Pressure in Liquids and Gases
3 marks · NormalA laboratory set of Magdeburg hemispheres has a surface area of 0.060 m². The atmospheric pressure is 100,000 Pa.
Calculate the total force acting on the outside of the hemispheres due to air pressure.
Explain why the hemispheres cannot be pulled apart until air is let back in.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) 6000 N · must show F = p × A, 100,000 Pa × 0.060 m²
(b) Because after air is removed from the inside, the pressure from the outside exerts a large force, holding the hemispheres together until the pressures are equal again. · must show Pressure only from outside, Large unbalanced force
Common mistakes: Uses wrong area or pressure value; Forgets to multiply
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Marking points
M1 (a) · 1 mark · States and/or uses F = p × A.
A1 (a) · 1 mark · Substitutes and evaluates 100,000 × 0.060 = 6000 N.
B1 (b) · 1 mark · Explains that only outside pressure acts, making hemispheres stick together.
Worked solution
a) F = p × A = 100,000 Pa × 0.060 m² = 6,000 N.
b) After air is removed from inside, only the outside atmospheric pressure acts on the hemispheres. This produces a strong net force holding the hemispheres together, which cannot be overcome until air is let back in, balancing the forces.