Posted by Sarah on .
MultipleConcept 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 1011 m. Determine the magnitude of the electron's centripetal acceleration.
MultipleConcept 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,
The force is wrong.
F=k2e^2/(3.09E11)^2
recheck your calcs. 
Physics 
Sarah,
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,
Your force is wrong. Are you using for k 9E9, e 1.6E19
so e sqared is 2.6 E38
So as an order of magnitude..
f= 10E10*2.5E38/10E22= E(10+2248)=Elarge number. Force is small. Check my thinking. 
Physics 
Sarah,
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,
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,
But when I do it the way I wrote it, my answer is 4.8 x 10 ^ 7.

Physics 
bobpursley,
You are most wrong.
YOu have in the numerator exponents 938
In the denominator exponents 22
moving the denominator to the numerator,
exponents 938+22 which is about 10^7