The density of water in a lake is 1000 kg/m³. At a depth of 25 m beneath the surface of the lake, the total pressure is 3.5 × 10⁵ Pa.

1. (a)State what is meant by pressure.[1]
2. (b)(i)The gravitational field strength is 10 N/kg. Determine: the pressure due to 25m of water[2]
3. (b)(ii)the atmospheric pressure.[1]
4. (c)(i)An underwater depth gauge contains a small cylinder as shown in Fig. 8.1. Gas is trapped inside the cylinder by a piston. The piston is free to move. [Figure 8.1] The outer face of the piston is in contact with the water. As the depth gauge is lowered into the water, the piston moves into the cylinder. This moves a needle on a dial to indicate the depth of the gauge in the water. Explain why the piston moves into the cylinder.[3]
5. (c)(ii)The temperature of the gas does not change as the piston moves into the cylinder. Explain, in terms of molecules, what happens to the pressure of the trapped gas as the piston moves into the cylinder.[3]
6. (c)(iii)At the surface of the water, the volume of the trapped gas in the depth gauge is Vo. On Fig. 8.2, sketch a graph to show how the volume of trapped gas decreases as the gauge is lowered into the water. [Figure 8.2][2]
7. (c)(iv)The instructions for the depth gauge state that, each time it is used, the needle of the dial must be re-set to zero at the surface of the water. Suggest one reason for this.[1]
8. (c)(v)The density of the air trapped in the depth gauge increases. The density of the water remains constant. Explain, in terms of the molecules of the water, why the density of the water remains constant.[2]

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