Electricity

• Current

  The rate of flow of charge
• potential difference (voltage)

  The work done in moving a charge
• Resistance

  How difficult it is for current to flow
• Resistance equation

  R=V/I
• Ohm's Law


  The current through an ohmic conductor is directly proportional to the potential difference through it
• Ohmic conductor


  A conductor that obeys Ohm's law
• State three things that the resistance of a material depends on

  Length, Area, Resistivity
• Resistivity


  The resistance of a 1m length with a 1m^2 cross-sectional area of material
• Units of Resitivity

  Ohm-metres
• How does a thermistors vary with temperature?

  As temperature increases, the the resistance of the thermistor decreases
• NTC thermistor

  Negative Temperature Coefficient thermistor
• Superconductor

  A material which has zero resistivity when the temperature is decreased to, or below, the material's critical temperature.
• Semiconductor

  a material that conducts current when energy is supplied to ut
• Two uses of superconducting wires

  Power cables, Electromagnets
• Resistivity formula

  p=RA/L
• Gradient of a Resistance-Length graph

  Resistivity over area
• Power


  the rate of transfer of energy
• EMF

  electromotive force
• EMF definition

  The amount of electrical energy the battery produces and transfers to each coulomb of charge
• Internal resistance

  loss of PD per unit current in the source when current passes through it
• Lost Volts

  The energy wasted per coulomb overcoming the internal resistance of a power source
• EMF formula

  EMF=I(R+r)
• Load resistance

  The total resistance of all the components in the external circuit
• Kirchoff's First Law


  The total current entering a junction equals the total current leaving it
• Kirchoff's Second Law


  The total EMF around a series circuit equals the sum of the potential difference across each component
• What is current defined as in a circuit?

  Rate of flow of charge
• In what unit is current measured?

  Amperes
• What are the two ways to describe current flow?

  • Electron flow: negative to positive
  • Conventional current flow: positive to negative
• What does the equation Q=It represent?

  Definition of current in a circuit
• How much current is 1 Ampere?

  1 Coulomb per second
• What is potential difference defined as?

  Work done per coulomb of charge
• In what unit is potential difference measured?

  Volts
• What is the equation for potential difference?

  V=W/Q
• How do you measure potential difference across a component?

  Use a voltmeter in parallel
• What is an ideal voltmeter's resistance?

  Infinite resistance
• What does resistance measure in a circuit?

  Difficulty for current to flow
• How is resistance related to potential difference and current?

  Directly proportional to potential difference
• What is the equation for resistance?

  R=V/I
• In what unit is resistance measured?

  Ohms
• What is an ohmic conductor?

  • Current flow is directly proportional to potential difference
  • Maintained at constant temperature