Current Electricity Study Notes

Key Concepts in Current Electricity

Definitions

• Current: The rate of flow of electric charge.

• Potential Difference (p.d.): The energy transferred per unit charge between two points in a circuit.

• Resistance: A measure of how difficult it is for charge carriers (such as electrons) to pass through a component in an electrical circuit.

• Resistivity: A material property that quantifies how easily a material conducts electricity, independent of its shape or size.

Ohm's Law

• States that for an ohmic conductor (a component that follows Ohm's law), the current ( $I$ ) is directly proportional to the potential difference ( $p.d.$ or voltage, $V$ ).

  • Mathematically, this is expressed as:
    $I ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } ext{ } V$

• The graph of current versus voltage (
  $I-V$
  graph) for an ohmic conductor is a straight line through the origin.

• Limitation: Ohm's Law is valid only when physical conditions are kept constant. The primary concern affecting this is heat generation in the conductor.

Types of Components

• Ohmic Conductor: A component that follows Ohm's Law. Examples include resistors that obey the linear
  $I-V$
  characteristics.

• Semiconductor Diode: A component that only allows current to flow in one direction and only when above a certain threshold voltage. On an
  $I-V$
  graph, it appears as a flat line followed by a linear region beyond the threshold voltage.

• Filament Lamp: A component containing a length of wire that heats up when current passes through it, producing light.

  • Behavior with Current:

  • At low currents, where heat is stable, Ohm's Law is obeyed.

  • At high currents, the resistance increases significantly due to heating, thus deviating from Ohm's Law.

• Superconductor: A material that has zero resistivity below a certain critical temperature (often very low).

  • Applications include creating strong magnetic fields, as they can accommodate very high currents without heat concerns due to zero resistance.

Potential Divider

• Definition: A potential divider is a configuration of resistors connected in series to produce a desired fraction of the source potential difference (
  $p.d.$
  ).

• Functionality: If the resistance of one resistor in the divider increases, the output potential difference across it decreases, illustrating the interdependence of resistances in such a setup.