Fig. 1 shows a car tyre immediately after a long motorway journey. The tyre contains air in a nearly fixed volume. A dashboard sensor reports that the tyre ...
Question 1 Report
Fig. 1 shows a car tyre immediately after a long motorway journey. The tyre contains air in a nearly fixed volume. A dashboard sensor reports that the tyre pressure has changed from 220 kPa at 17 degrees C to 250 kPa after the journey. The driver stops at a service area and considers releasing some air. The car is carrying a heavy box in its boot.
(a) State why the pressure gas in the tyre rises during the journey. [1]
(b) Explain the pressure increase using collisions of air molecules with the tyre wall. [3]
(c) Calculate the temperature of the tyre air in degrees C using p1/T1 = p2/T2. [4]
(d) State whether the driver should set the tyre pressure while the tyre is hot or after it has cooled to the recommended temperature. [1]
(e) Explain why releasing air while the tyre is hot can lead to low pressure later. [2]
(f) Describe two effects that a tyre with very low pressure may have on a car. [2]
(g) State which energy store of the gas increases when its temperature increases. [1]
(h) Complete the sentence: at constant volume, gas pressure is proportional to its absolute ________. [1]
(i) Explain why the heavy box can make correct tyre pressure especially important. [1]
Answer Details
(a) The tyre air is heated during the journey, for example by tyre flexing and road friction. [1]
(b) Heating gives air molecules greater kinetic energy, so they move faster. They collide with the tyre wall more frequently and with greater force. This raises the pressure. [3]
(c) Convert the initial temperature to kelvin:
\[T_1=17+273=290\text{ K}\]
\[T_2=\frac{250\times290}{220}=329.5\text{ K}\]
\[329.5-273=56.5\degree\text{C}\]
Tyre-air temperature \(\approx57\degree\text{C}\). [4]
(d) Set the tyre pressure after the tyre has cooled to the recommended temperature. [1]
(e) Releasing air while hot removes some gas at high pressure. When the remaining gas cools, its pressure falls and may become lower than the recommended value. [2]
(f) Two effects of very low tyre pressure include poorer handling, increased tyre wear, increased rolling resistance and energy use, and a greater risk of tyre damage or overheating. [2]
(g) The gas's kinetic energy store, or internal energy, increases. [1]
(h) At constant volume, pressure is proportional to absolute temperature. [1]
(i) The heavy box increases the force on the tyre, making an underinflated tyre more likely to deform or be damaged. [1]
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