Define light year and explain how and why light years are used to measure distances in the universe

Distances ANYWHERE could be described in terms of inches, feet,
yards, miles, kilometers, nanometers, centimeters, millimeters, or
even AU (Astronomical Units).  But if you tried to use any of these
to describe the distance of anything away from the Earth's surface, 
you'd be faced with a ridiculous number.  It would be very hard to
remember this number or tell other people about it, and impractical
to write this number down in your notebook.

For example: The average distance between Earth and the moon is

38,440,145,820 centimeters,

and the distance to the nearest star outside the solar system
is approximately

1,671,433,491,000 kilometers.

Isn't that ridiculous ! ?

Obviously, we need much bigger units to describe these distances,
so that we can talk about them in more convenient numbers.  so the
light year was invented to fill this need.  It's simply the distance that
light travels through vacuum in one year.

Definition of 1 light year = exactly 9,460,730,472,580,800 meters

Answerthe energy required is 1.60 × 10⁻¹⁸ j, to three significant figures.explanationwe define electric potential as the ratio electric potential energy to the charge,    then the following gives us the potential difference,      the formula  may be on your formula sheet.the change in electric potential energy would be the energy that would needed to move it from point a to point b. solving for that, we get    using q = 8.00  × 10⁻¹⁹ c and  δv = 2.00 v, we get    the energy required is 1.60 × 10⁻¹⁸ j, to three significant figures.