Topic 2 - Electricity

• Electrical component symbols

  Label the components

  A) Switch

  B) Lamp

  C) Fuse

  D) Diode

  E) Voltmeter

  F) Ammeter

  G) Resistor

  H) Variable resistor

  I) LDR

  J) Thermistor

  K) Cell

  L) Battery

  
• The flow of charge


  Current
• Coulombs (C)

  Charge (Q)
• Amperes (A)

  Current (I)
• Volts (V)

  Potential difference (V)
• Ohms ($\Omega$)


  Resistance (R)
• Q=It

  Charge = current x time
• Resistance with different components

  Label the graphs

  A) Constant temperature

  B) Filament lamp

  C) Diode

  
• V=IR

  Potential difference = current x resistance
• Light dependant resistor (LDR)


  More light = less resistance
• Diode


  Allows current to flow freely in one direction but has a very high resistance the opposite way so no current can flow
• Relation between current and resistance


  • as current increases, electrons (charge) have more energy
  • when electrons flow through a resistor, they collide with the atoms in the resistor
  • this transfers energy to the atoms, causing them to vibrate more
  • this makes it more difficult for electrons to flow through the resistor
  • more resistance = less current
• Relation between temperature and resistance


  • normal wires - atoms vibrate more making it difficult for electrons to pass through
  • more temperature = more resistance
  • thermistor - more temperature = less resistance
• Relation between length of wire and resistance


  • longer = more resistance = less current
  • in a longer wire, electrons have to make their way through more resistor atoms
• What does a sharper gradient on a voltage current graph show?


  Lower resistance
• Series circuit


  • closed circuit
  • current is the same everywhere
  • total resistance is the sum of resistance in each component
  • potential difference is shared across the whole circuit
• Parallel circuit


  • branched circuit
  • current is shared between each branch
  • potential difference is the same across branches
  • total resistance is less than the branch with the smallest resistance
• Mains electricity


  • AC supply
  • 50 Hz
  • 230 V
• Alternating current (AC)


  Current changes from positive to negative (charge changes direction)
• Direct current (DC)


  Charge goes one direction only
• Live wire (brown)


  • 230 V
  • carries alternating potential difference from the supply
  • current may still flow when the circuit off
• Neutral wire (blue)


  • 0 V
  • completes the circuit
• Earth wire (green and yellow)


  • 0 V
  • safety wire to stop shocks
  • connected to the metal casing and the earth
• E=Pt


  Energy transferred = power x time
• E=QV


  Energy transferred = charge x potential difference
• Step-up transformers


  Increase the potential difference so current decreases, resulting in less energy loss
• Step-down transformers


  Decrease the potential difference for customer safety
• Charge

  • opposite charges attract
  • like charges repel
  • same amounts of each cancel out
• Their electrons can flow, and are not fixed (they are delocalised)

  Conductors
• Their electrons cannot flow throughout the material, they are fixed

  Insulators
• Electrons from insulators transfer when rubbed together creating positively and negatively charged objects that attract


  Static electricity
• Opposing charge flow

  Resistance
• Flow of electrons

  Electricity
• 1/Rt = 1/R1 + 1/R2 + ...

  Parallel
• Rt = R1 + R2 + ...

  Series
• 4 factors affecting resistance


  • temperature
  • light (LDR)
  • cross sectional area (width) of wire
  • length of wire
  • material of wire