A mercury barometer is used to measure atmospheric pressure. A student notices that the barometer reads 742 mm Hg742\text{ mm Hg}742 mm Hg instead of the true atmospheric pressure of 760 mm Hg760\text{ mm Hg}760 mm Hg because a small amount of air is trapped in the tube above the mercury column. Why does the trapped air cause the mercury column height to be lower than expected?
The trapped air molecules exert an upward force on the mercury column, reducing its height.
The trapped air molecules collide with the top surface of the mercury, exerting a downward pressure.
The trapped air cools the mercury column, causing it to contract.
The trapped air reacts with the mercury, reducing its density.
188 exam-style questions on CIE IGCSE Physics Pressure, covering Pressure. Each one has a worked solution and a mark scheme showing where the marks go.