Electricity

• Electrical Charge


  For charge to flow, circuit must be closed (no open switches) and there must be a source of potential difference (battery/cell)
• Electrical Current


  • Current is the flow of electrical charge
  • Greater the rate of flow of charge, greater current
• In a single closed loop, the current has the same value at any point
• Resistors


  • If the resistance is constant, an ohmic conductor, current is directly proportional to the potential difference, in this case the graph is linear
  • If the resistance of components such as lamps, diodes, thermistors and LDRs is not constant it changes with the current through the component, so the graph is nonlinear
• How resistance changes with current

  1. As current increases, electrons (charge) has more energy

  2. When electrons flow through a resistor, they collide with the atoms in the resistor

  3. This transfers energy to the atoms, causing them to vibrate more

  4. This makes it more difficult for electrons to flow through the resistor

  • so the resistance increases and current decreases.
• As resistance increases

  Current decreases
• How resistance changes with temperature

  In hotter temperatures the resistance of a thermistor is lower (temperature detectors/thermostats)
• How resistance changes with length

  The greater the length, the more resistance and the lower the current
• How resistance changes with light

  The greater the intensity of light, the lower the resistance of an LDR (Light Dependent Resistor)
• How resistance changes with voltage


  Diodes allow current to flow freely in one direction, but have a very high resistance in the opposite direction
• Series Circuits


  • components connected end to end
  • Can only switch them all off at once
  • potential difference is shared across the whole circuit
  • Current same through all parts of the circuit.
  • Total Resistance is the sum of the resistance in each component.
• Parallel Circuits


  • connected separately to power supply
  • can switch each component of individually.
  • PD is same across all branches PD power supply=PD of each branch
  • Current is shared between each branch, current through source= sum of current through each branch
  • Total resistance is less than the branch with the smallest resistance, more than 1 branch to take
• The total resistance for two resistors in parallel is less than the resistance of the smallest resistance
• Mains electricity


  • AC supply, 50 Hz, 230 V
  • AC is alternating current, which comes from the mains (current constantly varies)
  • DC is direct current, from cells and batteries (one direction)
• Wires in a plug

  • Live wire (brown, 230V)
  • Neutral wire (blue, 0V)
  • Earth wire (green and yellow stripes, 0V)
• Earth wire


  • Safety wire to stop the appliance becoming live
  • It is connected to the earth and to the casing
  • If the live wire touches the metal casing of the appliance, it will become live
• Energy transfers in everyday appliances


  • Electrical energy may be transferred by the appliance in different ways, e.g. kinetic energy for a motor, thermal energy in a kettle
  • Work done is when charge flows through a circuit, and is also equal to energy transferred
  • Power rating of an appliance shows the power it uses in Watts
• National Grid


  • System of cables and transformers linking power stations to consumers across the UK
  • Step-up transformers increase the potential difference from the power station to the National Grid, so current decreases and less energy is lost
  • Step-down transformers decrease the potential difference from the National Grid to consumers, for safety
• Charge

  • Positive and negative charges exist
  • Like charges repel, opposite charges attract
• Live wire:

  • Brown, at 230V
  • carries alternating PD from supply
  • Dangerous even when off as current may still be flowing through it.
• insulators dont conduct electricity
• conductors can conduct electricity
• power is the energy transferred per second, directly proportional to current and voltage