Electrical Circuits

• Conventional current


  The flow of positive charge from the positive terminal, through the external circuit, to the negative terminal
• Potential difference


  The work done per unit positive charge
• Current


  The amount of charge passing a point in a circuit every second
• EMF


  The total energy supplied per coulomb of charge by the cell
• Resistance


  A material's opposition to the flow of electric current
• Ohm's law


  Current through a conductor is directly proportional to the potential difference across the conductor at constant temperature

  
• Kirchhoff's Law of Voltage


  Around any closed loop in circuit, the sum of the voltage drops is equal to the EMF

  
• Kirchhoff's Law of Current


  The sum of the current flowing into any point equals the sum of the current flowing out from the point
• Electrical Work


  The work done on a charged particle by an electric field
• Electrical Power


  The rate at which work is done
• The potential difference across a resistor is the work done or energy transferred by the charges moving through the resistor
• Voltmeters must be connected in parallel because they need to detect the difference in potential by comparing the potential before and after a component

  
• Voltmeters must have a very high resistance so that no current flows through them
• Electrical Current


  The amount of charge passing a point in a circuit every second
• Ammeters must be connected in series with the component in question and have zero resistance so that they do not have any effect on the circuit

  
• Relationship shown by the graph


  Current through a conductor is directly proportional to the potential difference across the conductor at constant temperature

  
• The gradient of a V vs I graph will always give the resistance of a resistor

  
• Ohmic resistors


  Resistors where the potential difference across them is directly proportional to the current through them at constant temperature

  
• The graph with the steeper gradient indicates the resistor with the higher resistance

  
• The graph of a weaker resistor would have a shallower gradient than the other two resistors
• If the potential difference across the test resistor and the resulting current through it are directly proportional, the resistor is Ohmic and obeys Ohm's Law

  
• Non-ohmic resistors


  Resistors where the resistance changes as the voltage and current change
• For non-ohmic conductors, a graph of voltage against current will not be a straight-line, indicating that the resistance is not constant

  
• The gradient of a V vs I graph gives the resistance
• The gradient of an I vs V graph gives the conductance
• Increasing gradient of a V vs I graph means increasing resistance
• Increasing gradient of an I vs V graph means increasing conductance
• Ohmic resistors have a constant conductance at constant temperature
• The indicating variables are the resistor values
• The relationship between potential difference and current for the two resistors is V = IR

  
• The gradient of the V vs I graph gives the resistance.

  
• Conductance


  The inverse of resistance
• The greater the conductance of a wire, the lower the resistance of the wire in a non-linear way
• Factors influencing the resistance of a wire


  • Temperature
  • Type of material
  • Length
  • Diameter/cross-sectional area
• Temperature effect on resistance


  The higher the temperature, the lower the current due to increased vibration of metal atoms making it more difficult for electric charges to flow
• Material effect on resistance


  The type of material used affects current flow, as not all metals are equally good conductors
• Length effect on resistance

  A longer length makes it more difficult for current to flow, as there is more material to travel through, increasing resistance
• Diameter/cross-sectional area effect on resistance

  The thicker the wire, the more charge can travel simultaneously through a given length, decreasing resistance
• Factors influencing the flow of current in a conductor


  • Temperature
  • Type of material
  • Length
  • Diameter/cross-sectional area
• Temperature


  The higher the temperature the lower the current