Coulomb's law

q

{\displaystyle q}

) should have units of force*
*Electric field (

E

{\displaystyle E}

) should have units of force per area (the same as stress)*
*Electric constant (

ϵ
0

{\displaystyle \epsilon\_0}

) should have units of force per area (the same as bulk modulus)*
*Magnetic field (

B

{\displaystyle B}

) should have units of force per velocity per time^2*
*Magnetic constant (

μ
0

{\displaystyle \mu\_0}

) should have units of 1/(force per time^2)*
See this for more details

'*Coulomb's law**, or** Coulomb's inverse-square law**, is an experimental law of physics that quantifies the amount of force between two stationary, electrically charged particles. The electric force between charged bodies at rest is conventionally called*** **electrostatic force *or*** *Coulomb force**. The law was first discovered in 1785 by French physicist Charles-Augustin de Coulomb, hence the name. Coulomb's law was essential to the development of the theory of electromagnetism, maybe even its starting point,'*** as it made it possible to discuss the quantity of electric charge in a meaningful way.

The law states that the magnitude of the electrostatic force of attraction or repulsion between two point charges is directly proportional to the product of the magnitudes of charges and inversely proportional to the square of the distance between them,

F

=

k
e

q
1

q
2

r
2

=

1

4
π

ε
0

q
1

q
2

r
2

Here, *k*e is Coulomb's constant (*k*e ≈ 8.988×109 N⋅m2⋅C−2), *q*1 and *q*2 are the signed magnitudes of the charges, and the scalar *r* is the distance between the charges.

The force is along the straight line joining the two charges. If the charges have the same sign, the electrostatic force between them is repulsive; if they have different signs, the force between them is attractive.

Being an inverse-square law, the law is analogous to Isaac Newton's inverse-square law of universal gravitation, but gravitational forces are always attractive, while electrostatic forces can be attractive or repulsive. Coulomb's law can be used to derive Gauss's law, and vice versa. In the case of a single stationary point charge, the two laws are equivalent, expressing the same physical law in different ways. The law has been tested extensively, and observations have upheld the law on the scale from 10−16 m to 108 m.

## Vector form of the law

Coulomb's law in states that the electrostatic force

F
1

{\displaystyle \boldsymbol{F}\_1}

 experienced by a charge,

q
1

{\displaystyle q\_1}

 at position

r
1

{\displaystyle \boldsymbol{r}\_1}

, in the vicinity of another charge,

q
2

{\displaystyle q\_2 }

 at position

r
2

{\displaystyle \boldsymbol{r}\_2}

, in a vacuum is equal to

F
1

=

q
1

q
2

4
π

ε
0

r
1

−

r
2

r
1

−

r
2

3

=

q
1

q
2

4
π

ε
0

r
^

12

r

12

2

where

r

12

=

r
1

−

r
2

{\displaystyle \boldsymbol{r}\_{12} = \boldsymbol{r}\_1 - \boldsymbol{r}\_2}

 is the vectorial distance between the charges,

r

^

12

=

r

12

r

12

{\displaystyle \widehat {\mathbf{r}}\_{12}=\frac{\mathbf{r}\_{12}}{|\mathbf{r}\_{12}|}}

 a unit vector pointing from

q
2

{\displaystyle q\_2 }

 to

q
1

{\displaystyle q\_1}

, and

ε
0

{\displaystyle \varepsilon\_0}

 the electric constant.

The vector form of Coulomb's law is simply the scalar definition of the law with the direction given by the unit vector,

r

^

12

{\displaystyle \widehat {\mathbf{r}}\_{12}}

, parallel with the line *from* charge

q
2

{\displaystyle q\_2 }

 *to* charge

q
1

{\displaystyle q\_1}

. If both charges have the same sign (like charges) then the product

q
1

q
2

{\displaystyle q\_1q\_2}

 is positive and the direction of the force on

q
1

{\displaystyle q\_1}

 is given by

r

^

12

{\displaystyle \widehat {\mathbf{r}}\_{12}}

; the charges repel each other. If the charges have opposite signs then the product

q
1

q
2

{\displaystyle q\_1q\_2}

 is negative and the direction of the force on

q
1

{\displaystyle q\_1}

 is

−

r
^

12

{\displaystyle -\hat{\boldsymbol{r}}\_{12}}

; the charges attract each other.

The electrostatic force

F
2

{\displaystyle \boldsymbol{F}\_2}

 experienced by

q
2

{\displaystyle q\_2 }

, according to Newton's third law, is

F
2

=
−

F
1

{\displaystyle \boldsymbol{F}\_2=-\boldsymbol{F}\_1}

.

### System of discrete charges

The law of superposition allows Coulomb's law to be extended to include any number of point charges. The force acting on a point charge due to a system of point charges is simply the vector addition of the individual forces acting alone on that point charge due to each one of the charges. The resulting force vector is parallel to the electric field vector at that point, with that point charge removed.

The force

F

{\displaystyle \boldsymbol{F}}

 on a small charge

q

{\displaystyle q}

 at position

r

{\displaystyle \boldsymbol{r}}

, due to a system of

N

{\displaystyle N}

 discrete charges in vacuum is

F
(
r
# )

q

4
π

ε
0

∑

# i
1

N

q
i

r
−

r
i

r
−

r
i

3

=

q

4
π

ε
0

∑

# i
1

N

q
i

R
i

^

R
i

2

{\displaystyle \boldsymbol{F}(\boldsymbol{r}) = {q\over4\pi\varepsilon\_0} \sum\_{i=1}^N q\_i

,
where

q
i

{\displaystyle q\_i}

 and

r
i

{\displaystyle \boldsymbol{r}\_i}

 are the magnitude and position respectively of the *i*th charge,

R
^

i

{\displaystyle \hat {\boldsymbol{R}}\_i}

 is a unit vector in the direction of

R
i

=
r
−

r
i

{\displaystyle \boldsymbol{R}\_i=\boldsymbol{r}-\boldsymbol{r}\_i}

, a vector pointing from charges

q
i

{\displaystyle q\_i}

 to

q

{\displaystyle q}

.

### Continuous charge distribution

In this case, the principle of linear superposition is also used. For a continuous charge distribution, an integral over the region containing the charge is equivalent to an infinite summation, treating each infinitesimal element of space as a point charge

d

q

{\displaystyle \mathrm{d}q}

. The distribution of charge is usually linear, surface or volumetric.

For a linear charge distribution (a good approximation for charge in a wire) where

λ
(

r
′

)

{\displaystyle \lambda(\boldsymbol{r}')}

 gives the charge per unit length at position

r
′

{\displaystyle \boldsymbol{r}'}

, and

d

ℓ
′

{\displaystyle \mathrm{d}\ell'}

 is an infinitesimal element of length,

d

q
′

=
λ
(

r
′

)

d

ℓ
′

.

{\displaystyle \mathrm{d}q' = \lambda(\boldsymbol{r'})\,d\ell'.}

For a surface charge distribution (a good approximation for charge on a plate in a parallel plate capacitor) where

σ
(

r
′

)

{\displaystyle \sigma(\boldsymbol{r}')}

 gives the charge per unit area at position

r
′

{\displaystyle \boldsymbol{r}'}

, and

d

A
′

{\displaystyle \mathrm{d}A'}

 is an infinitesimal element of area,

d

q
′

=
σ
(

r
′

)

d

A
′

.

{\displaystyle \mathrm{d}q' = \sigma(\boldsymbol{r'})\,dA'.}

For a volume charge distribution (such as charge within a bulk metal) where

ρ
(

r
′

)

{\displaystyle \rho(\boldsymbol{r}')}

 gives the charge per unit volume at position

r
′

{\displaystyle \boldsymbol{r}'}

, and

d

V
′

{\displaystyle \mathrm{d}V'}

 is an infinitesimal element of volume,

d

q
′

=
ρ
(

r
′

)

d

V
′

.

{\displaystyle \mathrm{d}q' = \rho(\boldsymbol{r'})\,dV'.}

The force on a small test charge

q

{\displaystyle q}

 at position

r

{\displaystyle \boldsymbol{r}}

 in vacuum is given by the integral over the distribution of charge:

F
(
r
# )

q

4
π

ε
0

∫

d

q
′

r
−

r
′

r
−

r
′

3

.

{\displaystyle \boldsymbol{F}(\boldsymbol{r}) = {q\over 4\pi\varepsilon\_0}\int \mathrm{d}q' {\boldsymbol{r} - \boldsymbol{r'} \over |\boldsymbol{r} - \boldsymbol{r'}|^3}.}

## Coulomb's constant

Main articles: Coulomb's constant
Coulomb's constant is a proportionality factor that appears in Coulomb's law as well as in other electric-related formulas. Denoted

k
e

{\displaystyle k\_e}

, it is also called the electric force constant or electrostatic constant hence the subscript

e

{\displaystyle e}

. When the electromagnetic theory is expressed in the International System of Units, force is measured in newtons, charge in coulombs and distance in meters. Coulomb's constant is given by

k
e

=

1

4
π

ε
0

{\displaystyle k\_e = \tfrac{1}{4\pi\varepsilon\_0}}

. The constant

ε
0

{\displaystyle \varepsilon\_0}

 is the vacuum electric permittivity (also known as "electric constant") in

C
2

⋅

m

−
2

⋅

N

−
1

{\displaystyle C^2 \cdot m^{-2} \cdot N^{-1}}

. It should not be confused with

ε
r

{\displaystyle \varepsilon\_r}

, which is the dimensionless relative permittivity of the material in which the charges are immersed, or with their product

ε
a

=

ε
0

ε
r

{\displaystyle \varepsilon\_a=\varepsilon\_0\varepsilon\_r}

, which is called "absolute permittivity of the material" and is still used in electrical engineering.

Prior to the 2019 redefinition of the SI base units, the Coulomb constant was considered to have an exact value:

k
e

=

1

4
π

ε
0

=

c
0
2

μ
0

4
π

=

c
0
2

×

10

−
7

H

m

−
1

=
8.987

551

787

368

176

4
×

10
9

N

m
2

C

−
2

.

{\displaystyle \begin{align}
k\_\text{e} &= \frac{1}{4\pi\varepsilon\_0}=\frac{c\_0^2\mu\_0}{4\pi}=c\_0^2\times 10^{-7}\ {H\, m}^{-1}\\
 &= 8.987\,551\,787\,368\,176\,4\times 10^9\ {N\, m^2\, C}^{-2}.
\end{align}}

Since the 2019 redefinition, the Coulomb constant is no longer exactly defined and is subject to the measurement error in the fine structure constant. As calculated from CODATA 2018 recommended values, the Coulomb constant is

k
e

=
8.987

551

792

3

(
14
)
×

10
9

N

m
2

C

−
2

.

{\displaystyle k\_\text{e} = 8.987\,551\,792\,3\,(14)\times 10^9\ {N\, m^2\, C}^{-2} .}

In Gaussian units and Lorentz–Heaviside units , which are both CGS unit systems, the constant has different, dimensionless values.

In electrostatic units or Gaussian units the unit charge (*esu* or statcoulomb) is defined in such a way that the Coulomb constant disappears, as it has the value of one and becomes dimensionless.

k
e

=
1

{\displaystyle k\_\text{e} = 1}

 (Gaussian units).
In Lorentz–Heaviside units, also called *rationalized* *units*, the Coulomb constant is dimensionless and is equal to

k
e

=

1

4
π

{\displaystyle k\_\text{e} = \frac{1}{4\pi} }

 (Lorentz–Heaviside units)
Gaussian units are more amenable for microscopic problems such as the electrodynamics of individual electrically charged particles. SI units are more convenient for practical, large-scale phenomena, such as engineering applications.

## Limitations

There are three conditions to be fulfilled for the validity of Coulomb's inverse square law:

1. The charges must have a spherically symmetric distribution (e.g. be point charges, or a charged metal sphere).
2. The charges must not overlap (e.g. they must be distinct point charges).
3. The charges must be stationary with respect to each other.

The last of these is known as the electrostatic approximation. When movement takes place, Einstein's theory of relativity must be taken into consideration, and a result, an extra factor is introduced, which alters the force produced on the two objects. This extra part of the force is called the magnetic force, and is described by magnetic fields. For slow movement, the magnetic force is minimal and Coulomb's law can still be considered approximately correct, but when the charges are moving more quickly in relation to each other, the full electrodynamics rules (incorporating the magnetic force) must be considered.

## Electric field

Main articles: Electric field

An electric field is a vector field that associates to each point in space the Coulomb force experienced by a unit test charge. The strength and direction of the Coulomb force

F

{\textstyle \mathbf F}

 on a charge

q
t

{\textstyle q\_t}

 depends on the electric field

E

{\textstyle \mathbf{E}}

 established by other charges that it finds itself in, such that

F

=

q
t

E

{\textstyle \mathbf{F} = q\_t \mathbf{E}}

. In the simplest case, the field is considered to be generated solely by a single source point charge. More generally, the field can be generated by a distribution of charges who contribute to the overall by the principle of superposition.

If the field is generated by a positive source point charge

q

{\textstyle q}

, the direction of the electric field points along lines directed radially outwards from it, i.e. in the direction that a positive point test charge

q
t

{\textstyle q\_t}

 would move if placed in the field. For a negative point source charge, the direction is radially inwards.

The magnitude of the electric field **E** can be derived from Coulomb's law. By choosing one of the point charges to be the source, and the other to be the test charge, it follows from Coulomb's law that the magnitude of the electric field **E** created by a single source point charge *Q* at a certain distance from it *r* in vacuum is given by

E

=

k
e

q

r
2

A system *N* of charges

q
i

{\displaystyle q\_i}

 stationed at

r
i

{\textstyle \boldsymbol{r}\_i}

 produces an electric field whose magnitude and direction is, by superposition

E
(
r
# )

1

4
π

ε
0

∑

# i
1

N

q
i

r
−

r
i

r
−

r
i

3

{\displaystyle \boldsymbol{E}(\boldsymbol{r}) = {1\over4\pi\varepsilon\_0} \sum\_{i=1}^N q\_i
\frac{\boldsymbol{r}-\boldsymbol{r}\_i}{|\boldsymbol{r}-\boldsymbol{r}\_i|^3}}

## Atomic forces

See also: Coulomb explosion
Coulomb's law holds even within atoms, correctly describing the force between the positively charged atomic nucleus and each of the negatively charged electrons. This simple law also correctly accounts for the forces that bind atoms together to form molecules and for the forces that bind atoms and molecules together to form solids and liquids. Generally, as the distance between ions increases, the force of attraction, and binding energy, approach zero and ionic bonding is less favorable. As the magnitude of opposing charges increases, energy increases and ionic bonding is more favorable.

## Relation to Gauss's law

### Deriving Gauss's law from Coulomb's law

Strictly speaking, Gauss's law cannot be derived from Coulomb's law alone, since Coulomb's law gives the electric field due to an individual point charge only. However, Gauss's law *can* be proven from Coulomb's law if it is assumed, in addition, that the electric field obeys the superposition principle. The superposition principle says that the resulting field is the vector sum of fields generated by each particle (or the integral, if the charges are distributed smoothly in space).

E

(

r

# )

q

4
π

ε
0

e

r

r
2

{\displaystyle \mathbf{E}(\mathbf{r}) = \frac{q}{4\pi \varepsilon\_0} \frac{\mathbf{e}\_r}{r^2}}

where

**e***r* is the radial unit vector,
r is the radius, |**r**|,
*ε*0 is the electric constant,
q is the charge of the particle, which is assumed to be located at the origin.
Using the expression from Coulomb's law, we get the total field at **r** by using an integral to sum the field at **r** due to the infinitesimal charge at each other point **s** in space, to give

E

(

r

# )

1

4
π

ε
0

∫

ρ
(

s

)
(

r

−

s

)

r

−

s

3

d

3

s

{\displaystyle \mathbf{E}(\mathbf{r}) = \frac{1}{4\pi\varepsilon\_0} \int \frac{\rho(\mathbf{s})(\mathbf{r}-\mathbf{s})}{|\mathbf{r}-\mathbf{s}|^3} \, \mathbf{d}^3 \mathbf{s}}

where ρ is the charge density. If we take the divergence of both sides of this equation with respect to **r**, and use the known theorem

∇
⋅

(

r

r

3

)

=
4
π
δ
(

r

)

{\displaystyle \nabla \cdot \left(\frac{\mathbf{r}}{|\mathbf{r}|^3}\right) = 4\pi \delta(\mathbf{r})}

where *δ*(**r**) is the Dirac delta function, the result is

∇
⋅

E

(

r

# )

1

ε
0

∫
ρ
(

s

)

δ
(

r

−

s

)

d

3

s

{\displaystyle \nabla\cdot\mathbf{E}(\mathbf{r}) = \frac{1}{\varepsilon\_0} \int \rho(\mathbf{s})\, \delta(\mathbf{r}-\mathbf{s})\, \mathbf{d}^3 \mathbf{s}}

Using the "sifting property" of the Dirac delta function, we arrive at

∇
⋅

E

(

r

# )

ρ
(

r

)

ε
0

,

{\displaystyle \nabla\cdot\mathbf{E}(\mathbf{r}) = \frac{\rho(\mathbf{r})}{\varepsilon\_0},}

which is the differential form of Gauss' law, as desired.

Note that since Coulomb's law only applies to stationary charges, there is no reason to expect Gauss's law to hold for moving charges based on this derivation alone. In fact, Gauss's law does hold for moving charges, and in this respect Gauss's law is more general than Coulomb's law.

### Deriving Coulomb's law from Gauss's law

Strictly speaking, Coulomb's law cannot be derived from Gauss's law alone, since Gauss's law does not give any information regarding the curl of **E** (see Helmholtz decomposition and Faraday's law). However, Coulomb's law *can* be proven from Gauss's law if it is assumed, in addition, that the electric field from a point charge is spherically symmetric (this assumption, like Coulomb's law itself, is exactly true if the charge is stationary, and approximately true if the charge is in motion).

∮
S

E

⋅
d

A

=

Q

ε
0

{\displaystyle \oint\_S\mathbf{E}\cdot d\mathbf{A} = \frac{Q}{\varepsilon\_0} }

By the assumption of spherical symmetry, the integrand is a constant which can be taken out of the integral. The result is

4
π

r
2

r

^

⋅

E

(

r

# )

Q

ε
0

{\displaystyle 4\pi r^2\hat{\mathbf{r}}\cdot\mathbf{E}(\mathbf{r}) = \frac{Q}{\varepsilon\_0}}

where **r̂** is a unit vector pointing radially away from the charge. Again by spherical symmetry, **E** points in the radial direction, and so we get

E

(

r

# )

Q

4
π

ε
0

r

^

r
2

{\displaystyle \mathbf{E}(\mathbf{r}) = \frac{Q}{4\pi \varepsilon\_0} \frac{\hat{\mathbf{r}}}{r^2}}

which is essentially equivalent to Coulomb's law. Thus the inverse-square law dependence of the electric field in Coulomb's law follows from Gauss' law.

### Quantum field theory

The **Coulomb potential** admits continuum states (with E > 0), describing electron-proton scattering, as well as discrete bound states, representing the hydrogen atom. It can also be derived within the non-relativistic limit between two charged particles, as follows:

Under Born approximation, in non-relativistic quantum mechanics, the scattering amplitude

A

(

p
→

⟩
→

p
→

′

⟩
)

{\textstyle \mathcal{A}(| \overrightarrow{p} \rangle \to | \overrightarrow{p}'\rangle)}

 is:

A

(

p
→

⟩
→

p
→

′

⟩
)
−
# 1
2
π
δ
(

E
p

−

E

p
′

)
(
−

i

)
∫

d

3

r
V
(

r
→

)

e

−

i

(

p
→

−

p
→

′

)

r
→

{\displaystyle \mathcal{A}(| \overrightarrow{p} \rangle \to | \overrightarrow{p}'\rangle) - 1 = 2\pi \delta(E\_p - E\_{p'})(-\mathrm{i})\int \mathrm{d}^3r V(\vec r)\mathrm{e}^{-\mathrm{i}(\vec p - \vec p')\vec r}}

This is to be compared to the :

∫

d

3

k

(
2
π

)
3

e

i

k

r
0

⟨

p
′

,
k

S

p
,
k
⟩

{\displaystyle \int \frac{\mathrm{d}^3k}{(2\pi)^3} \mathrm{e}^{\mathrm{i}k r\_0}\langle p',k |S| p,k \rangle}

where we look at the (connected) S-matrix entry for two electrons scattering off each other, treating one with "fixed" momentum as the source of the potential, and the other scattering off that potential.
Using the Feynman rules to compute the S-matrix element, we obtain in the non-relativistic limit with

m
0

≫

p
→

{\displaystyle m\_0 \gg | \vec p|}

⟨

p
′

,
k

S

p
,
k
⟩

c
o
n
n

=
−

i

e
2

p
→

−

p
→

′

2

−

i

ϵ

(
2
m

)
2

δ
(

E

p
,
k

−

E

p
′

,
k

)
(
2
π

)
4

δ
(

p
→

−

p
→

′

)

Comparing with the QM scattering, we have to discard the

(
2
m

)
2

{\displaystyle (2m)^2}

 as they arise due to differing normalizations of momentum eigenstate in QFT compared to QM and obtain:

∫
V
(

r
→

)

e

−

i

(

p
→

−

p
→

′

)

r
→

d

3

# r

e
2

p
→

−

p
→

′

2

−

i

ϵ

{\displaystyle \int V(\vec r)\mathbf{e}^{-\mathbf{i}(\vec p - \vec p')\vec r}\mathbf{d}^3r = \frac{e^2}{| \vec p -\vec p'|^2 - \mathbf{i}\epsilon}}

where Fourier transforming both sides, solving the integral and taking

ϵ
→
0

{\displaystyle \epsilon \to 0}

 at the end will yield

V
(
r
# )

e
2

4
π
r

{\displaystyle V(r) = \frac{e^2}{4\pi r}}

as the Coulomb potential.

However, the equivalent results of the classical Born derivations for the Coulomb problem are thought to be strictly accidental.

The Coulomb potential, and its derivation, can be seen as a special case of the Yukawa potential, which is the case where the exchanged boson – the photon – has no rest mass.

## Simple experiment to verify Coulomb's law

It is possible to verify Coulomb's law with a simple experiment. Consider two small spheres of mass

m

{\displaystyle m}

 and same-sign charge

q

{\displaystyle q}

, hanging from two ropes of negligible mass of length

l

{\displaystyle l}

. The forces acting on each sphere are three: the weight

m
g

{\displaystyle mg}

, the rope tension

T

{\displaystyle \mathbf T }

 and the electric force

F

{\displaystyle \mathbf F }

. In the equilibrium state:

T

sin
⁡

θ
1

=

F

1

{\displaystyle \mathbf T \sin \theta\_1 =\mathbf F\_1}

and

T

cos
⁡

θ
1

=
m
g

{\displaystyle \mathbf T \cos \theta\_1 = mg}

Dividing (**1**) by (**2**):

sin
⁡

θ
1

cos
⁡

θ
1

=

F
1

m
g

⇒

F
1

=
m
g
tan
⁡

θ
1

{\displaystyle \frac {\sin \theta\_1}{\cos \theta\_1 }=\frac {F\_1}{mg}\Rightarrow F\_1= mg \tan \theta\_1 }

Let

L

1

{\displaystyle \mathbf L\_1}

 be the distance between the charged spheres; the repulsion force between them

F

1

{\displaystyle \mathbf F\_1}

, assuming Coulomb's law is correct, is equal to

F
1

=

q
2

4
π

ε
0

L
1
2

{\displaystyle F\_1 = \frac{q^2}{4 \pi \varepsilon\_0 L\_1^2}}

so:

q
2

4
π

ε
0

L
1
2

=
m
g
tan
⁡

θ
1

{\displaystyle \frac{q^2}{4 \pi \varepsilon\_0 L\_1^2}=mg \tan \theta\_1 \,\!}

If we now discharge one of the spheres, and we put it in contact with the charged sphere, each one of them acquires a charge

q
2

{\textstyle \frac{q}{2}}

. In the equilibrium state, the distance between the charges will be

L

2

<

L

1

{\textstyle \mathbf L\_2 < \mathbf L\_1}

 and the repulsion force between them will be:

F
2

=

(

q
2

)

2

4
π

ε
0

L
2
2

=

q
2

4

4
π

ε
0

L
2
2

{\displaystyle F\_2 = \frac{{(\frac{q}{2})}^2}{4 \pi \varepsilon\_0 L\_2^2}=\frac{\frac{q^2}{4}}{4 \pi \varepsilon\_0 L\_2^2} \,\!}

We know that

F

2

=
m
g
tan
⁡

θ
2

{\displaystyle \mathbf F\_2 = mg \tan \theta\_2}

 and:

q
2

4

4
π

ε
0

L
2
2

=
m
g
tan
⁡

θ
2

{\displaystyle \frac{\frac{q^2}{4}}{4 \pi \varepsilon\_0 L\_2^2}=mg \tan \theta\_2}

Dividing (**4**) by (**5**), we get:

(

q
2

4
π

ε
0

L
1
2

)

(

q
2

4

4
π

ε
0

L
2
2

)

=

m
g
tan
⁡

θ
1

m
g
tan
⁡

θ
2

⇒
4

(

L
2

L
1

)

2

=

tan
⁡

θ
1

tan
⁡

θ
2

{\displaystyle \frac{\left(\cfrac{q^2}{4 \pi \varepsilon\_0 L\_1^2} \right)}{\left(\cfrac{\frac{q^2}{4}}{4 \pi \varepsilon\_0 L\_2^2}\right)} = \frac{mg \tan \theta\_1}{mg \tan \theta\_2} \Rightarrow 4 {\left( \frac {L\_2}{L\_1} \right) }^2 = \frac{ \tan \theta\_1}{ \tan \theta\_2}}

Measuring the angles

θ
1

{\displaystyle \theta\_1}

 and

θ
2

{\displaystyle \theta\_2}

 and the distance between the charges

L

1

{\displaystyle \mathbf L\_1}

 and

L

2

{\displaystyle \mathbf L\_2}

 is sufficient to verify that the equality is true taking into account the experimental error. In practice, angles can be difficult to measure, so if the length of the ropes is sufficiently great, the angles will be small enough to make the following approximation:

tan
⁡
θ
≈
sin
⁡
# θ

L
2

ℓ

=

L

2
ℓ

⇒

tan
⁡

θ
1

tan
⁡

θ
2

≈

L
1

2
ℓ

L
2

2
ℓ

{\displaystyle \tan \theta \approx \sin \theta= \frac{\frac{L}{2}}{\ell}=\frac{L}{2\ell} \Rightarrow \frac{ \tan \theta\_1}{ \tan \theta\_2} \approx \frac{\frac{L\_1}{2\ell}}{\frac{L\_2}{2\ell}}}

Using this approximation, the relationship (**6**) becomes the much simpler expression:

L
1

2
ℓ

L
2

2
ℓ

≈
4

(

L
2

L
1

)

2

⇒

{\displaystyle \frac{\frac{L\_1}{2\ell}}{\frac{L\_2}{2\ell}}\approx 4 {\left ( \frac {L\_2}{L\_1} \right ) }^2 \Rightarrow \,\!}

L
1

L
2

≈
4

(

L
2

L
1

)

2

⇒

L
1

L
2

≈

4
3

{\displaystyle \frac{L\_1}{L\_2}\approx 4 {\left ( \frac {L\_2}{L\_1} \right ) }^2 \Rightarrow \frac{L\_1}{L\_2}\approx\sqrt[3]{4} \,\!}

In this way, the verification is limited to measuring the distance between the charges and check that the division approximates the theoretical value.

## See also

* Biot–Savart law
* Darwin Lagrangian
* Electromagnetic force
* Gauss's law
* Method of image charges
* Molecular modelling
* Newton's law of universal gravitation, which uses a similar structure, but for mass instead of charge
* Static forces and virtual-particle exchange