# Homework Help: Magnitude of the electric force a hydrogen nucleus exerts on electron?

1. Aug 11, 2014

### needingtoknow

1. The problem statement, all variables and given/known data

What is the magnitude of the electric force a hydrogen nucleus exerts on its only orbiting electron in the Bohr model?

3. The attempt at a solution

Fe = kQq/r^2

Since there is one proton and one electron. Q and q are equal to each other: 1.6 x 10^-19.
k = 9 x 10^9
and radius = 40 x 10^-19.

All the values are given in the question.

The answer is 10^-7 but I got 1.7 x 10^-7.

Why did they completely disregard the 1.7 ?

2. Aug 11, 2014

### Simon Bridge

It's called an "order of magnitude" calculation.
Though it may be a typo - depends: who is "they"?

Note: don't forget the units.

3. Aug 14, 2014

### needingtoknow

Thank you I shall look into that! Sorry they is the people who wrote the solution manual and I will remember to include units next time!

4. Aug 15, 2014

### Simon Bridge

No worries - also remember, when you refer to the work of others, to include their names and the title of the work in question. Otherwise the reference is meaningless. ;)

Note: if you use k=9x10^9 SI units, then your answer should be to 1 sig fig too.

5. Jun 11, 2017

### PlancksApprentice

Isn't the radius supposed to be (5.3 x 10-11)?? How did you get (40 x 10-19)??

I solved the problem the same way you did so (except that i switched the radius) and found that the answer came to be -0.82 x 10-7 C.

6. Jun 11, 2017

### Simon Bridge

Bohr radius (order e-11) would be a good choice, yes... you could akso have used e-10 (angstrom = order of magnitude size of an atom) OPs order e-19 meters would be inside the nucleus.

Note. Dont forget units, and justify/critique guessed numbers using a physical reference. Youd probably get away with it is a secondary course but college usually penalises you if you use the correct value without indicating why. IRL if you do this you dont get published/paid.