Fig. 13.1 shows an open U-tube manometer containing mercury, connected to a gas supply on the left side. The right side is open to the atmosphere. The atmos...
Question 1 Report
Fig. 13.1 shows an open U-tube manometer containing mercury, connected to a gas supply on the left side. The right side is open to the atmosphere. The atmospheric pressure is 760 mmHg. The mercury level on the gas side is 50 mm lower than the mercury level on the open side. The gas supply provides a constant pressure to the left arm of the manometer. The mercury density is uniform throughout. The student reads the height difference and uses it to calculate the gas pressure. She then considers what would happen if the gas pressure were increased further.
(a) State on which side of the manometer the mercury level is higher. [1]
(b) Determine the pressure of the gas supply in mmHg. [2]
(c) Explain what happens to the mercury levels if the gas pressure is increased above its current value. [3]
Answer Details
(a) Which side has the higher mercury level [1]
The mercury level is higher on the open (right) side. [1]
This tells us that the gas pressure is greater than atmospheric pressure. The gas pushes the mercury down on its side, forcing it up on the open side.
(b) Pressure of the gas supply [2]
When the gas pressure exceeds atmospheric pressure, the mercury is pushed down on the gas side and up on the open side. The height difference represents the excess pressure above atmospheric.
Gas pressure = atmospheric pressure + height difference [1]
Gas pressure = 760 + 50 = 810 mmHg [1]
If the gas pressure were less than atmospheric, the mercury would be higher on the gas side, and we would subtract the height difference instead.
(c) What happens if gas pressure is increased further [3]
- The mercury on the gas (left) side is pushed further down because the increased gas pressure exerts a greater force on the mercury surface. [1]
- The mercury on the open (right) side rises further to compensate, since the mercury is an incompressible liquid and must be displaced from one arm to the other. [1]
- The height difference between the two mercury levels increases. This larger height difference balances the greater excess of gas pressure above atmospheric pressure. The new height difference in mmHg equals the new gas pressure minus 760 mmHg. [1]
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