CP9 Electricity

• Conventional current direction = positive to negative

  Flow of electrons = negative to positive
• Diode - only allows current to flow 1 way

  
• Resistors - reduce current increasing time for current to flow

  
• Cell = 1

  Battery = 2+

  Both power supplies to the circuit

  
• Series = only one path around the circuit

  • If one bulb brakes circuit goes out
  • adding bulb(s) dims all others
• Parallel = multiple paths for the current to follow

  • if one bulb brakes other stay lit
  • adding bulb(s) doesn't affect brightness
• Voltmeters - always attached in parallel
• Current = rate of flow of electrons or charged particles. Measured in amperes (A)
• Ammeters - connected in series
• Electric charge is measured in Coulombs (C)
• Faster electrons move, larger the current
• Current = Charge / Time

  $I=$$\frac{Q}{T}$

  Measured in: $A=$$\frac{C}{T}$
• Potential Difference = voltage, measured in volts (V)
• Potential Difference = Energy Transferred / Charge

  $V=$$\frac{E}{Q}$

  Measured in: $V=$$\frac{J}{C}$
• Current:

  Series - same all the way

  Parallel - shared by loops (according to resistance)
• Potential Difference:

  Series - shared by components

  Parallel - same across all elements
• Resistance:

  Series - Sum of individual resistors

  Parallel - Less than smallest individual resistor
• Resistance = difficulty of passing an electric current through a component
• The bigger the resistance, the bigger the potential difference needed to produce a current
• Resistance is measured in Ohms, $\Omega\$
• Resistance = Potential Difference / Current

  $R=$$\frac{V}{I}$

  Measured in: $\Omega=$$\frac{V}{A}$
• The longer the wire, the higher the resistance
• What is this graph: Fixed Resistor (ohmic)

  
• What is this graph: Filament Bulb (non-ohmic)

  
• What is this graph: LED / diode (non-ohmic)

  
• For IV graphs: If gradient increases, the resistance decreases (and vice versa)
• The heating affect:

  • as current flows, electrons collide with metal ions
  • transfers energy to the ions which vibrate more vigorously
  • wire gets hotter
  • harder for electrons to pass ions
  • resistance increases
• Thermistor - a resistor depending on temperature
• Thermistor:

  Low temperature = high resistance = increased current

  (and vice versa)
• LDR = light depending resistor = resistor depending on light intensity
• LDR:

  Very bright = low resistance = increased current

  (and vice versa)
• Thermistor/LDR:

  At a constant temperature/light intensity - constant resistance
• Thermistor =

  
• LDR =

  
• The steeper the gradient on an IV graph (for Thermistor/LDR) the lower the resistance -

  
• Thermistor used for:

  • temperature control in ovens
  • heating thermostats
• LDR used for:

  • security lighting (comes on in dark)
  • Safes/burglar alarms (broken open = alarm, properly opened = deactivated)
• Power = rate of transferring energy or doing work
• Power = work done / time

  $P=$$\frac{W}{T}$

  Measured in: $W=$$\frac{J}{T}$
• Electrical Power = Current$^2$ x Resistance

  $P=$$R\times I^2$

  Measured in: $W=$$\Omega\times C^2$