Resistors

• for some resistors, the value of R can change as the current changes, but for others it remains constant
• the current through an ohmic conductor at a constant temperature is direcly proportional to the potential difference across the resistor
• the resistance of components such as lamps, diodes, thermistors and LDRs is not constant, it changes with the current through the component
• The resistance of a filament lamp increases as the temperature of the filament increases
• the current through a diode flows in one direction only
• the diode has a very high resistance in the reverse direction to the current which flows only one way through it
• the resistance of a thermistor decreases as the temperature increses
• thermistors are used in:

  • ovens
  • fridges
  • fire alarms
  • digital thermometers
  • boilers
• the resistance of an LDR decreases as light intensity increases
• investigate current - voltage graphs:

  • connect the circuit
  • ensure that the power supply is set to 0 at the start
  • record the reading on the voltmeter and ammeter
  • use the variable resistor to alter the potential difference
  • record the new readings on the voltmeter and ammeter
  • repeat pets three and four, each time increasing the pd slightly
  • reverse the power supply connections and repeat steps two to six
  • plot a graph od current again pd
  • repeat experiment with fixed resistor as a bulb