Question

A thunder was heard by Brandon exactly 5 seconds after a lightning flushed. Assuming that the temperature of the air was equal to 26°C, how far was the lightning?

Answer 1

Explanation:

Assuming that the gas behaves ideally and that the process is reversible, we have that the entropy change in an isothermal reversible process is

ΔS=nRln

V

1

V

2

If the process is isothermal then we can use the ideal gas equation to get

ΔS=nRln

P

1

P

2

Now in order to use the formula, we need to determine the moles in 1 L of air at 25°C and an initial pressure of 760 torr (=1 atm). Using the ideal gas law,

n=

RT

PV

=

0.08205×298

1×1

=0.0409mol

Substituting in entropy formula,

ΔS=nRln

P

1

P

2

=0.0409×8.314×ln

758

760

=0.0009JK

−1