Electricity: I-V Characteristics

Electricity: I-V Characteristics

Understanding I-V Characteristics

  • The I-V characteristics of a component describe how the current (I) through it varies with the voltage (V) across it.
  • An I-V graph allows us to visualise the relationship between current and voltage for a given component.
  • Components that obey Ohm’s law, such as a resistor at a constant temperature, will have an I-V characteristic that is a straight line passing through the origin, showing a direct proportionality between I and V.

Diodes and their I-V Characteristics

  • A diode is a component that lets current flow in one direction only. It has a very high resistance in the reverse direction.
  • The I-V characteristic of a diode is not a straight line. It starts at the origin and stays at zero until a certain voltage, called the threshold or forward voltage, is reached.
  • After reaching the threshold voltage, the current increases very rapidly for a small increase in voltage in the forward direction.

Filament Lamps and their I-V Characteristics

  • A filament lamp is a non-ohmic conductor because its resistance increases as the current increases.
  • As current increases, the filament gets hotter, causing the ions in the filament to vibrate more and leading to more collisions with flowing electrons.
  • This results in increased resistance and an I-V characteristic that is a curve, rather than a straight line.

Resistors at Constant Temperature

  • A resistor at constant temperature obeys Ohm’s law, and therefore has an I-V characteristic that is a straight line through the origin.
  • This linearity indicates that the resistance is constant for any voltage, current, and temperature conditions.

Thermistors and their I-V Characteristics

  • A thermistor is a temperature-dependent resistor. Its resistance decreases as temperature increases.
  • The I-V characteristic of a thermistor is typically a curve due to this temperature-dependency.
  • Thermistors are often used as temperature sensors in digital thermometers, heating systems, and car engines.

Light-Dependent Resistors (LDRs) and their I-V Characteristics

  • A Light-Dependent Resistor (LDR) is a device whose resistance decreases when the intensity of light falling on it increases.
  • The I-V characteristic of an LDR is influenced by light intensity, making it ideal for use in light-sensing applications, such as streetlights and light-sensitive timing devices.