i/v characteristics

• the term 'i/v characteristic' refers to a graph of current against potential difference which shows how the current flowing through a component changes as the potential difference across it is increased
• for ohmic conductors, the i/v graph is a straight line that goes through the origin. this is because at constant temperature, the current through an ohmic conductor is directly proportional to the voltage
• this is the i/v characteristic for a filament lamp.

  
• in a filament lamp, the filament is just a coiled up length of metal wire, meaning current flowing through the lamp increases its temperature, so the resistance increases
• the resistance of a metal increases as the temperature increases
• semiconductors aren't as good at conducting electricity as metals, as there are far fewer charge carriers available. however, if energy is supplied to the semiconductor, more charge carriers can be released. this means that they make great sensors for detecting changes in the environment
• a thermistors is a resistor with a resistance that depends on its temperature. we only need to know about NTC (negative temperature coefficient) thermistors - the resistance decreases as temperature increases
• this is the I/V characteristic for a thermistor

  
• for NTC thermistors, increasing the current through the thermistor increases its temperature. the increasing gradient of this characteristic graph tells you that the resistance is decreasing

  
• warming NTC thermistors gives more electrons enough energy to escape from their atoms. this means that there are more charge carriers available, so resistance is lower
• diodes are designed to let current flow in one direction only

  • forward bias is the direction in which the current is allowed to flow
  • most diodes require a threshold voltage of about 0.6V in the forward direction before they will conduct
  • in reverse bias, the resistance of the diode is very high and the current that flows is very small