Heat Engines and Thermal Efficiency| Grade 9 Science Quarter 4 Week 7

Summary

This video lesson from a YouTube channel focuses on the principles of heat engines and thermal efficiency. It begins by explaining the concept of heat transfer, which is the movement of energy from a high-temperature object to a low-temperature one, and how it can be used to perform work. The video then delves into the three methods of heat transfer: conduction, convection, and radiation. It introduces heat engines as devices that convert thermal energy into mechanical work, with examples such as geothermal power plants. The lesson also covers the workings of combustion engines, including the four-stroke cycle in gasoline engines. Thermal efficiency is discussed, highlighting that not all thermal energy can be converted into useful work due to waste heat. The video presents the Carnot efficiency formula, which shows that the efficiency of an ideal heat engine depends on the temperature difference between the hot and cold reservoirs. Finally, it works through sample problems to illustrate how to calculate the efficiency of heat engines, providing a clear understanding of the topic.

Takeaways

• 🔥 **Heat Transfer**: Heat transfer occurs when there's a temperature change, moving from a high-temperature object to a low-temperature one, and can happen through conduction, convection, and radiation.
• 🚀 **Heat Engines**: A heat engine is a device that uses thermal energy or heat to perform work, taking in heat from a high-temperature source and releasing some as waste heat to a low-temperature sink.
• 🔩 **Engine Cycles**: Heat engines operate in cycles where heat is added, some is used to do work, and the rest is removed at a lower temperature, exemplified by geothermal power plants and combustion engines.
• 🏎️ **Combustion Engines**: There are two types of combustion engines: external (like steam engines) and internal (like gasoline or diesel engines), with the latter being common in vehicles.
• 🔬 **Thermal Efficiency**: The efficiency of a heat engine is the ratio of useful work done to the heat input, with a maximum theoretical limit set by the temperatures of the hot and cold reservoirs.
• 📉 **Waste Heat**: Not all absorbed heat can be converted into useful work; the rest is lost as waste heat, which is inevitable and limits engine efficiency.
• 💯 **Efficiency Calculation**: Efficiency is calculated as the work done divided by the input heat, minus the exhaust heat, and can also be determined by temperature measurements of the hot and cold reservoirs.
• ⚙️ **Four Stroke Cycle**: In a gasoline engine, the four strokes are intake, compression, power, and exhaust, which define the operation of the engine.
• 🔢 **Sample Problem**: The script provides a method to calculate the efficiency of a heat engine using given values of energy input and waste heat.
• 📐 **Carnot Efficiency**: According to Carnot, the efficiency of an ideal heat engine depends only on the temperature difference between the hot and cold reservoirs.
• 🌡️ **Temperature Scale**: Efficiency calculations use absolute temperatures, measured on the Kelvin scale.
• 🎓 **Educational Content**: The script is educational, aimed at teaching students about heat engines, their operation, and the principles of thermal efficiency.

Q & A

What is the main topic of the week 7 science lesson for grade 9?

The main topic of the week 7 science lesson for grade 9 is heat engines and thermal efficiency.

How is heat transfer related to work?

Heat transfer is related to work because it involves the release of heat that can be used to perform work, such as in heat engines like geothermal power plants.

What are the three methods of heat transfer?

The three methods of heat transfer are conduction, convection, and radiation.

How does a heat engine work?

A heat engine works by taking in heat from a high-temperature reservoir, using the absorbed energy to perform useful work, and then releasing waste heat to a low-temperature reservoir.

What is the difference between an external and internal combustion engine?

An external combustion engine burns fuel outside the engine, such as in a steam engine, while an internal combustion engine burns fuel inside the engine, like in gasoline or diesel engines.

What are the four strokes of a four-stroke gasoline engine?

The four strokes of a four-stroke gasoline engine are the intake stroke, compression stroke, power stroke, and exhaust stroke.

Why can't a heat engine be 100% efficient?

A heat engine can't be 100% efficient because some of the thermal energy is inevitably lost as waste heat due to interactions like friction, and not all heat can be converted into useful work.

What is the formula for calculating the efficiency of a heat engine?

The efficiency of a heat engine is calculated as efficiency equals (input heat - exhaust heat) divided by input heat times 100 percent.

How does the temperature difference between the hot and cold reservoirs affect the efficiency of a heat engine?

According to Carnot's principle, a heat engine operating between two reservoirs with a higher temperature difference is more efficient than one operating between reservoirs with nearly the same temperatures.

What is the maximum efficiency of a steam engine that receives steam at 600 Kelvin and exhausts to a condenser at 350 Kelvin?

The maximum efficiency of this steam engine is 41.67%, calculated using the formula efficiency = 1 - (Tc/Th) * 100%, where Tc is the temperature of the cold reservoir and Th is the temperature of the hot reservoir.

What is the efficiency of a gasoline engine that receives 193 joules of energy from combustion and loses 125 joules by heat to exhaust during one cycle?

The efficiency of the gasoline engine is 35.23%, calculated using the formula efficiency = 1 - (Qc/Qh) * 100%, where Qc is the energy removed by heat and Qh is the energy added by heat.

Outlines

00:00

🔥 Introduction to Heat Engines and Thermal Efficiency

This paragraph introduces the topic of heat engines and thermal efficiency. It explains the relationship between heat and temperature, and how heat transfer can lead to energy transfer. The focus is on how heat can be converted into work, with examples from everyday life like boiling water. Three methods of heat transfer are discussed: conduction, convection, and radiation. The concept of a heat engine is introduced, which is a device that uses thermal energy to perform work. The limitations of heat engines are also mentioned, with an explanation of waste heat and the efficiency of converting heat into useful work. The paragraph concludes with an introduction to geothermal power plants as an example of heat engines.

05:02

🚗 Combustion Engines and Their Efficiency

The second paragraph delves into the specifics of combustion engines, which are a type of heat engine. It differentiates between external and internal combustion engines, providing examples of each. The four-stroke cycle of a gasoline engine is described in detail, including the intake, compression, power, and exhaust strokes. The concept of thermal efficiency is further explored, emphasizing that not all thermal energy can be converted into useful work, with the remainder being waste heat. The inefficiency of car engines is highlighted, noting that they are only about 30% efficient. The paragraph also discusses the theoretical maximum efficiency of heat engines based on the temperature difference between the hot and cold reservoirs, as postulated by Sadi Carnot. An equation for calculating efficiency is provided, and a sample problem is solved to illustrate the concept.

10:02

🧮 Calculating Thermal Efficiency with Sample Problems

The final paragraph provides a step-by-step guide to calculating the thermal efficiency of heat engines using sample problems. The first sample problem involves a gasoline engine with given values for energy input from combustion and heat loss to exhaust, using the previously introduced efficiency formula. The second sample problem concerns a steam engine operating between two temperature reservoirs, and it demonstrates how to calculate the maximum efficiency using temperature measurements. The paragraph concludes with a reminder to round off final answers and an encouragement to apply the knowledge gained to solve similar problems. The video ends with a prompt for viewers to like, share, subscribe, and engage with the content, and a thank you note to the viewers for their support.

Keywords

💡Heat Engines

Heat engines are devices that convert thermal energy into mechanical work. In the context of the video, it is central to understanding how geothermal power plants and other similar systems operate. The script explains that a heat engine takes in heat from a high-temperature source and uses some of that energy to perform work, with the rest being lost as waste heat to a lower temperature reservoir.

💡Thermal Efficiency

Thermal efficiency refers to the ratio of useful work output to the heat input of a system. It is a measure of how effectively a heat engine can convert heat energy into work. The video discusses the concept in relation to car engines, which are noted to be only about 30% efficient, meaning that a significant portion of the heat energy is lost as waste.

💡Heat Transfer

Heat transfer is the process by which thermal energy is moved from one system to another. The video explains that heat transfer occurs when there is a temperature difference, causing energy to flow from the hotter object to the cooler one. The script provides examples such as boiling water on a stove, illustrating how the heat from the stove is transferred to the water.

💡Conduction

Conduction is a method of heat transfer that occurs through direct contact between materials with different temperatures. The video script uses the example of a rod on firewood to illustrate conduction, where heat is transferred from the hotter part of the rod to the cooler parts through direct contact.

💡Convection

Convection is another method of heat transfer that involves the movement of fluids, such as water or air, to distribute heat. The video mentions boiling water and steam as examples of convection, where heat is transferred through the movement of the fluid itself.

💡Radiation

Radiation is the transfer of heat through electromagnetic waves. Unlike conduction and convection, radiation does not require a medium and can occur in a vacuum. The video gives the example of a microwave oven, which uses radiation to heat food.

💡Combustion Engine

A combustion engine is a type of heat engine that generates heat by burning a fuel in combination with an oxidizer, such as air. The video differentiates between external and internal combustion engines, with the latter being common in automobiles. The script explains the four-stroke cycle of a gasoline engine as an example.

💡Waste Heat

Waste heat is the portion of heat energy that is not converted into useful work by a heat engine. It is often lost due to inefficiencies such as friction. The video emphasizes that waste heat is an unavoidable byproduct of heat engine operation and significantly limits their efficiency.

💡Temperature Reservoirs

Temperature reservoirs are the high and low temperature sources from which a heat engine absorbs and releases heat, respectively. The video explains that the efficiency of a heat engine depends on the temperature difference between these reservoirs, with a larger difference leading to higher efficiency.

💡Carnot Efficiency

Carnot efficiency is a theoretical maximum efficiency for a heat engine, named after the French scientist Sadi Carnot. The video script mentions that according to Carnot, the efficiency of an ideal heat engine depends only on the temperatures of the hot and cold reservoirs, and not on the specific properties of the engine or the working substance.

💡Sample Problem

The sample problem provided in the video is an example of how to calculate the efficiency of a heat engine using the given formula. It demonstrates the application of the concept of thermal efficiency in a practical scenario, where the engine's efficiency is calculated based on the energy input from combustion and the energy lost to exhaust.

Highlights

Heat transfer can be used to do work, involving the release of heat

Heat transfer and energy transformation make heat engines like geothermal power plants work

Heat is related to temperature and heat transfer may change an object's temperature

Heat transfer occurs when there is a temperature change, transferring energy from high to low temperature objects

Three methods of heat transfer: conduction, convection, and radiation

A heat engine is a device that converts thermal energy into mechanical work

Not all heat absorbed by a heat engine can be converted into useful work - some is lost as waste heat

The energy converted into useful mechanical work is the difference between heat input and heat output

Examples of heat engines include power plants like geothermal

A combustion engine produces heat using a combustion process with fuel and an oxidizer

There are two classes of combustion engines: external (e.g. steam engine) and internal (e.g. gasoline or diesel)

In a four-stroke gasoline engine, the piston undergoes intake, compression, power, and exhaust strokes

Thermal efficiency is the ratio of useful work output to the thermal energy input

Not all thermal energy can be converted into useful work - some is lost as waste heat

Car engines are only around 30% efficient, with 70% of thermal energy lost as waste heat

It's impossible to construct a 100% efficient heat engine that fully converts all heat into useful work

The efficiency of ideal heat engines depends only on the temperature difference between the hot and cold reservoirs

Efficiency can be calculated as (work done / input heat) x 100% or using temperature measurements

Sample problem: Calculate the efficiency of a gasoline engine given energy input and heat loss to exhaust

Sample problem: Calculate the maximum efficiency of a steam engine given temperatures of hot and cold reservoirs

Transcripts

good day students welcome back to my

strong techie youtube channel we are now

in week 7 of grade 9 science quarter 4

lesson if you haven't watched our

previous lesson for the past weeks pause

the video and check the links in the

description box below this week's lesson

is all about heat engines and thermal

efficiency interesting right now let's

check out our learning objectives infer

that heat transfer can be used to do

work and that work involves the release

of heat explain how heat transfer and

energy transformation make heat engines

like geothermal power plants work

if you want to know more about this

lesson please keep on watching

[Music]

in your grade 7 science you have learned

that heat is related to temperature heat

transfer may change one's temperature

this change in temperature either a

decrease or an increase means that there

is an energy transfer in the form of

heat this video focuses on heat and work

and how heat can be turned into work and

how work involves the release of heat in

our everyday life heat transfer is

always evident many times we experience

heat transfer when we do majority of our

household chores and even when we go on

sports but what exactly is heat transfer

whenever there is a temperature change

heat transfer takes place it is the

transfer of energy from a high

temperature object to a low temperature

object an example of heat transfer is

boiling of water the hot stove heats the

pot and the pot heats the water inside

once transfer it can no longer be called

heat it becomes the internal energy of

the body transfer of energy from hot

objects to cool objects stops when the

two attain the same temperature the

objects are said to be in thermal

equilibrium so many processes involve

heat transfer it is hard to imagine a

situation where no heat transfer occurs

there are three methods of heat transfer

the first one is the conduction it is

the transfer of heat due to direct

contact between two objects or materials

with different temperatures the process

of heat transfer in solids is called

conduction one example is the rod on

firewood second method of heat transfer

is convection it is the transfer of heat

from one location to the other by the

movement of fluids examples are boiling

of waters and the steam the third method

of heat transfer is what we call

radiation it is the transfer of heat by

electromagnetic

wave and another example is the

microwave oven

now let us talk about engines

one such object that allows us to

produce mechanical work from type of

energy is called an engine if the energy

that was used to perform work was

thermal energy or heat then the engine

is called a heat engine to perform work

heat is taken in by the engine from a

heat source also called the high

temperature reservoir the energy

absorbed by the heat engine is used to

perform useful work however not all the

heat absorbed by the engine can be

converted into useful work

there will always be a portion of heat

that will be lost as a result of other

interactions like friction this lost

heat is called a waste heat this waste

heat goes to the low temperature

reservoir or the heat sink of that heat

engine the energy converted as useful

mechanical work is equal to the

difference in the heat input from high

temperature super server and the heat

output that was received by the low

temperature reservoir so work is equal

to heat input minus heat output again

class heat engine is a device which

converts thermal energy to mechanical

energy examples of heat engine are power

plants such as geothermal there are

three things that happen in the full

cycle of a heat engine first one

heat is added it is an input heat which

is relatively high temperature second

some of that energy from that input heat

is used to do work and third the rest of

the heat is removed at a relatively cold

temperature furthermore class a common

type of heat engine is called combustion

engine in a combustion engine heat is

produced using a combustion process

which in turn makes use of a fuel and an

oxidizer for that fuel-like air there

are two classes of combustion engines

the first one is external combustion

engine burning a fuel takes place

outside the engine examples are steam

and the piston engine that is shown in

your screen right now the second one is

the internal combustion engine burning a

fuel takes place inside the cylinder or

turbine engine examples are gasoline or

diesel engine most automobiles make use

of either a d cell or a gasoline engine

in the case of gasoline engines most

have four cylinders each containing a

piston each piston undergoes a series of

four movements or strokes as shown in

your screen right now is a four stroke

cycle in a gasoline engine the first

stroke is the intake stroke the intake

valve opens allowing the cylinder to

receive the fuel air mixture as the

piston moves downward the second one is

the compression stroke the piston moves

up compressing the fuel air mixture the

third one is the power stroke the spark

plug at the top of the cylinder causes

the mixture to ignite and combust making

its temperature high and the last one is

the exhaust stroke the combusted gases

are pushed out of the open exhaust bulb

through an upward motion on the piston

now let us talk about thermal efficiency

the thermal energy produced from the

combustion of fuel air mixture is

transformed into mechanical energy which

moves the car however not all thermal

energy is converted into useful work

this thermal energy which is not

converted to useful work is called waste

heat these heat losses are unavoidable

and greatly limit the efficiency of heat

engine for example the engines of cars

are only 30 percent efficient this means

that for every 100 joules of thermal

energy produced by a combustion of

gasoline only 30 joules are used to

actually move the car therefore it is

impossible to construct a heat engine

that is 100 efficient which can fully

convert all the heat into a useful work

an engine that converts energy into more

work and less weight is said to be more

efficient however according to sadi

carnot he found out that while it is

true that we can express efficiency in

terms of work the efficiency of ideal

heat engines depends only on the

temperatures of the hot and cold

reservoir according to him an engine

operating between two reservoirs of

higher temperature difference is more

efficient than an engine operating

between reservoirs of nearly the same

temperatures now let us have the

equation for efficiency it is calculated

as efficiency equals work done divided

by input heat times a hundred percent

but since work is just the input heat

minus the exhaust heat the equation

becomes efficiency equals the input heat

minus the exhaust heat divided by the

input heat times a hundred percent

simplifying this equation we have input

heat divided by input heat that is one

and we have the remaining minus exhaust

heat divided by input heat times a

hundred percent where q sub c is the

energy removed by heat or energy in cold

reservoir and q sub h is the energy

added by heat or energy in hot reservoir

the equation for efficiency can also be

modified to use temperature measurement

instead of the energy values therefore

we can say that efficiency is equal to 1

minus p sub c where t sub c is the

absolute temperature in cold reservoir

divided by t sub h where it is the

absolute temperature in hot reservoir

times a hundred percent take note class

the temperatures are the absolute

temperatures on the kelvin scale now let

us have a sample problem sample problem

number one what is the efficiency of a

gasoline engine that receives 193 joules

of energy from combustion

and lose 125 joules by heat to exhaust

during one cycle the given are q sub c

or the energy removed by heat which is

125 joules q sub h which is the energy

added by the heat and that is 193 joules

and we are looking for its efficiency we

are going to use this formula the one

that we had a while ago now let us

substitute our given to our formula and

we have one minus we have q sub c

divided by q sub h times a hundred

percent following the pemdas rule we

need first to calculate the numbers

inside the parentheses and do the

division first before the subtraction

125 joules divided by

193 joules we have

0.6476 and so on next subtract 1 minus

0.6476

we have

0.3523 and so on do not forget to

multiply it to 100

the product is 35.23

do not forget to round off your final

answer now let us have sample problem

number two suppose a steam engine

receives steam at 600 kelvin the engine

uses a part of these thermal energy for

work it exhausts the rest to a condenser

at the temperature of 350 kelvin what is

the maximum efficiency of this steam

engine our given we have the temperature

in cold reservoir which is 350 kelvin

and the absolute temperature in the hot

reservoir which is 600 kelvin and we are

looking for its efficiency our formula

the one that we had a while ago now let

us substitute our given to our formula

efficiency is equal to 1 minus 350

kelvin divided by 600 kelvin times

hundred percent again we need to follow

the pemdas rule divide first 350 divided

by 600 and this is the quotient next one

minus

0.583 and so on we have the difference

of

zero point four one six six and so on

then multiply it to one hundred percent

we have the product of forty one point

sixty seven percent do not forget to

round off your final answer and that is

how you are going to solve problems

involving thermal efficiency and that's

it for our lesson this week i hope you

learned something new again if this

video helped you please do like share

subscribe and hit the bell button to

keep you updated for my future videos

comment now for a shout out

shout out to gerlain bailon

john paul imessolania riya solomon

raniel de vera again from the bottom of

my heart thank you all so much for

watching you may also subscribe to lady

alchemist youtube channel for more

advanced chemistry lessons and to kcmctv

youtube channel for some inspirational

spoken poetry videos bye class and see

you on my next video

[Music]

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