resistors

• • For some RESISTORS, the RESISTANCE remains CONSTANT but in others it can CHANGE as the current changes.

  The resistance of many COMPONENTS like lamps, diodes, thermistors & LDRs is NOT constant.

  • It also changes with the current through the component.
• I-V graphs:

  To see how RESISTANCE changes with current in a component.

  
• Three types of I-V graphs:

  • ohmic conductors
  • filament lamp
  • diode

  
• The CURRENT through an ohmic conductor (at a constant temperature) is DIRECTLY PROPORTIONAL to the POTENTIAL DIFFERENCE across the resistor.

  • This is shown on the graph as a STRAIGHT LINE through the ORIGIN.
  • This means that the RESISTANCE remains CONSTANT as the current changes.

  
• The STEEPER the line, the LOWER the RESISTANCE

  
• FILAMENT LAMP:

  At low potential differences, there's a straight portion on the graph, meaning the resistance is CONSTANT.

  • The graph curves at higher potential differences.

  
• FILAMENT LAMP:

  As the potential difference INCREASES, the gradient (steepness) of line DECREASES, showing that RESISTANCE is INCREASING.

  • Because the FILAMENT in the bulb gets HOTTER, which happens as more current passes through it.

  
• Filament lamp:

  The filament is a METAL, so it contains METAL IONS & DELOCALISED ELECTRONS in its structure.

  • When the temperature of the filament INCREASES, the metal ions VIBRATE leading to MORE COLLISIONS between the IONS & ELECTRONS which INCREASES RESISTANCE.

  
• Diodes allow current to flow in only ONE DIRECTION & have a HIGH RESISTANCE in the opposite direction.

  
• On an I-V graph, current flowing in the CORRECT direction is shown by a sharp increase in current on the POSITIVE AXIS. (Right hand side of the graph)

  • When the diode is connected in the OPPOSITE DIRECTION (left hand side of the graph), the graph is FLAT at 0, indicating NO CURRENT flowing.

  
• LIGHT-DEPENDENT RESISTORS (LDRs) graphs:

  • In BRIGHT LIGHT, the resistance of an LDR decreases, allowing more current to pass through.
  • In DARKNESS, the resistance increases, reducing the current flow.

  
• LDRs are widely used in devices that react to light conditions like:

  • AUTOMATIC NIGHT LIGHTS
  • OUTDOOR LIGHTING
  • BURGLAR DETECTORS

  
• THERMISTOR graphs:

  • When it's HOT, the resistance of a thermistor decreases, letting more current through.
  • In COOL environments, the resistance increases, reducing the current.

  
• Thermistors are used in TEMPERATURE DETECTORS like:

  • CAR ENGINE SENSORS
  • ELECTRONIC THERMOSTATS

  
• The current increases when the potential difference increases, which causes the temperature of the filament to increase, so the resistance increases.

  
• potential difference is directly proportional to current

  
• Thermistor C, because the change is resistance is greatest between 0 & 25C.

  
• As temperature increases, all of the ions in the metal vibrate faster

  • It makes it harder for electrons to pass along the wire, so the resistance increases
• When the resistance stays constant, when the voltage increases, the current increases proportionally
• As the p.d. increases like using a bigger battery

  • the current in the circuit increases
  • the negative part on the graph shows the same thing but the battery is attached the opposite way round
  • so the voltage & current are effectively negative

  
• Higher currents cause wires to heat up, if they’re left on for a while

  • As the temperature increases, the resistance increases
• If the circuit contains a smaller resistor, then it needs a small p.d. to drive a large current, if the temp. stays constant

  • A bigger resistor would have a less steep line
  • a smaller resistor would have a steeper line

  
• Filament lamps:

  Light bulbs that contain a very thin metal filament

  • As the current flows through the filament, the wire heats up, until its so hot that it emits light
  • but it increases the resistance, so the curve gets less steep as we increase the current
  • shows that less current can flow per unit of p.d. at these higher voltages
  • because the temp & resistance are so much higher

  
• Diodes:

  devices that only allow current to flow in one direction

  • which is why they only show current when the p.d. is positive.
  • They have a really high resistance in the reverse direction
  • so no current can flow in that direction

  
• The straight lines are in circuits that contain only wires or resistors, because the resistance stays constant the entire time.