# 18.4 Coulomb’s law  (Page 3/7)

 Page 3 / 7

As the example implies, gravitational force is completely negligible on a small scale, where the interactions of individual charged particles are important. On a large scale, such as between the Earth and a person, the reverse is true. Most objects are nearly electrically neutral, and so attractive and repulsive Coulomb forces nearly cancel. Gravitational force on a large scale dominates interactions between large objects because it is always attractive, while Coulomb forces tend to cancel.

## Test prep for ap courses

For questions 25–27, suppose that the electrostatics force between two charges is F .

What will be the force if the distance between them is halved?

1. 4 F
2. 2 F
3. F /4
4. F /2

(a)

Which of the following is false?

1. If the charge of one of the particles is doubled and that of the second is unchanged, the force will become 2 F .
2. If the charge of one of the particles is doubled and that of the second is halved, the force will remain F .
3. If the charge of both the particles is doubled, the force will become 4 F .
4. None of the above.

Which of the following is true about the gravitational force between the particles?

1. It will be 3.25×10 −38 F .
2. It will be 3.25×10 38 F .
3. It will be equal to F .
4. It is not possible to determine the gravitational force as the masses of the particles are not given.

(d)

Two massive, positively charged particles are initially held a fixed distance apart. When they are moved farther apart, the magnitude of their mutual gravitational force changes by a factor of n . Which of the following indicates the factor by which the magnitude of their mutual electrostatic force changes?

1. 1/ n 2
2. 1/ n
3. n
4. n 2
1. What is the electrostatic force between two charges of 1 C each, separated by a distance of 0.5 m?
2. How will this force change if the distance is increased to 1 m?

(a) 3.60×10 10 N, (b) It will become 1/4 of the original value; hence it will be equal to 8.99×10 9 N

1. Find the ratio of the electrostatic force to the gravitational force between two electrons.
2. Will this ratio change if the two electrons are replaced by protons? If yes, find the new ratio.

## Section summary

• Frenchman Charles Coulomb was the first to publish the mathematical equation that describes the electrostatic force between two objects.
• Coulomb's law gives the magnitude of the force between point charges. It is
$F=k\frac{|{q}_{1}{q}_{2}|}{{r}^{2}},$

where ${q}_{1}$ and ${q}_{2}$ are two point charges separated by a distance $r$ , and $k\approx 8.99×{10}^{9}\phantom{\rule{0.25em}{0ex}}\text{N}·{\text{m}}^{2}/{\text{C}}^{2}$

• This Coulomb force is extremely basic, since most charges are due to point-like particles. It is responsible for all electrostatic effects and underlies most macroscopic forces.
• The Coulomb force is extraordinarily strong compared with the gravitational force, another basic force—but unlike gravitational force it can cancel, since it can be either attractive or repulsive.
• The electrostatic force between two subatomic particles is far greater than the gravitational force between the same two particles.

## Conceptual questions

[link] shows the charge distribution in a water molecule, which is called a polar molecule because it has an inherent separation of charge. Given water's polar character, explain what effect humidity has on removing excess charge from objects.

Determine the total force and the absolute pressure on the bottom of a swimming pool 28.0m by 8.5m whose uniform depth is 1 .8m.
for the answer to complete, the units need specified why
That's just how the AP grades. Otherwise, you could be talking about m/s when the answer requires m/s^2. They need to know what you are referring to.
Kyle
Suppose a speck of dust in an electrostatic precipitator has 1.0000×1012 protons in it and has a net charge of –5.00 nC (a very large charge for a small speck). How many electrons does it have?
how would I work this problem
Alexia
how can you have not an integer number of protons? If, on the other hand it supposed to be 1e12, then 1.6e-19C/proton • 1e12 protons=1.6e-7 C is the charge of the protons in the speck, so the difference between this and 5e-9C is made up by electrons
Igor
what is angular velocity
angular velocity can be defined as the rate of change in radian over seconds.
Fidelis
Why does earth exert only a tiny downward pull?
hello
Islam
Why is light bright?
an 8.0 capacitor is connected by to the terminals of 60Hz whoes rms voltage is 150v. a.find the capacity reactance and rms to the circuit
thanks so much. i undersooth well
what is physics
is the study of matter in relation to energy
Kintu
physics can be defined as the natural science that deals with the study of motion through space,time along with its related concepts which are energy and force
Fidelis
a submersible pump is dropped a borehole and hits the level of water at the bottom of the borehole 5 seconds later.determine the level of water in the borehole
what is power?
power P = Work done per second W/ t. It means the more power, the stronger machine
Sphere
e.g. heart Uses 2 W per beat.
Rohit
A spherica, concave shaving mirror has a radius of curvature of 32 cm .what is the magnification of a persons face. when it is 12cm to the left of the vertex of the mirror
did you solve?
Shii
1.75cm
Ridwan
my name is Abu m.konnek I am a student of a electrical engineer and I want you to help me
Abu
the magnification k = f/(f-d) with focus f = R/2 =16 cm; d =12 cm k = 16/4 =4
Sphere
what do we call velocity
Kings
A weather vane is some sort of directional arrow parallel to the ground that may rotate freely in a horizontal plane. A typical weather vane has a large cross-sectional area perpendicular to the direction the arrow is pointing, like a “One Way” street sign. The purpose of the weather vane is to indicate the direction of the wind. As wind blows pa
hi
Godfred
Godfred
If a prism is fully imersed in water then the ray of light will normally dispersed or their is any difference?
the same behavior thru the prism out or in water bud abbot
Ju
If this will experimented with a hollow(vaccum) prism in water then what will be result ?
Anurag