Comprehensive Notes on Electric Charge, Current, Voltage, and Resistance

Fundamental Concepts of Electric Charge

• Electrified bodies exert an electric force on each other; the magnitude and nature of this force depend on their electric charge ($+\text{ or }-$).

• An atom consists of three primary subatomic particles:

  • Protons: positively charged particles ($+$).

  • Electrons: negatively charged particles ($-$).

  • Neutrons: neutral particles with zero net electric charge ($0$).

• Electric charge ($Q$) is a fundamental physical quantity describing the electrical state of a body.

• The standard SI unit for electric charge is the Coulomb ($[Q] = \text{C}$).

• Electric charge cannot be smaller than the elementary charge ($e$), which is the absolute charge of a single electron or proton:

  • Elementary charge value: $e = 1.6 \times 10^{-19}\,\text{C}$

  • An electron carries a negative charge of $-e$ ($-1.6 \times 10^{-19}\,\text{C}$).

  • A proton carries a positive charge of $+e$ ($+1.6 \times 10^{-19}\,\text{C}$).

Electric Current and Voltage

• Electric current ($I$) is defined as the ordered, directional movement of charged particles.

• Electric current corresponds to the number of charged particles (total charge) passing through a conductor per unit time ($1\,\text{second}$):

  • Mathematical definition: $I = \frac{Q}{t}$

  • SI unit of electric current: Ampere ($[I] = \text{A}$).

• The primary cause of electric current flow is electric voltage ($U$):

  • SI unit of electric voltage: Volt ($[U] = \text{V}$).

• Conventional electric current direction: Defined as flowing from the positive pole (+$) to the negative pole (-) of a voltage source.\n* Physical electron movement: The actual motion of electrons is in the exact opposite direction, moving from the negative pole to the positive pole.\n\n# Measurement of Electric Current\n\n* Physical effects of electric current: The passage of electric current through a conductor affects its magnetic properties and increases its temperature. Measuring these physical changes allows for the determination of the magnitude of electric current.\n* Ammeter (ampérmetr): An instrument specifically designed for measuring electric current.\n * Connection guidelines: An ammeter must be connected in series within the electric circuit. Its positive electrode must be connected toward the positive pole of the battery/source.\n * Measurement range adjustment: Prior to connecting the instrument, set the ammeter to its maximum measurement range and gradually decrease the range as needed to prevent destroying or damaging the instrument.\n* Multimeter (multimetr): A versatile instrument used to measure multiple electrical quantities within a circuit (e.g., model VOREL 81780).\n\n\n\n# Ohm's Law and Electrical Resistance\n\n* The magnitude of electric current in a circuit depends directly on the magnitude of the electric voltage that causes it.\n* Ohm's Law: The electric current passing through a conductor is directly proportional to the voltage applied across the conductor.\n* Electrical resistance (R): The ratio of electric voltage to electric current represents a physical property of a conductor known as electrical resistance:\n * Formula: R = \frac{U}{I}\n * SI unit of electrical resistance: Ohm ([R] = \Omega).\n* Resistance measurement methods:\n * Measured directly using an ohmmeter.\n * Measured indirectly using a voltmeter (connected in parallel across the component) and an ammeter (connected in series).\n\n\n\n# Factors Affecting Electrical Resistance and Resistors\n\n* Current control: By modifying or controlling the magnitude of resistance in a circuit, the magnitude of electric current can be adjusted accordingly.\n* Resistor (rezistor): A circuit component specifically characterized by its electrical resistance.\n* Physical factors influencing resistance: Conductor resistance depends on the conductor material, length (l$), and cross-sectional area ($S).\n* Resistor materials: Resistors are manufactured from various metallic alloys designed to have a high resistivity (\rho).\n* Conductor resistance formula: R = \rho \cdot \frac{l}{S}$$

• Potentiometer (potenciometr): An electrical component designed with variable resistance to enable adjustable control over circuit resistance and current.