  # Physics official question: what the *****?

<p>i bought the college boards review for all subject tests and while taking the physics test, i came across this question: The radius of the first Bohr orbit of an electron in a hydrogen atom is about 10^-11 meter. The radius of the nucleus is about 10^-15 meter. If a model of the hydrogen atom were built with the diameter of the electron orbit equal to the width of a classroom (10 meters), which of the following would most closely represent the size of the nucleus? A) the chair you are sitting in<br>
C)the eraser on the end of a new pencil
D) the point of a ball point pen
E) A red blood cell
This is a question from a released test. How the **** are we supposed to know this? D is correct btw.</p>

<p>Hm, it seems like C to me because that would be 0.01 meters.</p>

<p>It's actually a relatively simple ratio, completely unrelated to physics (well, it's a physics application, but requires no prior physics knowledge, just some math and familiarity with the metric system).</p>

<p>The model scales 1/10^11 to 10, which is multiplication by 10^12. So multiply the other number by the same factor: (1/10^15)*10^12= 10^-3=1/1000. One thousandth of a meter is a millimeter, which most closely resembles the tip of a ball-point pen.</p>

<p>Ha, there was a question like this in a chemistry competition that I did. I thought it was cool.</p>

<p>This is actually a fermi question, which (according to my teacher) is a fairly common physics question. It tests your ability to accurately estimate within powers of 10.</p>

<p>Is it D? I dont know, but it seems like the best logical answer since 10^-15 is small, but not small enought to be the size of a red blood cell.</p>

<p>It's essentially asking this: "which of the following objects is 10000 times smaller than the width of this room?" Or, as somebody mentioned, "which of these objects is 1 mm (1/1000 meters) in diameter?" The pen tip is right. A, B, C are too big; E is way too small.</p>

<p>order of magnitudes...</p>

<p>Relatively simple. Just ratios etc.</p>

<p>They use</p>

<p>10^-11:10</p>

<p>Therefore the Nucleus will be</p>

<p>[(10^-15)/(10^-11)]*10=0.001m=0.1cm which is approximately the size of the point of a ball point pen.</p>

<p>There is very little difference between C and D</p>