10TH GRADE · ELECTROSTATICS

Charge and electric field

Law of conservation of charge, Coulomb's law, tension and superposition.

26 minutes21 theory cards

LESSON MATERIAL

Basic Concepts

Law of conservation of electric charge

The algebraic sum of the charges of an electrically isolated system remains constant.

Charge can transfer between bodies and be redistributed, but this does not change the total charge of an isolated system.

Coulomb's law

The law of interaction of stationary point electric charges.

The force is directed along the line between the charges, depends on their signs and decreases in inverse proportion to the square of the distance.

Electric field

A form of matter through which electric charges interact with each other.

The field exists around the charge and at each point it determines what force would act on a unit positive test charge.

Principle of field superposition

The field of several sources is equal to the vector sum of the fields of each source.

The contribution of each source is calculated separately, and then the tension vectors are added up, taking into account their directions.

LESSON MATERIAL

Physical quantities

Electric charge

q

A quantity that determines the electrical interaction of a body.

Unit: pendant · $\mathrm{Kl}$

First electric charge

q_1

Charge of the first interacting body.

Unit: pendant · $\mathrm{Kl}$

Second electric charge

q_2

Charge of the second interacting body.

Unit: pendant · $\mathrm{Kl}$

Coulomb constant

k

Electrical interaction coefficient in vacuum.

Unit: newton square meter per square pendant · $\frac{\mathrm{N}\cdot\mathrm{m}^2}{\mathrm{Kl}^2}$

Relative dielectric constant

\varepsilon

How many times is the field in the medium weaker than the field in vacuum with equal free charges?

Unit: dimensionless quantity

Electrical constant

\varepsilon_0

Fundamental constant characterizing the electric field in a vacuum.

Unit: farad per meter · $\frac{\mathrm{F}}{\mathrm{m}}$

Electric field strength

E

Force acting on a unit positive test charge.

Unit: newton per coulomb · $\frac{\mathrm{N}}{\mathrm{Kl}}$

Field strength in vacuum

E_0

Electric field strength in the absence of a dielectric medium.

Unit: newton per coulomb · $\frac{\mathrm{N}}{\mathrm{Kl}}$

LESSON MATERIAL

Lesson formulas

Law of conservation of charge

q_1+q_2+\ldots+q_n=\mathrm{const}

The algebraic sum of charges of an isolated system is constant.

Coulomb's law

F=k\frac{\lvert q_1q_2\rvert}{\varepsilon r^2}

The strength of charge interaction depends on the charges, distance and properties of the medium.

Definition of tension

E=\frac{F}{q}

Tension is equal to the force per unit positive test charge.

Tension units

1\,\frac{\mathrm{N}}{\mathrm{Kl}}=1\,\frac{\mathrm{V}}{\mathrm{m}}

A newton per coulomb is equal to a volt per meter.

Point charge field

E=k\frac{\lvert q\rvert}{\varepsilon r^2}

The intensity of a point charge decreases in inverse proportion to the square of the distance.

Dielectric constant across fields

\varepsilon=\frac{E_0}{E}

Dielectric constant shows the field weakening in the medium.

Superposition of tensions

\vec{E}=\vec{E_1}+\vec{E_2}+\ldots

The intensity of several sources is equal to the vector sum of the individual intensities.

Coulomb constant

k=\frac{1}{4\pi\varepsilon_0}=9\cdot10^9\,\frac{\mathrm{N}\cdot\mathrm{m}^2}{\mathrm{Kl}^2}

The Coulomb constant specifies the strength of electrical interaction in a vacuum.

Electrical constant

\varepsilon_0=8{,}85\cdot10^{-12}\,\frac{\mathrm{F}}{\mathrm{m}}

The electric constant characterizes the electric field in a vacuum.

Related formulas

Electric chargeq, Q · Cl Electric field strengthE · V/m, N/Cl

PRACTICE

Tasks on the topic

Electrostatics10 class · 12 problems