Electricity and circuits

• Nucleus


  Central part of an atom containing protons and neutrons
• Sodium atom has 11 protons and 12 neutrons
• Electron


  Negatively charged particle found in the shells surrounding the nucleus, with negligible mass relative to the proton
• A diagram of a sodium atom is shown (not to scale)
• Particles in an atom

  • Proton
  • Neutron
  • Electron
• Proton


  Positively charged particle in the nucleus, with a mass relative to the proton of 1
• Neutron


  Uncharged particle in the nucleus, with a mass relative to the proton of 1
• Shells containing electrons


  Electrons are found at different distances from the nucleus, in shells
• Sodium is a metal
• Free electrons in metals

  Metals have electrons in the outer shell that are only weakly attracted to the nucleus, and can easily be removed
• When a battery is attached to a metal wire

  The voltage 'pushes' the free electrons around the circuit
• The conventional direction of current is from the positive terminal to the negative terminal of the battery, thanks to Benjamin Franklin
• Common circuit symbols

  • Switch (open)
  • Lamp
  • Cell
  • Ammeter
  • Battery
  • Voltmeter
• Series circuit


  Components are connected in a single route for the current to take around the circuit
• Parallel circuit


  Components are connected with junctions that allow the current to take different routes
• In a series circuit, lamps cannot be switched on and off individually, and if one lamp fails they will all switch off
• In a parallel circuit, each lamp can be switched separately
• Electric current


  The rate of flow of electric charge, measured in amperes (amps, A)
• Current is conserved in a circuit - the total amount of current stays the same on its journey around the circuit
• In a parallel circuit, current splits at a junction to travel along different branches, but the total amount entering the junction is the same as the total amount leaving
• Potential difference


  The 'push' that causes current to flow in a circuit, also called voltage
• For a current to flow, there must be a potential difference and a closed circuit
• The bigger the potential difference across a component, the bigger the current
• In a parallel circuit, the potential difference across each branch is the same
• Potential difference is measured in volts (V) using a voltmeter, which is connected in parallel
• Electric charge


  The fundamental property of matter that is the source of electric fields and electric current, measured in coulombs (C)
• One coulomb is the charge that passes a point in a circuit when there is a current of 1 amp for 1 second
• In metals, the current is a flow of electrons, each with a very tiny negative charge of less than one millionth of a millionth of a millionth of a coulomb
• Electric current


  The rate of flow of electric charge, measured in amperes (A)
• Energy transfer in a circuit

  The cell transfers energy to the charge, which then has the potential to transfer energy to other components
• Electrical resistance


  A measure of how much a component or circuit opposes the flow of electric current, measured in ohms (Ω)
• Resistors in series


  The total resistance of the circuit is increased because the pathway becomes harder for current to flow through
• Resistors in parallel


  The total resistance of the circuit is less than the resistance of the individual resistors, because there are more paths for the current
• A variable resistor can be used to change the current in a circuit
• Measurements of current and potential difference can be used to calculate the resistance of an unknown component
• Filament lamp


  As potential difference increases, current increases but the resistance also increases due to heating of the filament
• Diode


  Has a low resistance if the potential difference is in one direction but a very high resistance if the potential difference is in the opposite direction
• Thermistors have high resistance at low temperatures but the resistance decreases as temperature increases
• Components mentioned


  • Resistor
  • Filament lamp
  • Diode
  • Light-dependent resistor (LDR)
  • Thermistor
• A circuit can be used to explore how the resistance of a lamp changes as the potential difference across it is changed