Electric circuits

• Electrical current


  Electrical current is the rate of flow of charge
• Current equation


  I = Q / t
• Voltage


  Voltage is the work done per unit charge
• Voltage equation


  V = W/Q
• Current, Voltage and Resistance equation

  V = IR
• Ohms law


  If Ohms law is obeyed the current and voltage passing through a component are directly proportional when at a constant temperature
• Current in a series circuit

  In a series circuit current is the same throughout
• Law distribution of current in a series circuit is a consequence of

  The law of conservation of charge. Charge cannot be created or destroyed and so in a closed loop the flow of charge must be the same throughout
• Current in a parallel circuit

  In a parallel circuit the current is split between the different branches. The current entering each branch must be the same as the current leaving it.
• Conservation of charge in parallel circuits

  Charge cannot be created or destroyed meaning the total current leaving the source must equal the sum of currents in all the individual branches
• The distribution of potential differences in a circuit is a consequence of

  The law of conservation of energy
• Potential differences in a series circuit

  The total potential difference is split across all the components in a series circuit, in the ratio of their resistances
• Potential difference in a parallel circuit

  The sum of potential differences in each branch of a parallel circuit, is the same and will be equal to the potential difference of the source
• Power, Current and Potential difference equation

  P = IV
• Explain the IV characteristics for a filament bulb

  • The higher the current the higher the temperature of the filament
  • The higher the temperature the higher the kinetic energy of the metal ions so they vibrate more
  • This makes it harder for the current to flow and so the resistance of the bulb increases
• Three factors affecting resistance of a wire

  • Length
  • Cross sectional area
  • Resistivity
• What will increasing the length of the conductor do to the resistance

  The longer the conductor is the higher its resistance as long as all other factors remain constant
• What will increasing the cross sectional area of a conductor do to the resisitance

  The larger the cross sectional area of a conductor the lower its resistance assuming all other factors remain constant
• Resistivity


  A characteristic property of a material that determines its resistance to current flow
• Resistivity equation


  R = pl/A
• Current equation with charge carrier

  I = nAvq
• Potential along a uniform current carrying wire


  The potential will increase as you move along the wire
• Electromotive force (E.M.F)


  The electromotive force is the amount of energy that is transferred to each Coulomb of charge that passes through it
• Terminal potential difference


  The terminal potential difference is the actual potential difference that is produced across the terminals of the supply and is provided to the circuit
• Internal resistance


  The internal resistance of a power supply is the electrical resistance when a current flows due to the materials within the supply itself
• Semiconductor 


  A semi conductor is a component that can change its resistance based on external conditions
• Potential divider


  A potential divider is a pair of resistors used to split the potential difference over them in a given ration
• Temperature sensor circuit creation

  Create a potential divider circuit containing a resistor
• Finding lost volts for a supply

  Use Ir where r is internal resistance and I is current
• Resistance of an ideal voltmeter

  Infinite
• Which way does conventional current flow

  Positive to negative
• Kirchoffs second law (Voltage)

  Sum of the EMFs is equal to the sum of the potential drops around a closed circuit loop
• Increasing resistance in a metal wire explanation

  • More frequent collisions between conduction electrons and lattice ions
  • Therefore kinetic energy is transferred from electrons to the lattice ions
  • This causes the temperature of the wire to increase, meaning resistance has also increased
• NTC thermistor changing with temperature explanation

  • A thermistor is a semi conductor
  • As the thermistor heats up more conduction electrons are released
  • From the equation, I = nAvQ the current will therefore also increase
  • Using R = V/I, for a constant potential difference, the resisitance of the thermistor will decrease
• Light sensor circuit creation

  A potential divider circuit including a LDR
• Ideal resisitance of an ammeter

  Zero
• Three precautions for finding resistivity of a wire

  • Measure the diameter with a micrometer in three different places and orientations and find the mean diameter
  • Keep the current of the wire low
  • Avoid parallax error by keeping the wire close to the meter rule and take the reading with your eye directly above the fiducial mark
• Charge carrier


  A charged particle which is free to move
• What does Kirchoffs second law depend on

  Conservation of energy