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Chapter 20: Problem 10
Under what circumstances is the path of a charged particle a parabola? A circle?
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DNA fragments introduced into an electrophoresis apparatus (see Application, page 360 ) generally carry negative charges equivalent to two extra electrons per base pair of nucleotides in the fragment. The table below shows the forces on several DNA fragments in an electrophoresis apparatus, as a function of the number of base pairs. Plot these data, establish a best-fit line, and use the resulting slope to determine the strength of the electric field in the electrophoresis apparatus. $$\begin{array}{|l|l|l|l|l|l|l|} \hline \text { Base pairs } & 400 & 800 & 1200 & 2000 & 3000 & 5000 \\ \hline \text { Force (pN) } & 0.235 & 0.472 & 0.724 & 1.15 & 1.65 & 2.87 \\ \hline \end{array}$$
Earth carries a net charge of about \(-5 \times 10^{5} \mathrm{C}\). How many more electrons are there than protons on Earth?
A \(9.5-\mu \mathrm{C}\) charge is at \(x=15 \mathrm{cm}, y=5.0 \mathrm{cm}\) and a \(-3.2-\mu \mathrm{C}\) charge is at \(x=4.4 \mathrm{cm}, y=11 \mathrm{cm} .\) Find the force on the negative charge.
A 5.0 - \(\mu\) m strand of DNA carries charge \(+e\) per nm of length. Treating it as a charged line, what's the electric field strength \(25 \mathrm{nm}\) from the DNA, not near either end?
Conceptual Example 20.1 shows that the gravitational force between an electron and a proton is about \(10^{-40}\) times weaker than the electric force between them. since matter consists largely of electrons and protons, why is the gravitational force important at all?
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