The Pressure Law (Edexcel IGCSE Physics (Modular): Unit 2): Revision Note

The pressure inside a bicycle tyre is 5.10 × 10⁵ Pa when the temperature is 279 K. After the bicycle has been ridden, the temperature of the air in the tyre is 299 K. Calculate the new pressure in the tyre, assuming the volume is unchanged.

Answer:

Step 1: Choose the correct ideal gas law

• Volume is constant, so the pressure law must be used

$\frac{p_1}{T_1}=\frac{p_2}{T_2}$

Step 2: Write down the known quantities

• p₁ = 5.10 × 10⁵ Pa

• T₁ = 279 K

• T₂ = 299 K

Step 3: Rearrange for p₂ and substitute values into the pressure law

• To make p₂ the subject, multiply both sides by T₂ to cancel out the T₂ in the fraction under p₂

$\frac{p_1}{T_1}\times T_2=\frac{p_2}{\cancel{T_2}}\times \cancel{T_2}$

$\frac{p_1}{T_1}\times T_2=p_2$

• Substitute the known quantities

$p_2=\frac{5.10\times {10}^5}{279}\times 299$

$p_2=5.47\times {10}^5\mathrm{Pa}$

Remember when using gas laws the temperature T must always be in kelvin (K)!