Electric Potential, Resistance, Conductance, and Types of Currents

  • Current (I) flows in a conductor if enough electron-moving force is applied to cause an ordered flow of electrons. This electron-moving force is called electromotive force (EMF) (or electric pressure, electric potential, or potential difference [PD]), and its unit of measurement is the Volt (V).
  • The unit of measurement of current is the Ampere (A).
  • When EMF is applied to create a current flow, there is an opposing force to this current flow in the conductor. This opposing force is called resistance (R), and its unit of measurement is Ohm (Ω).
  • Ampere is the number of electrons flowing orderly through a point in a circuit in one second. The number of these electrons are measured as Coulomb (C). One Coulomb is 6.25 × 1018 electrons, and 1 Coulomb-per-second is called an Ampere.
  • Any material with free electrons that can conduct current is called a conductor.
  • Any material without free electrons cannot conduct current, and is thus called a non-conductor or insulator. Glass is a good insulator, while silver is a good conductor.
  • The simple electric circuit has 4 main components:
  1. Source of EMV.
  2. Conducting wires.
  3. Load.
  4. Control device, which is usually a switch.
Schematic diagram of a simple electric circuit showing a switch, conducting wire, and 2 loads (marked in X) connected in series, and a source of EMF as a battery.
  • There are 4 types of current:
  1. Direct current (DC): Current flows in the same direction with no variation in its strength (amplitude) and EMF.
  2. Varying DC: DC whose current and voltage varies, though neither falls to zero.
  3. Pulsating DC: DC whose current and voltage varies, with either periodically falling to zero.
  4. Alternating Current (AC): The direction of ordered electron flow reverses periodically, and this is associated with changes in current amplitude.
  • The resisting ability (or resistance) of a wire is determined by the following 4 attributes:
  1. Material the wire is made of.
  2. Length of wire.
  3. Temperature of wire.
  4. Cross-sectional area (or thickness) of the wire.
  • Resistance in the wire can be calculated using this formula:

Resistance (in Ω) = {ρ (resistivity of material) × Length (in meters)} / Cross-sectional Area (in square-meters)

= Resistance (R)= ρL/A.

ρ is the resistivity of material of the wire, and its unit of measurements in ohm-meter (Ωm).

  • The reciprocal of resistance is called conductance, and it is the measure of the ease of current flow in a material. Conductance (G) is calculated as 1/R and its unit of measurement is the siemens (S).

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