# 13.10 Applications of electrostatics  (Page 4/14)

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Strategy

To solve an integrated concept problem, we must first identify the physical principles involved and identify the chapters in which they are found. Part (a) of this example asks for weight. This is a topic of dynamics and is defined in Dynamics: Force and Newton’s Laws of Motion . Part (b) deals with electric force on a charge, a topic of Electric Charge and Electric Field . Part (c) asks for acceleration, knowing forces and mass. These are part of Newton’s laws, also found in Dynamics: Force and Newton’s Laws of Motion .

The following solutions to each part of the example illustrate how the specific problem-solving strategies are applied. These involve identifying knowns and unknowns, checking to see if the answer is reasonable, and so on.

Solution for (a)

Weight is mass times the acceleration due to gravity, as first expressed in

$w=\text{mg}.$

Entering the given mass and the average acceleration due to gravity yields

$w=\left(\text{4.00}×{\text{10}}^{-\text{15}}\phantom{\rule{0.25em}{0ex}}\text{kg}\right)\left(9\text{.}\text{80}\phantom{\rule{0.25em}{0ex}}{\text{m/s}}^{2}\right)=3\text{.}\text{92}×{\text{10}}^{-\text{14}}\phantom{\rule{0.25em}{0ex}}\text{N}.$

Discussion for (a)

This is a small weight, consistent with the small mass of the drop.

Solution for (b)

The force an electric field exerts on a charge is given by rearranging the following equation:

$F=\text{qE}.$

Here we are given the charge ( $3.20×{10}^{–19}\phantom{\rule{0.25em}{0ex}}\text{C}$ is twice the fundamental unit of charge) and the electric field strength, and so the electric force is found to be

$F=\left(3.20×{\text{10}}^{-\text{19}}\phantom{\rule{0.25em}{0ex}}\text{C}\right)\left(3\text{.}\text{00}×{\text{10}}^{5}\phantom{\rule{0.25em}{0ex}}\text{N/C}\right)=9\text{.}\text{60}×{\text{10}}^{-\text{14}}\phantom{\rule{0.25em}{0ex}}\text{N}.$

Discussion for (b)

While this is a small force, it is greater than the weight of the drop.

Solution for (c)

The acceleration can be found using Newton’s second law, provided we can identify all of the external forces acting on the drop. We assume only the drop’s weight and the electric force are significant. Since the drop has a positive charge and the electric field is given to be upward, the electric force is upward. We thus have a one-dimensional (vertical direction) problem, and we can state Newton’s second law as

$a=\frac{{F}_{\text{net}}}{m}.$

where ${F}_{\text{net}}=F-w$ . Entering this and the known values into the expression for Newton’s second law yields

$\begin{array}{lll}a& =& \frac{F-w}{m}\\ & =& \frac{\text{9.60}×{\text{10}}^{-\text{14}}\phantom{\rule{0.25em}{0ex}}\text{N}-\text{3.92}×{\text{10}}^{-\text{14}}\phantom{\rule{0.25em}{0ex}}\text{N}}{\text{4.00}×{\text{10}}^{-\text{15}}\phantom{\rule{0.25em}{0ex}}\text{kg}}\\ & =& \text{14}\text{.}2\phantom{\rule{0.25em}{0ex}}{\text{m/s}}^{2}.\end{array}$

Discussion for (c)

This is an upward acceleration great enough to carry the drop to places where you might not wish to have gasoline.

This worked example illustrates how to apply problem-solving strategies to situations that include topics in different chapters. The first step is to identify the physical principles involved in the problem. The second step is to solve for the unknown using familiar problem-solving strategies. These are found throughout the text, and many worked examples show how to use them for single topics. In this integrated concepts example, you can see how to apply them across several topics. You will find these techniques useful in applications of physics outside a physics course, such as in your profession, in other science disciplines, and in everyday life. The following problems will build your skills in the broad application of physical principles.

## Unreasonable results

The Unreasonable Results exercises for this module have results that are unreasonable because some premise is unreasonable or because certain of the premises are inconsistent with one another. Physical principles applied correctly then produce unreasonable results. The purpose of these problems is to give practice in assessing whether nature is being accurately described, and if it is not to trace the source of difficulty.

## Problem-solving strategy

To determine if an answer is reasonable, and to determine the cause if it is not, do the following.

1. Solve the problem using strategies as outlined above. Use the format followed in the worked examples in the text to solve the problem as usual.
2. Check to see if the answer is reasonable. Is it too large or too small, or does it have the wrong sign, improper units, and so on?
3. If the answer is unreasonable, look for what specifically could cause the identified difficulty. Usually, the manner in which the answer is unreasonable is an indication of the difficulty. For example, an extremely large Coulomb force could be due to the assumption of an excessively large separated charge.

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