10TH GRADE · DIRECT CURRENT

Current, resistance and Ohm's law

Current strength and density, resistance, emf and Ohm's law.

28 minutes25 theory cards

LESSON MATERIAL

Basic Concepts

Electric current

Ordered movement of electrically charged particles.

For a stable current, free charge carriers and an electric field are needed; the direction of the current is taken according to the movement of positive charges.

Ohm's law

The relationship between current strength and voltage and resistance of a circuit section.

For a section of a circuit under constant conditions, the current increases with voltage and decreases with resistance: I = U/R.

Electromotive force

The work of external forces to transfer a single positive charge inside the source.

Despite the name, EMF is not a force, but the energy that a source imparts to a unit charge; it is measured in volts.

LESSON MATERIAL

Physical quantities

Current Density

j

Current strength per unit cross-sectional area.

Unit: ampere per square meter · $\frac{\mathrm{A}}{\mathrm{m}^2}$

Electrical resistance

R

A measure of a conductor's resistance to electric current.

Unit: ohm · $\mathrm{Om}$

Resistivity

\rho

A characteristic of a material that determines the resistance of a conductor.

Unit: ohm meter · $\mathrm{Om}\cdot\mathrm{m}$

Conductor length

l

The length of the conductive section.

Unit: meter · $\mathrm{m}$

Conductor cross-sectional area

S

The cross-sectional area of the conductor.

Unit: square meter · $\mathrm{m}^2$

Electromotive force

\mathcal{E}

Work done by external forces on a unit charge.

Unit: volt · $\mathrm{V}$

Internal source resistance

r

Resistance to the movement of charges inside the source.

Unit: ohm · $\mathrm{Om}$

Short circuit current

I_{\text{kz}}

Current strength at negligible external resistance.

Unit: ampere · $\mathrm{A}$

The work of outside forces

A_{\text{st}}

Work of non-electrostatic forces inside a source.

Unit: joule · $\mathrm{Dzh}$

LESSON MATERIAL

Lesson formulas

Determination of current strength

I=\frac{q}{t}

The current strength is equal to the charge passing through the section per unit time.

Current Density

j=\frac{I}{S}

Current density is equal to the current divided by the cross-sectional area.

Unit of current density

[j]=1\,\frac{\mathrm{A}}{\mathrm{m}^2}

Current density is measured in amperes per square meter.

Stress through work

U=\frac{A}{q}

Voltage is equal to the work done by electrical forces on a unit charge.

Definition of resistance

R=\frac{U}{I}

Resistance is equal to the ratio of voltage to current.

Ohm definition

1\,\mathrm{Om}=1\,\frac{\mathrm{V}}{\mathrm{A}}

One ohm corresponds to a current of one ampere at a voltage of one volt.

Homogeneous conductor resistance

R=\rho\frac{l}{S}

Resistance depends on the material, length and cross-sectional area of the conductor.

Resistivity units

[\rho]=\mathrm{Om}\cdot\mathrm{m}=\frac{\mathrm{Om}\cdot\mathrm{mm}^2}{\mathrm{m}}

Resistivity is expressed in ohm-meters or ohm-square millimeters per meter.

Ohm's law for a circuit section

I=\frac{U}{R}

Current is directly proportional to voltage and inversely proportional to resistance.

Ohm's law for a complete circuit

I=\frac{\mathcal{E}}{R+r}

The current of the complete circuit is determined by the emf and the sum of the external and internal resistances.

Short circuit current

I_{\text{kz}}=\frac{\mathcal{E}}{r}

During a short circuit, the current is limited by the internal resistance of the source.

Determination of EMF

\mathcal{E}=\frac{A_{\text{st}}}{q}

EMF is equal to the work done by external forces on a unit charge.

EMF unit

[\mathcal{E}]=1\,\mathrm{V}

Electromotive force is measured in volts.

Related formulas

WorkA · J Electrical resistanceR · Ohm Resistivityρ · Ohm m Electromotive forceε · B

PRACTICE

Tasks on the topic

Direct current10 class · 11 problems