4.3

• in a series circuit the current at every point is the same. the p.d splits.
• in a parallel circuit, the p.d across devices is equal, the current splits
• the p.d at the terminals of a battery. decreases slightly when the current is drawn from it
• series circuit - R = R1 + R2
• Parallel circuits - 1/R = 1/R1 + 1/R2
• Parallel circuits:

  • the current from the source is larger that the current in each branch
  • the combined resistance of resistors is less than that of either resistor by itself
• Advantages of parallel circuits

  • p.d across each lamp is fixed so the lamp shines with the same brightness irrespective of how many other lamps are switched on
  • each lamp can be turned on and off independently
• • in a filament lamp, when current increases, it heats up and resistance increases
  • in a semiconductor diode, resistance decreases when temperature increases
  • in a thermistor, resistance decreases when external temperature rises
• V1 = V(R1/R1 + R2)

  V2 = V(R2/R1 + R2)

  V1/V2 = R1/R2
• LDR

  when light falls, resistance of the LDR increases and the voltage decreases
• Thermistor

  resistance decreases when temperature increases
• Relays

  1. allows small current provided by switching circuit to control the larger current needed to operate a device.
  2. if output of the switching circuit is high, a small current flows to the relay which closes the main switch
• LED

  1. Forward biased - cathode connected to negative terminal (emits red, yellow or green light)
  2. no light emitted in reverse bias
  3. must have suitable resistor in series to limit the current
• Semiconductor diode

  1. lets current pass in one direction only
  2. conducts when anode goes to the positive terminal and is forward biased
  3. useful as a rectifier for changing a.c to d.c