Understanding Electrical Current, Resistance, and Conductivity
Electrical Current
Current, or electric current, is the flow of electric charge through a material over time. This flow is due to the movement of electrons within the material. Current is measured in amperes (A), which represents coulombs per second (C/s) in the International System of Units. Electric current generates a magnetic field, a principle used in electromagnets. Current intensity is measured using a galvanometer, calibrated in amperes and called an ammeter, placed in series with the conductor.
Ohm’s Law
Ohm’s Law states that the current flowing through a conductor is directly proportional to the voltage and inversely proportional to the resistance, as long as the temperature remains constant. This relationship is expressed by the equation:
I = V / R
Where:
- I represents the current in amperes.
- V represents the potential difference in volts.
- R represents the resistance in ohms (Ω).
Ohm’s law states that R remains constant regardless of the current. This law is named after German physicist Georg Ohm, who described this relationship in 1827 based on experiments with simple circuits.
The Ampere
The ampere (A) is the unit of electric current in the International System of Units, named after André-Marie Ampère. One ampere is defined as the constant current that, if maintained in two straight, parallel conductors of infinite length and negligible cross-section, placed one meter apart in a vacuum, would produce a force of 2 × 10-7 newtons per meter of length.
The ampere is a base unit, along with the meter, second, and kilogram. It is defined without reference to the quantity of electric charge. The coulomb, the unit of charge, is a derived unit defined as the amount of charge moved by a current of one ampere in one second.
1 A = 1 C/s
Types of Current
- Alternating Current (AC): AC periodically reverses direction, causing the current to flow alternately in one direction and then the other.
- Direct Current (DC): In DC, electrons flow continuously in the same direction, maintaining a constant polarity.
Electrical Conductivity and Resistivity
Electrical conductivity is a material’s ability to conduct electric current. This involves the flow of charged particles, either electrons (in metals) or ions (in electrolytes). Conductivity (σ) is the inverse of resistivity (ρ):
σ = 1 / ρ
Conductivity is measured in siemens per meter (S/m) or Ω-1•m-1. It represents the proportionality between the electric field (E) and the current density (J):
J = σE
Conductivity should not be confused with conductance (G), which is the ease with which current flows between two points in a circuit. Conductance is the inverse of resistance: G = 1/R.
Electrical Resistivity
Electrical resistivity (ρ) represents the difficulty electrons encounter when moving through a material. It is measured in ohm-meters (Ω•m). A high resistivity indicates a poor conductor, while a low resistivity indicates a good conductor. The resistivity of metals generally increases with temperature, while the resistivity of semiconductors decreases with increasing temperature.
Electrical Resistance
Electrical resistance is a measure of an object’s opposition to current flow. Discovered by Georg Ohm in 1827, resistance is analogous to friction in mechanics. It is measured in ohms (Ω). Resistance can be measured using an ohmmeter. The reciprocal of resistance is conductance, measured in siemens.
For many materials under constant conditions, resistance is independent of the current and voltage. According to Ohm’s law, resistance is defined as the ratio of voltage to current:
R = V / I
Materials are classified as conductors, insulators, or semiconductors based on their resistance. Some materials exhibit superconductivity at certain temperatures, where resistance becomes virtually zero.