P2 - Electricity

• Electric current


  The flow of electrical charge
• Equation linking charge, current and time

  Q = I t

  Charge (Coulombs), Current (Amperes), Time (Seconds)
• Current is the same at all points in a closed loop
• Factors current in a circuit depends on

  1. Potential Difference (V)

  2. Resistance (R)
• Equation to calculate potential difference if current and resistance are known

  V = I R

  Potential Difference (V), Current (A), Resistance (Ω)
• Ohmic Conductor


  A conductor for which current and potential difference are directly proportional

  Resistance remains constant as current changes

  Temperature must be constant
• Components for which resistance is not constant as current changes

  • Lamps
  • Diodes
  • Thermistors
  • Light Dependant Resistors (LDRs)
• As temperature of a filament lamp increases

  Resistance increases

  Ions in metal have more energy, so vibrate more, causing more collisions with electrons as they flow through the metal, creating greater resistance to current flow
• Current flow through a diode

  The current only flows in one direction

  Resistance is very high in the other direction, preventing current flow
• As temperature of a thermistor increases


  Resistance decreases
• When a thermistor may be used

  • In a thermostat to turn a heater on below a certain temperature
  • In a freezer to turn on a cooler when the temperature becomes too high
• As light intensity on a LDR decreases


  Resistance increases
• Application of a LDR


  • Street lights often use LDRs

  When light levels become too low, the light gains sufficient current to turn on
• What Circuit Symbol is this?

  Cell

  
• What Circuit Symbol is this?

  Battery

  
• What Circuit Symbol is this?

  Switch Open

  
• What Circuit Symbol is this?

  Switch closed

  
• What Circuit Symbol is this?

  Filament lamp

  
• What Circuit Symbol is this?

  Fuse

  
• What Circuit Symbol is this?

  LED

  
• What Circuit Symbol is this?

  Resistor

  
• What Circuit Symbol is this?

  Variable Resistor

  
• What Circuit symbol is this?

  Diode

  
• What Circuit Symbol is this?

  LDR

  
• What Circuit Symbol is this?

  Thermistor

  
• Series


  Same loop
• Parallel


  Adjacent loop
• How the potential difference across two components varies when connected in series and parallel


  1. Series: Total P.D is shared between each component

  2. Parallel: P.D across each component is the same
• Total resistance of two resistors connected in parallel


  Less than the smallest of the two individual resistances
• Total resistance of two resistors connected in series


  Equal to the sum of the two individual resistances
• Current in a series circuit


  • The same at all positions since the charge only has one path to flow through
• Current in a parallel circuit


  • Shared between the different branches, when the charge reaches a junction it splits
• How to connect an ammeter in a circuit to measure current

  Ammeters should be connected in series with the component that they are measuring current through
• How to connect a voltmeter in a circuit to measure potential difference


  Voltmeters should be connected in parallel to the component that they are measuring the potential difference of
• Advantage of connecting lamps in parallel

  If one lamp blows, the rest will be unaffected and can still receive current (i.e. the circuit is still complete)
• What is Potential Difference?

  Voltage
• Electrical Charge


  For charge to flow, circuit must be closed (no open switches) and there must be a source of potential difference (battery/cell)
• Electrical Current


  • Current is the flow of electrical charge
  • Greater the rate of flow of charge, greater current
• In a single closed loop, the current has the same value at any point
• Resistors


  • If the resistance is constant, an ohmic conductor, current is directly proportional to the potential difference, in this case the graph is linear
  • If the resistance of components such as lamps, diodes, thermistors and LDRs is not constant it changes with the current through the component, so the graph is nonlinear