2.1.5 Resistors

Resistors

Resistors

Overview

• Resistance and Energy:
  • The higher the resistance, the more energy is transferred as the current moves.
  • Ohm's Law:
  • Current through a resistor is directly proportional to the potential difference (P.D.).
  • The resistance is constant and does not change if we increase the current.
  • This kind of resistor is an ohmic conductor.
  • The resistance will only stay constant if the temperature is constant.

How does the resistance change?

• With current
  • As current increases, electrons (charge) have more energy
  • When electrons flow through a resistor, they collide with the atoms in the resistor
  • This transfers energy to the atoms, causing them to vibrate more
  • This makes it more difficult for electrons to flow through the resistor
  • So resistance increases, and current decreases
• With temperature
  • Normal wires - See above, the same process occurs as atoms vibrate when hot
  • Thermistor
  • In hotter temperatures, the resistance is lower
  • These are often used in temperature detectors/thermostats
• With length
  • The greater the length, the more resistance and the lower the current
  • Electrons have to make their way through more resistor atoms, so it is harder than using a shorter wire
• With light
  • LDR (Light Dependent Resistor)
  • The greater the intensity of light, the lower the resistance
  • So the resistance is greatest when it is dark
  • These are used in automatic night lights.
• With Voltage
  • Diodes
  • The diode allows current to flow freely in one direction
  • In the opposite direction, it has a very high resistance so no current can flow

Resistance of a Filament Lamp

Overview

• Filament Lamp:
• The current through a filament lamp is not directly proportional to the P.D.
• As the filament gets hot, its resistance increases.
• At high temperatures, the atoms in the filament vibrate more.
• Current and Electrons:
• Electrons in the current collide more with the atoms.
• More energy is needed to push the current through the filament.
• As the P.D. increases, the current no longer increases as much.

Graphs

• Ohm's Law (Resistors):
  • The linear relationship between current (A) and P.D. (V).
• Filament Lamp:
  • Non-linear relationship between current (A) and P.D. (V).

Diodes and LEDs

Diode

• Current Flow:

• The current through the diode flows in one direction only.

• The diode has a very high resistance in the reverse direction.

• Forward and Reverse Direction:

• No current in reverse.

• Current flows in a forward direction with the potential difference (P.D.).

  

• Usage:

  • Diodes are extremely useful for controlling the flow of current in circuits.

Light-emitting diode (LED)

• Function:

• Like a normal diode, it only allows current to flow in the forward direction.

• An LED gives off light when a current flows through.

  

• Efficiency:

  • LEDs are an extremely energy-efficient source of light.