Posted by **Sarah** on Wednesday, February 11, 2009 at 10:45pm.

Multiple-Concept Example 3 provides some pertinent background for this problem. Suppose a single electron orbits about a nucleus containing two protons (+2e), as would be the case for a helium atom from which one of the naturally occuring electrons is removed. The radius of the orbit is 3.09 x 10-11 m. Determine the magnitude of the electron's centripetal acceleration.

Multiple-Concept Example 3 tells you that the electron mass is 9.11 x 10 ^ -31 kg.

I know what I have to do...

First, use Coulomb's law to find the Force. Then, divide by mass to find the acceleration. But the answer Im getting is HUGE.

This is what I found....

F= 6.94 x 10 ^ -4.

Then, a= 7.62 x 10 ^26.

That is the wrong answer. Please help!!

- Physics -
**bobpursley**, Wednesday, February 11, 2009 at 10:48pm
The force is wrong.

F=k2e^2/(3.09E-11)^2

recheck your calcs.

- Physics -
**Sarah**, Wednesday, February 11, 2009 at 10:50pm
I redid it and got F= 4.82 x 10 ^ 7. Is that correct? Then a = 5.29 x 10 ^ 23. Isnt that answer too big?

- Physics -
**bobpursley**, Wednesday, February 11, 2009 at 10:53pm
Your force is wrong. Are you using for k 9E9, e 1.6E-19

so e sqared is 2.6 E-38

So as an order of magnitude..

f= 10E10*2.5E-38/10E-22= E(10+22-48)=E-large number. Force is small. Check my thinking.

- Physics -
**Sarah**, Wednesday, February 11, 2009 at 11:02pm
Im not sure where you got those numbers from. I thought it would be...

F = (8.99 x 10 ^ 9)(1.6 x 10 ^ -19)(3.2 x 10 ^ -19) / (3.09 x 10 ^ -11) ^ 2

You have to have (3.2 x 10 ^ -19) because the proton has a charge of +2e. Right? Or is that still wrong?

- Physics -
**bobpursley**, Wednesday, February 11, 2009 at 11:10pm
yes, I did it only in ORDER OF MAGNITUDE, to show you that your force was way off. You need to do it accurately.

- Physics -
**Sarah**, Wednesday, February 11, 2009 at 11:17pm
But when I do it the way I wrote it, my answer is 4.8 x 10 ^ 7.

- Physics -
**bobpursley**, Wednesday, February 11, 2009 at 11:24pm
You are most wrong.

YOu have in the numerator exponents 9-38

In the denominator exponents -22

moving the denominator to the numerator,

exponents 9-38+22 which is about 10^-7

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