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The net work done by a cyclical process is the area inside the closed path on a PV size 12{ ital "PV"} {} diagram, such as that inside path ABCDA in [link] . Note that in every imaginable cyclical process, it is absolutely necessary for heat transfer from the system to occur in order to get a net work output. In the Otto cycle, heat transfer occurs along path DA. If no heat transfer occurs, then the return path is the same, and the net work output is zero. The lower the temperature on the path AB, the less work has to be done to compress the gas. The area inside the closed path is then greater, and so the engine does more work and is thus more efficient. Similarly, the higher the temperature along path CD, the more work output there is. (See [link] .) So efficiency is related to the temperatures of the hot and cold reservoirs. In the next section, we shall see what the absolute limit to the efficiency of a heat engine is, and how it is related to temperature.

The figure shows four diagrams a, b, c, and d representing four stages of a four stroke gasoline engine. The construction of the engine has the base chamber whose cross section is in the shape of a square with flat corners, the top portion of the chamber is extended into a cylindrical section. The cylindrical section ends in the upper section with two valves, an inlet and an outlet. The cylindrical section has a movable cylinder with a piston attached to it. The piston is connected to the crank shaft in the base gas chamber. There is a spark plug on the top most part of the cylinder between the two valves. The four parts of the diagram show various stages of this four stoke engine. Part a of the diagram shows the air fuel mixture enters through the inlet valve in the upper section of the engine. The outlet valve is shown to be closed. The air and fuel is shown to exert a pressure on the piston acting downward. This force is shown to move the rotating crank shaft in clockwise direction in the gas chamber. This is the intake stroke. Part b of the diagram shows the compression stroke. Both the inlet and outlet valves are closed. The air and fuel mixture is shown to be compressed. The piston is shown to rise up as shown by a vertically pointing arrow. The piston is at the edge of the cylinder near the valves. The crankshaft in the gas chamber has shown to complete one complete cycle of rotation in the gas chamber. Part c of the diagram shows the power stroke. It has two parts, first the ignition stroke. This shows ignition of the fuel in the cylinder and pressure buildup in the region. Then in the second part the piston is shown to descend down the cylinder moving the crankshaft in the gas chamber in the clockwise direction. Part d of the figure shows the exhaust stroke. The piston expels the hot gas by moving upward and the gas is expelled through the exhaust valve.
In the four-stroke internal combustion gasoline engine, heat transfer into work takes place in the cyclical process shown here. The piston is connected to a rotating crankshaft, which both takes work out of and does work on the gas in the cylinder. (a) Air is mixed with fuel during the intake stroke. (b) During the compression stroke, the air-fuel mixture is rapidly compressed in a nearly adiabatic process, as the piston rises with the valves closed. Work is done on the gas. (c) The power stroke has two distinct parts. First, the air-fuel mixture is ignited, converting chemical potential energy into thermal energy almost instantaneously, which leads to a great increase in pressure. Then the piston descends, and the gas does work by exerting a force through a distance in a nearly adiabatic process. (d) The exhaust stroke expels the hot gas to prepare the engine for another cycle, starting again with the intake stroke.
Part a of the figure shows a graph of pressure P versus volume V for an Otto cycle. The pressure P is along the Y axis and the volume V is along the X axis. The graph shows a complete cycle A B C D.  The path begins at point A; the curve rises upward from point A to point B along the direction of the negative X axis. This is marked as an adiabatic process. Then the curve rises vertically up from point B to point C in a direction perpendicular to the X axis. Then the curve moves smoothly down to point D along the direction of the positive X axis. This is also marked as an adiabatic process. The last part of the curve drops vertically down from point D back to point A. The path A B is slightly lower than path C D. Heat Q sub h is shown to enter the system as shown by a bold arrow to the curve B C. Heat Q sub c is shown to leave the system as shown by a bold arrow near D A. The area inside A B C D is shaded, and the area of the shaded region is shown proportional to the work W. Part b of the diagram shows an internal combustion engine represented as a circle. The hot reservoir is a rectangular section at the top of the circle shown at temperature T sub h. A cold reservoir is shown as a rectangular section in the bottom part of the circle at temperature T sub c. Heat Q sub h is shown to enter the heat engine, as shown by a bold arrow. Work W is produced as output, shown to leave the system, and the remaining heat Q sub c is returned back to the cold reservoir as shown by bold arrow toward it.
PV size 12{ ital "PV"} {} diagram for a simplified Otto cycle, analogous to that employed in an internal combustion engine. Point A corresponds to the start of the compression stroke of an internal combustion engine. Paths AB and CD are adiabatic and correspond to the compression and power strokes of an internal combustion engine, respectively. Paths BC and DA are isochoric and accomplish similar results to the ignition and exhaust-intake portions, respectively, of the internal combustion engine’s cycle. Work is done on the gas along path AB, but more work is done by the gas along path CD, so that there is a net work output.
Part a of the figure shows a graph of pressure P versus volume V for an Otto cycle. The pressure P is along the Y axis and the volume V is along the X axis. The graph shows a complete cycle A B C D. The path begins at point A; the curve rises upward from point A to point B along the direction of the negative X axis. This is marked as an adiabatic process. Then the curve rises vertically up from point B to point C in a direction perpendicular to the X axis. Then the curve moves smoothly down to point D along the direction of the positive X axis. This is also marked as an adiabatic process. The last part of the curve drops vertically down from point D back to point A. The path A B is much lower than path C D, which shows that the starting temperature of path C D is higher than A B. Heat Q sub h prime enters the system as shown by a bold arrow to the curve B C. Heat Q sub c prime leaves the system, as shown by a bold arrow near D A. The area inside A B C D is shaded. Part b of the diagram shows an internal combustion engine represented as a circle. The hot reservoir is a rectangular section at the top of the circle shown at temperature T sub h prime. A cold reservoir is shown as a rectangular section in the bottom part of the circle at temperature T sub c prime. Heat Q sub h prime enters the heat engine as shown by a bold arrow, a work W prime is produced as output, shown to leave the system, and the remaining heat Q sub c prime is returned back to the cold reservoir, as shown by a bold arrow toward it.
This Otto cycle produces a greater work output than the one in [link] , because the starting temperature of path CD is higher and the starting temperature of path AB is lower. The area inside the loop is greater, corresponding to greater net work output.

Section summary

  • The two expressions of the second law of thermodynamics are: (i) Heat transfer occurs spontaneously from higher- to lower-temperature bodies but never spontaneously in the reverse direction; and (ii) It is impossible in any system for heat transfer from a reservoir to completely convert to work in a cyclical process in which the system returns to its initial state.
  • Irreversible processes depend on path and do not return to their original state. Cyclical processes are processes that return to their original state at the end of every cycle.
  • In a cyclical process, such as a heat engine, the net work done by the system equals the net heat transfer into the system, or W = Q h Q c , where Q h is the heat transfer from the hot object (hot reservoir), and Q c is the heat transfer into the cold object (cold reservoir).
  • Efficiency can be expressed as Eff = W Q h size 12{ ital "Eff"= { {W} over {Q rSub { size 8{h} } } } } {} , the ratio of work output divided by the amount of energy input.
  • The four-stroke gasoline engine is often explained in terms of the Otto cycle, which is a repeating sequence of processes that convert heat into work.

Questions & Answers

Calculate the work done by an 85.0-kg man who pushes a crate 4.00 m up along a ramp that makes an angle of 20.0º20.0º with the horizontal. (See [link] .) He exerts a force of 500 N on the crate parallel to the ramp and moves at a constant speed. Be certain to include the work he does on the crate an
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What is thermal heat all about
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why uniform circular motion is called a periodic motion?.
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what is distance of A to B of the stations and what is the time taken to reach B from A
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the information provided is not enough
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there is enough information to calculate an AVERAGE acceleration
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mistake, there is enough information to calculate an average velocity
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what are the types of radioactivity
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It is the opposite of kinetic friction
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static fiction is friction between two surfaces in contact an none of sliding over on another, while Kinetic friction is friction between sliding surfaces in contact.
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I don't get it,if it's static then there will be no friction.
author
It means that static friction is that friction that most be overcome before a body can move
kingsley
static friction is a force that keeps an object from moving, and it's the opposite of kinetic friction.
author
It is a force a body must overcome in order for the body to move.
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what is wave
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what is force
Muhammed
force is something which is responsible for the object to change its position
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more technically it is the product of mass of an object and Acceleration produced in it
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wave is disturbance in any medium
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energy is distributed in any medium through particles of medium.
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we have to first figure out .... wats a particle accelerator first
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The resistive force of surface.
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on both a string and peldulum?
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spring*
Anya
Yea
yusuf
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yusuf
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Which is
yusuf
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I don't understand the basics of this group
Jimmy
teach him SHM init
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Simple harmonic motion
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how.an.equipotential.line is two dimension and equipotential surface is three dimension ?
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definition of mass of conversion
umezurike Reply
Force equals mass time acceleration. Weight is a force and it can replace force in the equation. The acceleration would be gravity, which is an acceleration. To change from weight to mass divide by gravity (9.8 m/s^2).
Marisa
how many subject is in physics
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the write question should be " How many Topics are in O- Level Physics, or other branches of physics.
effiom
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Praise what level are you
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If u are doing a levels in your first year you do AS topics therefore you do 5 big topic i.e particles radiation, waves and optics, mechanics,materials, electricity. After that you do A level topics like Specific Harmonic motion circular motion astrophysics depends really
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Heat nd Co for a level
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yh I need someone to explain something im tryna solve . I'll send the question if u down for it
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hallow
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please send the answer
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the range of objects and phenomena studied in physics is
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Practice Key Terms 4

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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