The Physics
Hypertextbook
Opus in profectus

# Coulomb's Law

## Problems

### practice

1. Compare the magnitude of the electrostatic and gravitational forces between…
1. an electron and proton in a hydrogen atom (the radius of the electron's orbit is about 0.053 nm)
2. two protons in a helium nucleus (the separation between them is about 1.2 fm)
3. the Earth and the moon (the separation between them can be found in many references)
Compile your results in a table like the one below.
electrostatic
force (FE)
gravitational
force (Fg)
order of magnitude
comparison (FE/Fg)
e and p+ in
hydrogen atom
p+ and p+ in
helium nucleus
Earth and moon
in space
2. If the moon were held in its orbit by an electrostatic rather than gravitational force…
1. What quantity of charge would be needed?
2. How many elementary charges is this?

If all of the charge came from the separation of hydrogen atoms into electrons and protons…

1. What mass of hydrogen would be required?
2. How many liters of H2 gas at STP would be required?
3. Given three charges in a standard coordinate system, calculate the magnitude and direction of the net electrostatic force on each.
1. +10 μC at (+0 m, +0 m)
2. −20 μC at (+0 m, +3 m)
3. +25 μC at (−4 m, +0 m)
4. Write something completely different.

### conceptual

1. How does…
1. the electrostatic force exerted by a proton on an electron compare to the electrostatic force exerted by an electron on a proton?
2. the gravitational force exerted by a proton on an electron compare to the gravitational force exerted by an electron on a proton?
2. If electric forces are so much more powerful than gravitational forces, why do we feel the Earth's gravitational force and not its electric force?