Question

A student (different from the one discussed earlier) wrote the following written explanation for why the shaving gel expanded when the vacuum pump was turned on.

(1) When the pump is turned on, air is removed from the glass jar and this causes the air pressure inside both the glass jar and the air pocket to decrease the same amount, while the air pressure outside the glass jar remains the same. (2) Thus the air pressure inside and outside the air pocket remain the same, and as a result the strength of the force due to the air pushing outward equals the strength of the force due to the air pushing in on the air pocket. (3) This means the forces on the air pocket are balanced, and by Newton’s law the surface then begins moving outward at a constant speed, making the air pocket expand. This happens inside each air pocket, so the whole shaving gel expands.

How would you evaluate this explanation?

	A.	It is accurate as is.
	B.	It is not accurate because sentence #1 is incorrect.
	C.	It is not accurate because sentence #2 is incorrect.
	D.	It is not accurate because sentence #3 is incorrect.
	E.	It is not accurate because more than one sentence is incorrect.

Answer 1

Yes shaving cream will expand surely but explanation is not correct.

Answer will be- Option (E). It is not accurate because more than one sentence is incorrect.

Option C .It is not accurate because sentence #2 is incorrect.

Option D. It is not accurate because sentence #3 is incorrect.

Explanation- The vacuum pump is turned on and the air inside of the bell jar is slowly pumped out. This causes a decrease in air pressure around the shaving cream. Since the air pressure inside the shaving cream has not changed, there is now a difference in pressure. The force on the molecules from the air inside is now greater than the force on the molecules from the outside. The shaving cream expands​​​​​​​