10TH GRADE · DIRECT CURRENT
Current, resistance and Ohm's law
Current strength and density, resistance, emf and Ohm's law.
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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.
Physical quantities
Current Density
Current strength per unit cross-sectional area.
Unit: ampere per square meter ·
Electrical resistance
A measure of a conductor's resistance to electric current.
Unit: ohm ·
Resistivity
A characteristic of a material that determines the resistance of a conductor.
Unit: ohm meter ·
Conductor length
The length of the conductive section.
Unit: meter ·
Conductor cross-sectional area
The cross-sectional area of the conductor.
Unit: square meter ·
Electromotive force
Work done by external forces on a unit charge.
Unit: volt ·
Internal source resistance
Resistance to the movement of charges inside the source.
Unit: ohm ·
Short circuit current
Current strength at negligible external resistance.
Unit: ampere ·
The work of outside forces
Work of non-electrostatic forces inside a source.
Unit: joule ·
Lesson formulas
Determination of current strength
The current strength is equal to the charge passing through the section per unit time.
Current Density
Current density is equal to the current divided by the cross-sectional area.
Unit of current density
Current density is measured in amperes per square meter.
Stress through work
Voltage is equal to the work done by electrical forces on a unit charge.
Definition of resistance
Resistance is equal to the ratio of voltage to current.
Ohm definition
One ohm corresponds to a current of one ampere at a voltage of one volt.
Homogeneous conductor resistance
Resistance depends on the material, length and cross-sectional area of the conductor.
Resistivity units
Resistivity is expressed in ohm-meters or ohm-square millimeters per meter.
Ohm's law for a circuit section
Current is directly proportional to voltage and inversely proportional to resistance.
Ohm's law for a complete circuit
The current of the complete circuit is determined by the emf and the sum of the external and internal resistances.
Short circuit current
During a short circuit, the current is limited by the internal resistance of the source.
Determination of EMF
EMF is equal to the work done by external forces on a unit charge.
EMF unit
Electromotive force is measured in volts.
Related formulas
The same formulas in the reference, with symbols and units.
Tasks on the topic
Statements, answer options and the walkthrough.