Heat engines and work
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Heat engines and work
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[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
A Carnot engine is a type of engine that allows for the maximum amount of thermodynamic energy to be transformed into work. Although it is physically impossible to create one, the Carnot model is useful for determining the maximum efficiency of an engine. Scientists interested in increasing the fuel efficiency of automobiles begin by developing a heat engine, the closest engineers have come to replicating a Carnot engine.
The heat engine includes a piston that can move up or down allowing work to be done on the gas or allow the gas to do work on the surroundings. Neon, a monatomic ideal gas, is placed in a sealed canister connected to the tightly fitted piston. The canister can be placed in a temperature controlled room which allows heat to be transferred to the gas or allows heat to flow out of the gas. At the start of each experimental trial the gas is reset to a temperature of 298K and atmospheric pressure (101,000Pa). Assume that + work refers to the case where the surroundings do work on the gas, and – work refers to the case where the gas does work on the surroundings.
Figure 1. A diagram of the heat engine the scientists are using.
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[question] => During a certain trial, the canister is placed in a room which is at a temperature of 350K, and the piston is seen to move upwards. What can be said about the heat (Q) and the work done on the gas (W) during this trial?
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[description] => Reason for the Correct Answer:
If heat flows into a gas, the heat Q will be positive. If heat flows out of a gas, the heat will be negative
If the gas is compressed, the work done on the gas is positive. If the gas expands, the work done on the gas is negative
Since heat flows in to the gas, and the gas expands, Q is + and W is –
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[each_answer] => A. Q is +, and W is –
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[each_answer] => B. Q is -, and W is +
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[each_answer] => C. Q is -, and W is –
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[each_answer] => D. Q is +, and W is +
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[question] => During a certain trial, the piston is prevented from moving and the canister is placed in a room which is at a temperature of 255K. During this trial what will happen to the average kinetic energy of the atoms of the gas in the canister and why?
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[answer] => 3
[description] => Reason for the Correct Answer:
If heat flows into a gas, the heat Q will be positive. If heat flows out of a gas, the heat will be negative
The gas always starts at room temperature 298K
Since the piston does not move, no work can be done
Since the room temperature is lower than the initial temperature of the gas heat will flow out of the gas which means the heat flow will be negative and the kinetic energy of the gas will go down
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[each_answer] => A. The average kinetic energy of the gas atoms will increase since the heat will be positive
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[each_answer] => B. The average kinetic energy of the gas atoms will decrease since the work done by the gas will be positive
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[each_answer] => C. The average kinetic energy of the gas atoms will decrease since the heat will be negative
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[each_answer] => D. The average kinetic energy of the gas atoms will decrease since the work done by the gas will be negative
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[question] => During a certain trial a researcher does 3.2J of work by pushing the piston down. During the time while this work is done, the room in which the canister is placed absorbs 4.5J of heat from the gas. What can be concluded about the motion of the gas molecules after this trial compared to the motion before the trial?
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[answer] => 1
[description] => Reason for the Correct Answer:
If heat flows into a gas, the heat Q will be positive. If heat flows out of a gas, the heat will be negative
If the gas is compressed, the work done on the gas is positive. If the gas expands, the work done on the gas is negative
The internal energy of the gas decreases here since the negative heat had more impact than the work done on the gas
Since the internal energy of the gas decreased, the average speed of the molecules will decrease
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[each_answer] => A. The average speeds of the gas molecules decrease
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[each_answer] => B. The average speeds of the gas molecules increase
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[each_answer] => C. The speeds of all gas molecules decrease
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[each_answer] => D. The speeds of the gas molecules does not change
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[question] => During a certain trial, the piston is prevented from moving and the canister is placed in a room, which is at a temperature of 450K. What can be concluded about the pressure of the gas?
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[answer] => 2
[description] => Reason for the Correct Answer:
The room is at a higher temperature so heat will flow into the gas
The piston cannot move so no work can be done
The internal energy of the gas will increase but the volume does not change
Since the internal energy of the gas will increase but the volume does not change, the pressure must increase from heat flowing into the gas
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[each_answer] => A. The pressure of the gas will decrease since the heat will be –
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[each_answer] => B. The pressure of the gas will increase since the heat will be +
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[each_answer] => C. The pressure of the gas will increase since the work done will be +
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[each_answer] => D. The pressure of the gas will not change since no work can be done
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[question] => During a certain trial, the pressure of the gas is tripled while the volume is cut in half. What can be said about the kinetic energy of the gas molecules?
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[answer] => 3
[description] => Reason for the Correct Answer:
PV=nRT
Think about what happens to the temperature if pressure is tripled and volume is cut in half
The temperature must increase, which means the average kinetic energy of the gas increases
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[each_answer] => A. The average kinetic energy of the gas molecules increases since the temperature of the gas decreases
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[each_answer] => B. The average kinetic energy of the gas molecules does not change since temperature will not change
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[each_answer] => C. The average kinetic energy of the gas molecules increases since the temperature of the gas increases
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[each_answer] => D. The average kinetic energy of the gas molecules decreases since the temperature of the gas decreases
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