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When gas in piston and heated but pressure remains constant. It is easier to explain in terms of the average force acting on a unit area.
2008PPp1q15
Answer: B
2017PPp1q15
Answer: A
In short: Temperature increases, Kinetic Energy increases, Rate of Collision decreases, Collision force on wall increases (due to higher speed), Average force per unit area remains constant, Pressure constant.
As piston is free to move, it will move to the right such that the pressure remains constant (equal to atmospheric pressure outside). As the piston moves to the right, the volume inside the piston increases. Surface area of piston in which the air molecules collide increases.
The rate of collision decreases as the number of molecules remains constant. With higher KE of molecules, the molecules will collide the wall with greater force. Though rate of collision decreases, with each collision having greater impact force, this makes average force per unit area acting on the wall of piston remains the same. Hence pressure remains constant.
Misconception: Many think that the rate of collision remains the same, which is wrong. Considering per unit area, if rate of collision remains the same, with the speed of the molecules increases, there will be greater collision force with the wall. This will result in the average force per unit area being higher, hence the pressure would increase.
If the rate of collision increased, with the speed of the molecules increases, there will be greater collision force with the wall. This will result in the average force per unit area being much higher, hence the pressure would increase even more.
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