Question

The earth surface or a point at infinity from the earth can be chosen as zero reference level of? (a) Electric P.E. (b) Kinetic Energy (c) Gravitational P.E. (d) All of the above.

Answer 1

I think `"C"`.

Explanation:

• We often define the surface of the earth as a point of `0` gravitational potential energy when dealing with objects near the earth's surface, such as a book sitting on a shelf, which has GPE `U=mgh`, where `h` is defined as the height of the book above Earth's surface.

• For GPE between two massive bodies, we further apply Newton's laws of gravitation. The way that gravitational potential energy is defined here is negative.

`U_g=-(Gm_1m_2)/r`

The negative potential energy means that the potential energy of two masses at separation r is less than their potential energy at infinite separation. The zero point of potential energy is defined at `r=oo`.

So it is certainly applicable to answer `"C"`.

• Kinetic energy is `0` for objects at rest, as `v=0`, and kinetic energy is defined by:

`K=1/2mv^2`

regardless of the object's position relative to the earth.

• The electric potential tends to be defined such that the negative potential is the electric field

`-DeltaV=E`

Answer 2

I think (a) Electric P.E.

Explanation:

I at first thought GPE. Then I reread the question. Since it says, that the zero point can be the Earth or a point at infinite distance. That is done with Electric P.E. It is true that a point at infinity from the earth could be chosen. However I see no advantage to that.

This well respected website discusses both options:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elepe.html
Look at the last 3 sentences in the section titled Zero Potential. Also look at the section titled Potential Reference at Infinity . Where they are letting the distance `r_b` increase to infinity, they are setting the reference equal to a point at infinity from the charge `Q`.

I hope this helps,
Steve