Electricity

• current is the rate of flow of charge round the circuit, measured in amps (A)
• voltage is what drives the current round the circuit, measured in Volts (V)
• Resistance is anything in the circuit that slows flow of charge.
• Direct current (d.c)

  • The electric current flows in only one direction.
  • Batteries and cells are sources of direct current.
• Alternating current (a.c)

  • The electric current repeatedly changes direction.
• Neutral wire is blue

  • The neutral wire is blue.
  • It is at or close to earth potential (which is 0V).
  • It completes the circuit, and carries current away from the appliance.
• Live wire is brown

  • The live wire is brown and carries a 230V potential.
  • It provides the alternating potential difference from the mains electricity supply.
• Earth wire has green and yellow stripes

  • The earth wire has green and yellow stripes.
  • It has a 0V potential and only carries a current if there is a fault.
  • It is a safety wire to stop the appliance becoming live.
• Metal cases

  • Lots of electrical appliances, such as toasters and microwave ovens, have metal casings.
  • If there is a fault in the appliance, one of the wires may touch the metal case. Metals conduct electricity, so anyone touching the case could then receive an electric shock.
• Earth wire

  • An earth wire is a low resistance wire that connects the metal case to the ground.
  • If the casing is touched by a wire inside the appliance, the current will flow through the earth wire.
  • Because the resistance of the earth wire is low, a very high current will flow through it, breaking the fuse in the device. This will stop the flow of current and make the appliance safe.
• Plastic insulation

  • Many appliances are made with plastic cases.
  • Plastic is an insulator, so the case provides an extra layer of insulation. The appliance is then said to be double insulated.
  • The symbol for double insulated is shown in the diagram.
  • Double insulated appliances do not need an Earth connection.
• Electrical hazards:

  1

  Frayed cables

  2

  Damaged plugs

  3

  Long cables

  4

  Water around sockets

  5

  Pushing metal objects into sockets
• Fuses

  • Fuses contain a thin wire that melts if the current flowing through an appliance is too high.
  • This breaks the circuit and stops the flow of electric current.
• Circuit breakers

  • Circuit breakers are special devices that switch off the flow of current if the current is higher than a certain value.
  • Circuit breakers do the same job as fuses but can be used lots of times.
• What are the advantages of circuit breakers over fuses?

  Fuses only work once. Once blown, they must be replaced. We can reset circuit breakers. Circuit breakers also break circuits faster than fuses. This reduces the risk of getting an electric shock even more.
• Fuse ratings

  • Fuse ratings tell us the level where a fuse will begin to melt. The fuse chosen should have a value just above the normal current that an electrical appliance uses.
  • For example, if the operating current is 3.9 A, and fuses with values of 3 A, 5 A or 13 A are available, the most suitable fuse is the 5 A fuse.
• The potential difference across the mains power supply in the UK is 230 V.
• potential difference is Measured in Volts

  • Potential difference (p.d.) is measured in volts (V) and is also called voltage.
  • The energy is transferred to the electrical components in a circuit when the charge carriers pass through them.
  • We use a voltmeter to measure potential difference (or voltage).
  • 1 Volt is the same as 1 Joule per Coulomb.
• Ammeters

  • An ammeter measures the flow of current that passes through it.
  • Ammeters have to be connected in series (in the same loop of the circuit) with the electrical component whose current you are measuring. For example component X above.
• Electrical conductors

  • An electrical conductor is material that allows electric currents (flow of electric charges) to pass through it.
  • All metals are electrical conductors.
• Electrical insulators

  • An electrical insulator is a material that doesn’t allow electric currents (flow of electric charges) to pass through it.
  • Plastic and glass are electrical insulators.
• Electrical conductors

  • An electrical conductor has lots of charges that are free to move.
  • In a metal, the charges that are free to move are electrons.
  • The electric current through the metal is the flow of these free electrons.
• Electrical insulators

  • An electrical insulator has no free electrons.
  • No charges are free to move and carry a current.