Electricity

• Electric current


  The flow of electrical charge
• Equation linking charge, current and time

  1. Q = I t

  2. Charge (Coulombs)

  3. Current (Amperes)

  4. Time (Seconds)
• Current is the same at all points in a closed loop
• Factors current in a circuit depends on

  • 1. Potential Difference (V)
  • 2. Resistance (R)
• Equation to calculate potential difference if current and resistance are known

  1. V = I R

  2. Potential Difference (V)

  3. Current (A)

  4. Resistance (Ω)
• Ohmic Conductor


  • A conductor for which current and potential difference are directly proportional
  • Resistance remains constant as current changes
  • Temperature must be constant
• Components for which resistance is not constant as current changes

  • Lamps
  • Diodes
  • Thermistors
  • Light Dependant Resistors (LDRs)
• Resistance of a filament lamp as temperature increases

  • Resistance increases
  • Ions in metal have more energy, so vibrate more, causing more collisions with electrons as they flow through the metal, creating greater resistance to current flow
• Current flow through a diode

  • The current only flows in one direction
  • Resistance is very high in the other direction, preventing current flow
• Resistance of a thermistor as temperature increases

  The thermistor's resistance decreases
• When a thermistor may be used

  • In a thermostat to turn a heater on below a certain temperature
  • In a freezer to turn on a cooler when the temperature becomes too high
• Resistance of a LDR as light intensity decreases


  The LDR's resistance increases
• Application for a LDR


  • Street lights often use LDRs
  • When light levels become too low, the light gains sufficient current to turn on
• Mains electricity


  a.c supply
• a.c.


  Alternating Current
• d.c.


  Direct Current
• Alternating current


  Current that continuously changes direction at a specific frequency
• Direct current


  One directional current flow
• Mains electricity in the UK is an a.c supply
• The frequency of the UK mains electricity supply is 50 Hz
• The voltage of the UK mains electricity supply is 230V
• Wires in cables connecting electrical appliances to the mains

  • Live wire
  • Neutral wire
  • Earth wire
• Earth wire insulation colour

  Green and Yellow Stripes
• When the Earth wire carries a current


  1. Under normal circumstances, no current flows through the Earth wire

  2. If a fault occurs in the appliance (such as a surge or the casing becoming live), current will flow to the ground
• The neutral wire is at 0 Volts
• The potential difference between the live and earth wires is 230 Volts
• Purpose of the neutral wire

  To complete the circuit by connecting the appliance back to the mains supply
• Where the Earth wire is connected for metal appliances

  1. Earth wire is connected to the metal casing of the appliance

  2. If live wire becomes loose and touches the casing, the current will flow through the Earth wire, preventing electrocution
• State two equations for the power of a circuit

  1. P = I V

  2. P = I² R
• State an equation linking energy transferred, power and time

  E = P t
• State an equation linking energy transferred, charge flow and potential difference

  E = Q V
• Amount of energy transferred by an appliance depends on

  • How long the appliance is being used for
  • The power of the appliance
• Describe the energy transfers in a battery powered torch

  1. Battery converts chemical energy into electrical energy

  2. Bulb converts electrical energy into light as well as waste energy in the form of heating
• Describe the energy transfers in a battery powered motor

  1. Battery converts chemical energy into electrical energy

  2. Motor converts electrical energy into kinetic energy as well as waste energy in the form of heating due to friction
• What three things determine the power of a circuit device

  • The potential difference across the circuit
  • The current through the circuit
  • The amount of energy transferred in a given time
• National Grid


  To link power stations to consumers so that they have access to a source of electricity
• Types of transformers used in the National Grid

  • Step-Up Transformers
  • Step-Down Transformers
• Where are step-up transformers found in the National Grid? What do they do?

  1. Step-Up Transformers are used when connecting power stations to transmission cables

  2. They increase the potential difference
• Where are step-down transformers found in the National Grid? What do they do?

  1. Step-Down Transformers are used in connecting transmission cables to domestic buildings (like houses)

  2. They decrease the potential difference
• Why do transmission lines transfer electricity at high potentials?

  • A high potential, results in a low current
  • The lower the current, the less energy that is wasted as heat
  • Therefore it is more efficient