coils The among of energy leaving the external coils

behind or underneath the refrigerator is greater behind or underneath the refrigerator is greater than the amount of energy removed from the food than the amount of energy removed from the food or from the air in the kitchen if the door is left or from the air in the kitchen if the door is left open.open.

The difference between the energy out and the The difference between the energy out and the energy in is the work done by the electricity energy in is the work done by the electricity supplied to the refrigerator. Energy, Wsupplied to the refrigerator. Energy, Winin, allows , allows compressor to remove heat from inside the compressor to remove heat from inside the refrigerator and transfer it to the kitchen.refrigerator and transfer it to the kitchen.

Heat PumpsHeat Pumps For a heat pump operating in the For a heat pump operating in the

cooling mode, “what you get” is cooling mode, “what you get” is energy removed from the cold energy removed from the cold reservoir. The most effective reservoir. The most effective refrigerator or air conditioner is one refrigerator or air conditioner is one that removes the greatest amount of that removes the greatest amount of energy from the cold reservoir in energy from the cold reservoir in exchange for the least amount of exchange for the least amount of work.work.

COP (cooling mode) =COP (cooling mode) = in

c

WQ

Heat PumpsHeat Pumps The greatest possible COP for a heat The greatest possible COP for a heat

pump in the cooling mode is that of a pump in the cooling mode is that of a heat pump whose working substance heat pump whose working substance is carried through a Carnot cycle in is carried through a Carnot cycle in reverse.reverse.

Carnot COP (cooling mode) =Carnot COP (cooling mode) =

ch

c

ch

c

in

c

TTT

QQQ

WQ

Second Law: RefrigeratorSecond Law: Refrigerator Second Law of Second Law of

Thermodynamics: It is Thermodynamics: It is not possible for heat not possible for heat to flow from a colder to flow from a colder body to a warmer body to a warmer body without any work body without any work having been done to having been done to accomplish this flow. accomplish this flow. Energy will not flow Energy will not flow spontaneously from a spontaneously from a low temperature low temperature object to a higher object to a higher temperature object.temperature object.

Second Law: RefrigeratorSecond Law: Refrigerator

Second Law: EntropySecond Law: Entropy

Second Law of Thermodynamics: In Second Law of Thermodynamics: In any cyclic process the entropy will any cyclic process the entropy will either increase or remain the same. either increase or remain the same.

Entropy: a measure of the amount of Entropy: a measure of the amount of energy which is unavailable to do energy which is unavailable to do work; a measure of the disorder of a work; a measure of the disorder of a system. system.

Entropy Entropy S =S =

KTQΔ

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Heat Engines Heat Pumps Physics Montwood High School R. Casao