Question

our galaxy is about 15 light years across and most energetic particles transversing it have energies of about 10^20 ev find the relativistic energy of the partical assuming it in a proton
1)16J
2)15J
3)10J
4)0J

Answer 1

So, we have energy

$$E=10^{20} eV$$

for one eV

$$1 eV=1.6\cdot 10^{-19} J$$

In joules this energy

$$E=10^{20} \cdot 1.6 \cdot 10^{-19} = 16 J$$

Answer 1) 16 J