circuits

• sensing circuit


  a circuit which contains a component with a resistance that changes with a change in the environment (for example, temperature or light level)
• an LDR is...


  a resistor that is dependant on light intensity
• LDR stands for...


  light dependent resistor
• in bright light, the LDR's resistance...

  falls
• in the dark, the LDR's resistance...


  rises
• uses of an LDR are...


  automatic night lamps, burglar detectors
• LDR symbol

  
• a thermistor is a...

  temperature dependent resistor
• in hot conditions, the resistance of a thermistor...

  falls
• in cold conditions, the resistance of a thermistor...

  rises
• uses of a thermistor are...

  fire alarms, electronic thermostats
• thermistor symbol

  
• sensing circuits can...


  be used to turn on or increase the power to components depending on the conditions they're in
• in series circuits, cell potential differences...

  add up: there is a bigger potential difference when more cells are in a series, provided that they are all connected the same way. If so, two batteries of voltage 1.5V will add up to 3V. However if they're in opposite directions, they will be subtracted, supplying a total of 0V
• how are series circuits connected?

  the different components are connected in a line, end to end, between the positive and negative of the power supply (except for voltmeters which are always in parallel).

  if you remove one component the circuit is broken and they all stop working
• if you remove a component in a series circuit...


  the circuit is broken and everything stops working
• in series circuits, cell potential difference...


  adds up: there is a bigger potential difference when more cells are in the series, provided that they are connected in the same way
• in series circuits, total potential difference...

  adds up: the total potential difference of the supply is shared between the various components, so the potential differences round the series circuit always add up to the source potential difference
• (series circuit) V total =

  V1 + V2 + ...
• in series circuits, current is...

  the same everywhere: the same current flows through all components
• (series circuit) I1 =

  I2 = I3 = ...
• in series circuits, resistance...

  adds up: this is because adding a resistor in series will make the resistors share the total potential difference, so the current through each resistor is also lower
• (series circuit) R total =

  R1 + R2 + ...
• in parallel circuits...


  each component is separately connected to the +ve and the -ive of the supply (except the ammeters which are always connected in series)
• in parallel circuits, if you remove a component...


  it hardly affects the others and everything else runs as normal
• in parallel circuits, potential difference is...

  the same across all components: all components get the full source potential difference, so the voltage is the same across all components
• (parallel circuits) V1 =

  V2 = V3 = ...
• in parallel circuits, current is...

  shared between branches: in parallel circuits there are junctions where the current either splits or re-joins, the total current going into a junction needs to be equal to the amount leaving it
• (parallel circuit) I total =


  I1 + I2 + ...
• in parallel circuits, adding a resistor...

  reduces the total resistance: if there are two resistors in a parallel circuit, the total resistance is less than the smaller of the two because current has more than one direction to go
• ammeters should be connected in...

  series, because the resistance of it is very small so it doesn't affect the current to be measured
• voltmeters should be connected in...

  parallel, because if it was connected in series then no current will be there in the circuit due to it's high resistance