P5 - Electricity in the home

• Direct current (d.c.)


  Current that flows in one direction only
• Alternating current (a.c.)


  Current that repeatedly reverses its direction of flow
• Mains circuit


  • Has a live wire, which is alternately positive and negative every cycle
  • Has a neutral wire at zero volts
• Peak potential difference


  The maximum voltage measured from zero volts
• Measuring frequency of a.c. supply

  1. Measure time period of waves

  2. Use equation: frequency = 1 / time taken for 1 cycle
• The casing of a mains plug or socket is made of hard-wearing electrical insulators
• What's inside a mains cable

  • Two or three insulated copper wires
  • Surrounded by an outer layer of rubber or flexible plastic material
• Colours of wires in a mains plug

  • Brown wire = live
  • Blue wire = neutral
  • Green and yellow striped wire = earth

  
• Three-pin plug


  • Longest pin is designed to make contact with the earth wire of a wall socket circuit
  • Plug contains a fuse between the live pin and the live wire

  
• Reason for brass pins in a three-pin plug

  Brass is a good conductor and doesn't rust or oxidise
• Cables that are worn away or damaged are dangerous
• If a live wire inside an appliance touches a neutral wire


  A very big current passes between the two wires at the point of contact, causing a short circuit
• Even if an appliance is switched off, never touch the wires inside the supply cable
• If a live wire inside the appliance touches a neutral wire, a very big current passes between the two wires at the point of contact. This is called a short circuit. Provided the fuse blows, it cuts the current off.
• Even if an appliance is switched off, never touch the wires inside the supply cable. People's bodies are at zero volts. If someone touches a live wire, a big potential difference will act across their body, causing a current to flow through them. They will suffer an electric shock, which could be lethal.
• Colour of wires in a mains plug

  Live wire - brown

  Neutral wire - blue

  Earth wire - striped green and yellow

  
• Why sockets are wired in parallel

  Sockets are wired in parallel so that if one socket is switched off, the others remain powered
• Why brass is better than copper for plug pins

  Brass, an alloy of copper and zinc, is harder and more durable than pure copper for the pins of a three-pin plug
• Cables that are worn away or damaged are dangerous because they can expose live wires and cause electric shocks
• Parts of a three-pin plug

  • Cable insulation

  2. Case

  3. Pin

  4. Wire

  
• Materials for parts of a three-pin plug

  • A. Brass

  B. Copper

  C. Rubber

  D. Stiff plastic
• Cable insulation


  Rubber or plastic
• Plug case


  Stiff plastic
• The three wires in a three-core mains cable are insulated to prevent short circuits and electric shocks
• How the metal case of an electrical appliance is connected to earth

  The earth wire in the mains cable is connected to the longest pin in the plug, which is then connected to the metal case of the appliance to provide a safe path to earth

  
• Cables joining the wall sockets in a house need to be thicker than the cables joining the light fittings because they need to carry more current
• Two-core cable


  Contains a live wire and a neutral wire
• Three-core cable


  Contains a live wire, a neutral wire, and an earth wire

  
• Whether an appliance should have a two-core or three-core cable depends on whether it has a metal case that needs to be earthed

  
• Sockets and plug cases are made of stiff plastic materials that enclose the electrical connections. Plastic is used because it is a good electrical insulator.
• A mains cable is made up of two or three insulated copper wires surrounded by an outer layer of flexible plastic material.
• In a three-pin plug or a three-core cable, the live wire is brown, the neutral wire is blue, and the earth wire is striped green and yellow.

  
• The earth wire is connected to the longest pin in a plug and is used to earth the metal case of a mains appliance.
• Resistor


  When energy is transferred to the resistor, it becomes hotter
• Energy transferred to a resistor

  1. Energy, E = charge flow, Q x potential difference, V

  2. Energy, E = power, P x time, t

  3. Energy, E = potential difference, V x current, I x time, t

  
• Energy transfer in a circuit

  Energy from the battery = energy transferred to the bulb + energy transferred to the variable resistor
• Each coulomb of charge leaves the battery with 12J of energy, transfers 10J of energy to the torch bulb, and transfers 2J of energy to the variable resistor
• The energy transferred to the bulb and the resistor increases their thermal energy stores
• Energy is transferred to the surroundings by both the bulb and the resistor
• The charge of an electron was first measured by J.J. Thompson


  About 120 years ago