Electricity AS

• current - rate of flow of charge - flow of electrons
• current is measured with an ammeter
• Kirchoff's first law:

  • the total current entering a junction is equal to the total current leaving the junction.
• mean drift velocity is the average velocity of charge carriers
• Volt - the potential difference between two points when 1 joule of energy is transferred per coulomb
• Kirchoff's second law:

  • The sum of emfs in a closed loop is equal to the sum of pd's.
• Electromotive force - the amount of energy supplied to each unit of charge in a circuit.
• potential difference - the amount of energy supplies by each unit of charge.
• thermionic emission - the process by which free electrons are emitted from the surface of a metal when external heat energy is applies.
• electronvolt - the work done when one electron is accelerated through a potential difference of 1 V.
• Ohm's law:

  • The potential difference across a fixed resistor is directly proportional to the current provided the temperature remains constant.
• The filament lamp does not obey Ohm's law as its resistance changes.
• resistivity - the resistance across opposite faces of a cube of side 1m. It is constant for a material if temperature is constant.
• superconductivity - a state where certain materials have zero resistivity. It occurs at and below a critical temperature which depends on the material.
• Metallic conductors:

  • resistance increases relatively slowly with temperature.
  • said to have a 'positive temperature coefficient'
  • positive ions within the conductor vibrate more with increasing temperature.
  • charge carriers cannot pass through the conductor as easily when a p.d is applied.
• Semi-conductors:

  • resistance decreases quickly with temperature.
  • has 'negative temperature coefficient'
  • the number of charge carriers increase far more rapidly with temperature than the impendence caused by the more quickly vibrating positive ions.
• Power is the rate at which energy is transferred.
• kilowatt hour - the electrical energy used by a device of power one kilowatt in one hour.
• NTCs are used as temperature sensors
• PTCs are used as reusable fuses
• In NTCs

  • When temperature increases
  • resistance decreases
• In PTCs

  • When temperature increases
  • resistance increases
• kilowatt hours = kilowatt x hours
• cost in pence = kilowatt hours x cost per kilowatt hour
• Internal resistance - the resistance of a power supply when it delivers current
• lost volts - the p.d dissipated across the internal resistance of a power source
• terminal p.d - the output p.d from a power source
• eV = 1/2 x mass x velocity^2
• resistors in parallel all have the same p.d.