Slide 1

Slide description:

Slide 2

Slide description:

Slide 3

Slide description:

Slide 4

Slide description:

Slide 5

Slide description:

Slide 6

Slide description:

Table "Types of current sources and the principle of their operation" Table "Types of current sources and the principle of their operation" Electrophoric machine Mechanical rotation of non-conducting disks with applied conductive sections, part of which on one of the disks is electrified by friction, leads to the accumulation of charges in a special device called a Leiden jar. Currently used primarily for demonstration experiments requiring the controlled generation of large (up to tens of thousands of volts) voltages Galvanic cell Two different materials are immersed in a solution or other conducting medium. Due to irreversible chemical reactions occurring at the “solution-solid” boundary, electrons or charged ions accumulate on the electrodes. In galvanic cells, the energy of chemical bonds accumulated during the synthesis of these substances is irreversibly converted into the energy of separated charges

Slide 7

Slide description:

Slide 8

Slide description:

A source of electric current and a metal conductor (resistor) form the simplest closed electrical circuit, in which the patterns of current flow were studied by G. Ohm. He showed that the current strength from a constant current source depends on the material of the conductor (r), its cross-sectional area (S) and length. A source of electric current and a metal conductor (resistor) form the simplest closed electrical circuit, in which the patterns of current flow were studied by G. Ohm. He showed that the current strength from a constant current source depends on the material of the conductor (r), its cross-sectional area (S) and length. The section of the circuit containing the resistor is called external, and the section containing the current source is called internal. Ohm's law for a closed (complete) circuit allows you to calculate the current strength for circuits containing a current source with a known emf and known characteristics of the external conductor:

Slide 10

Slide description:

• In order for the current to exist constantly, a device is needed that constantly moves charges from one ball to another (current source).
• In addition to Coulomb forces, other external forces act in it

• Nature is not electric.
• Inside the source they are directed against Coulomb forces.
• The work done by external forces along a closed trajectory is not zero
• Third-party forces are non-potential forces; their work depends on the shape of the trajectory.

• Inside the current source, charges move under the influence of external forces against Coulomb forces (electrons from a positively charged electrode to a negative one), and throughout the rest of the circuit they are driven by an electric field.

External forces set in motion charged particles inside all current sources: in generators in power plants,

in galvanic cells,

batteries, etc.

Alternator, Russia

Battery, Tyumen

Galvanic cells, USSR

The nature of outside forces

Current sources

Third party force

Power plant generator

The force exerted by a magnetic field on electrons in a moving conductor

Galvanic cell

(Volta element)

Chemical forces dissolving zinc in sulfuric acid solution

The action of external forces is characterized by an important physical quantity called electromotive force (abbreviated EMF).

Electromotive force in a closed loop is the ratio of the work done by external forces when moving a charge along the loop to the amount of charge:

EMF is expressed in volts: [Ɛ] = J/C = IN

• The inscription on the batteries 9 V means that external forces do 9 J of work when moving a charge of 1 C. from one pole to another.

Ohm's law for a closed circuit. Current sources. To obtain direct current in an electrical circuit, the charges must be subject to some forces other than the (Coulomb) forces of the electrostatic field. Such forces are called third-party forces. A characteristic of the action of external forces is electromotive force (EMF), which is numerically equal to the work of external forces to move a single positive (test) charge along a closed circuit or, in other words, is determined by the work of external forces to move a charge along a closed circuit, related to the value of this charge, EMF is measured in volts. The section of the circuit where there is an emf is called a non-uniform section of the circuit. Inside the source, charges move against Coulomb forces under the influence of external forces, and throughout the rest of the circuit they are driven by an electric field. Such sources can be galvanic cells, batteries, DC electric generators. The emf of the current source is equal to the electrical voltage at its terminals when the circuit is open. From the law of conservation of energy it follows that the work of external forces is equal to the amount of heat released in the circuit Q = I2? R0? ?t where R0 = R + r is the total resistance of the circuit, and R is the resistance of the external circuit, r is the internal resistance of the source. Then? ? I? ?t = I2? (R + r) ?t.

Slide 2

Third-party forces Electromotive force External part of the circuit Internal part of the circuit Current source Concepts and quantities:

Slide 3

Laws: Ohm for a closed circuit

Slide 4

Short circuit current Electrical safety rules in various rooms Fuses Aspects of human activity:

Slide 5

Electromotive force. Ohm's law for a closed circuit. Current sources. To obtain direct current in an electrical circuit, the charges must be subject to some forces other than the (Coulomb) forces of the electrostatic field. Such forces are called third-party forces. A characteristic of the action of external forces is electromotive force (EMF), which is numerically equal to the work of external forces to move a single positive (test) charge along a closed circuit or, in other words, is determined by the work of external forces to move a charge along a closed circuit, related to the value of this charge, EMF is measured in volts. The section of the circuit where there is an emf is called a non-uniform section of the circuit. Inside the source, charges move against Coulomb forces under the influence of external forces, and throughout the rest of the circuit they are driven by an electric field. Such sources can be galvanic cells, batteries, DC electric generators. The emf of the current source is equal to the electrical voltage at its terminals when the circuit is open. From the law of conservation of energy it follows that the work of external forces is equal to the amount of heat released in the circuit Q = I2 ∙ R0 ∙ ∆t where R0 = R + r is the total resistance of the circuit, and R is the resistance of the external circuit, r is the internal resistance of the source. Then ε ∙ I ∙ ∆t = I2 ∙ (R + r) ∆t

Slide 6

From here we obtain Ohm's law for a complete circuit: The current strength in a complete circuit is equal to the electromotive force of the source divided by the sum of the resistances of the external and internal sections of the circuit. In the case when the resistance of the external circuit tends to zero, a short circuit current appears in the circuit - the maximum possible current in a given source. Short circuit current is the maximum current that can be obtained from a given source with electromotive force and internal resistance r. For sources with low internal resistance, the short circuit current can be very high and cause destruction of the electrical circuit or source. For example, lead-acid batteries used in automobiles can have short-circuit currents of several hundred amperes. Short circuits in lighting networks powered from substations (thousands of amperes) are especially dangerous. To avoid the destructive effects of such large currents, fuses or special circuit breakers are included in the circuit. Galvanic cells have a small short-circuit current and therefore are not very dangerous for them.