circuits and safety

• Direct current (DC)


  The electrical current flowing in one direction, usually found in battery powered devices.

  
• Alternating Current (AC)


  Current that changes direction from positive to negative periodically.

  E.g UK mains supply (current from sockets in the home).

  
• Electrical appliances are usually connected with a three-core cable:

  A) Earth

  B) fuse

  C) neutral

  D) live
• Live Wire - Brown


  Carries the alternation potential difference from the supply and is always connected to the fuse.

  This wire has an electric potential of 230V
• Neutral wire - blue


  Completes the circuit and is approximately at the earth potential.

  An electric potential of 0V.
• Earth wire - green and yellow


  Used for safety, to stop the appliance from becoming live.

  An electric potential of 0V, and it only carries a current if there's a fault.
• Potential difference


  The difference between electric potentials
• Potential difference between LIVE WIRE (230V) and EARTH WIRE (0V)

  • 230V - 0V = 230V
• Connecting LIVE WIRE to EARTH WIRE


  Can pose serious dangers due to the large potential difference of 230V, which can result in a large current that can create a fire
• Potential difference between LIVE WIRE and a HUMAN

  • 230V - 0V = 230V
• Live wire may be dangerous even when a switch in the mains circuit is open


  It has a potential of 230V, so there will be a potential difference of 230V across your body, which can cause current to flow through you, leading to death
• EARTH WIRE


  • Crucial for safety
  • Normally carries no electricity during regular appliance operation
  • Electric potential is 0V because it's connected to the ground, making it the same electric potential as the Earth
• Metal cases on electrical appliances

  Can be a safety hazard if the live wire inside the appliance touches the metal case, causing the case to become live
• Touching a live metal case


  Can lead to electrocution
• Earth wire


  • Provides a safe path for the electricity
  • Offers a low resistance path to the Earth, which is important if a fault occurs
  • When a fault happens, allows current to flow to the ground, creating a surge of current in the earth wire and potentially also in the live wire
  • This surge can trigger a fuse to melt and break the circuit, which stops the flow of electricity and makes the appliance safe to touch
• Series circuits


  Components are connected in ONE LOOP
• Parallel circuits


  Components are connected in MULTIPLE LOOPS
• Series circuits


  • The CURRENT is the SAME through each component
• Parallel circuits


  • The CURRENT through the whole circuit is the SUM of the currents through the separate components
• Series circuits


  • The POTENTIAL DIFFERENCE supplied by the battery is SHARED between the components
• Parallel circuits


  • The POTENTIAL DIFFERENCE across each component is the SAME
• Calculating total resistance in series circuits


  ADDING the resistance of each component
• Increasing the number of resistors IN SERIES


  Increases the overall resistance, as the current now has MORE resistors to pass through
• Calculating total resistance in parallel circuits


  The TOTAL RESISTANCE of resistors in parallel is LESS THAN the SMALLEST individual resistor
• Adding resistors in parallel


  Creates an EXTRA PATH along which the current can flow, resulting in a LARGER TOTAL CURRENT and a LOWER TOTAL RESISTANCE in the circuit
• Mains electricity: Electrical appliances are usually connected with a three-core cable.

  A) Earth

  B) fuse

  C) neutral

  D) live

  E) cable grip

  F) fuse holder

  
• Explain why is would NOT be safe for a person to touch a metal case that is touching a live wire, if the earth wire is NOT connected.

  The person could get an electric shock because there's a current in the person.
• Explain why is would be safe for a person to touch a metal case that is touching a live wire, if the earth wire IS connected.


  The charge flows through the earth wire instead of the person, sot the fuse will break.