The two strands of the helix shaped dna molecule are held...

The two strands of the helix-shaped DNA molecule are held together by electrostatic forces as shown in the Figure 16-44 in the textbook. Assume that the net average charge (due to electron sharing) indicated on H and N atoms is 0.2e and on the indicated C and O atoms is 0.4e. Assume also that atoms on each molecule are separated by 1.0×10−10m. Part A Estimate the net force between a thymine and an adenine. For each bond (red dots) consider only the three atoms in a line (two atoms on one molecule, one atom on the other). Part B Estimate the net force between a cytosine and a guanine Part C Estimate the total force for a DNA molecule containing 105 pairs of such molecules.

Transcript

00:01 Alright, so let's say we have some molecules.

00:03 We have like an oxygen atom and a hydrogen ion and then a nitrogen ion.

00:12 And see, this distance we're told is 10 to the negative 10 meters.

00:17 This distance, we're told, is, let's see, 1 .8 times 10 to the negative 10 meters.

00:25 And then below that we have nitrogen to a hydrogen ion.

00:31 This distance is 10 to the negative 10 meters.

00:34 And then we have this interacting with the nitrogen ion, which is, let's see, this is 2 times 10 to the negative 10 meters.

00:46 And so we want to find, you know, if these are all contained or connected by one molecule, what is the net electrostatic force between them? so it'll be kulam's constant.

01:00 We'll have a constant term there.

01:01 And then we're also, we're told that the charges can be expressed in multiples of the electron charge.

01:06 So we're going to have an e squared here.

01:09 And then if we look at the charges we're going to have, so it'll be 0 .2 times the electron charge for the nitrogen ions and the hydrogen ions.

01:21 And then between those and the carbon atoms that's point four.

01:28 So it's 0 .2 times 0 .4.

01:30 If we're looking at, like, for example, this first pair, the oxygen -hydrogen pair, we'll have that, and then the distance between them is 1 .8 times 10 to the negative 10 meters squared.

01:43 And that's a force between two.

01:48 So that'll be an attractive force, and it'll be to the left.

01:51 So we'll put like a negative sign there.

01:54 And then if we look at our other forces, that's also going to be attractive.

01:58 So we'll put a positive sign for the southern one, but this should be 0 .2 squared...

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