circuit components

• the resistance of a fixed resistor does not change when current changes
• current through a fixed resistor is directly proportional to the potential difference across the resistor
• in what components does resistance change with current?

  • filament lamps
  • diodes
• why does resistance change with current in a filament lamp?

  • current flows through a filament lamp
  • filament heats up
  • some of the energy is transferred into the thermal energy store
  • so as potential difference across the filament increases, the temperature will also increase
  • resistance increases with temperature
• lower current flows through a filament at high potential differences
• the resistance of a diode depends on the direction of the current
• the current in a diode can only flow in one direction
• what is a thermistor?

  resistor that changes its resistance with temperature
• in low temperatures, resistance in a thermistor increases
• in high temperatures, resistance in a thermistor decreases
• thermistors can be used in circuits as temperature sensors or thermostats
• in an LDR low levels of light (light intensity) the resistance is high
• in an LDR high levels of light (light intensity) the resistance is low
• LDR's can be used in circuits that automatically turn on lights at night
• how can you build a circuit to test how resistance varies for different devices?

  • ammeter in series with device to measure current
  • voltmeter parallel with device to measure potential difference
  • resistance = potential difference ÷ current
  • if testing a filament light bulb/diode: use a variable resistor to change current, then draw a graph of current against potential difference
  • if testing a thermistor: vary temp of thermistor + draw a graph of resistance against temp
  • if teasing an LDR: vary amount of light hitting LDR + draw a graph of resistance against light intensity
• The current-potential difference graph for a resistor is a straight line as long as the __________ is constant.

  temperature