A nurse uses a sealed syringe to give a liquid medicine. Fig. 1 shows the cap fitted to the nozzle and the plunger being pushed inward. No liquid or air can...
Question 1 Report
A nurse uses a sealed syringe to give a liquid medicine. Fig. 1 shows the cap fitted to the nozzle and the plunger being pushed inward. No liquid or air can leave the syringe.
(a) Describe what happens to the volume of air trapped in the syringe when the plunger is pushed. [1]
(b) Explain why the pressure of the trapped air increases. [3]
(c) State what happens to the force needed to push the plunger if its area is unchanged and the pressure rises. [1]
(d) Calculate the pressure when a force of 45 N acts on a plunger area of 3.0 cm2. [3]
(e) State one safety reason for removing the cap before injecting liquid. [1]
Answer Details
(a) The volume of trapped air decreases. [1]
(b) Pushing the plunger traps the same number of gas particles in a smaller volume. [1] The particles hit the syringe walls more frequently. [1] This increases force per unit area, so the air pressure increases. [1]
(c) The force needed increases. [1] Since \(F=pA\), with plunger area unchanged, a larger pressure requires a larger force.
(d) Convert the area before using SI units:
\[3.0\text{ cm}^2=3.0\times10^{-4}\text{ m}^2\] [1]
\[p=\frac{F}{A}\] [1]
\[p=\frac{45\text{ N}}{3.0\times10^{-4}\text{ m}^2}=150\,000\text{ Pa}\]
The pressure is 150 000 Pa. [1]
(e) Removing the cap allows liquid to leave the syringe, preventing excessive pressure from building up. [1]
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