Pressure and depth

3 marks · Easy

Figure 1 shows a simple barometer using oil instead of mercury. The oil has a density of 900 kg/m³. The height of the oil column is 1.4 m. Take g = 10 N/kg.

Figure 1: Simple oil barometer

(a)

Calculate the atmospheric pressure in Pascals.

[2 marks]

(b)

Suggest why mercury is usually used in barometers rather than oil.

[1 mark]

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.

Show answer

(a) 12,600 Pa

(b) — · must show Mercury is much denser, so the column is much shorter for the same pressure, Oil would require a much taller column

Common mistakes: Forgetting to use correct units for density or height

Show mark scheme and worked solution

Marking points

M1 (a) · 1 mark · Substitutes correctly into p = h × ρ × g

A1 (a) · 1 mark · Calculates atmospheric pressure: 12,600 Pa

B1 (b) · 1 mark · Gives a valid reason (e.g., denser liquid means a shorter column is needed)

Worked solution

(a) p = h × ρ × g = 1.4 × 900 × 10 = 12,600 Pa

(b) Mercury is much denser than oil, so the barometer column needed for the same pressure is much shorter and easier to manage.