Mains electricity

• Source: 'Quote'
• Power is defined as the rate of energy transfer or the amount of energy transferred per second
• Factors determining the power of a device

  • Voltage (potential difference) of the device
  • Current of the device
• Unit of power

  Watt (W), which is the same as a joule per second (J/s)
• Calculating power of an electrical component

  Power = Voltage x Current
• A fuse is a safety device designed to cut off the flow of electricity to an appliance if the current becomes too large (due to a fault or a surge)
• Fuse sizes

  • 3 A
  • 5 A
  • 13 A
• Choosing the right fuse for an appliance

  I = P / V
• Rule of thumb for selecting a fuse


  Choose the next size up
• If the fuse current rating is too high, it will not be breaking the circuit in enough time before damage occurs
• Calculating Energy Transfers

  1. Work is done when charge flows through a circuit

  2. Energy transferred depends on current, potential difference, and time

  3. Energy transferred can be calculated using the equation E = P × t or E = I × V × t
• When charge flows through a resistor, the energy transferred is what makes the resistor hot
• When charge flows around a circuit for a given time, the energy supplied by the battery is equal to the energy transferred to all the components in the circuit
• Worked example

  • Calculate the energy transferred in 1 minute when a current of 0.7 A passes through a potential difference of 4 V: E = 0.7 × 4 × 60 = 168 J
• Mains electricity is potentially lethal
• Potential differences as small as 50 V can pose a serious hazard to individuals
• Common hazards of electrical safety

  • Damaged Insulation
  • Overheating of cables
  • Damp conditions
• Safety features in domestic appliances

  • Double insulation
  • Earthing
  • Fuses
  • Circuit breakers
• Insulation

  Wires are covered with an insulating material, such as rubber, to prevent the risk of electrocution
• The conducting part of a wire is usually made of copper or some other metal
• If the conducting part of a wire comes into contact with a person, this poses a risk of electrocution
• Wires are covered with an insulating material, such as rubber, for safety
• Some appliances do not have metal cases, so there is no risk of them becoming electrified
• Double insulated appliances have two layers of insulation: insulation around the wires themselves and a non-metallic case that acts as a second layer of insulation
• Double insulated appliances do not require an earth wire or have been designed so that the earth wire cannot touch the metal casing
• Earthing is used in many electrical appliances with metal cases to reduce the risk of electrocution
• The earth wire provides a low resistance path to the earth, causing a surge of current in the earth wire and live wire, which melts the fuse and cuts off the electricity supply to the appliance, making it safe
• Fuses and circuit breakers are safety devices designed to cut off the flow of electricity to an appliance if the current becomes too large due to a fault or surge
• Fuses consist of a glass cylinder containing a thin metal wire that melts if the current becomes too large, breaking the circuit and stopping the current
• A circuit breaker consists of an automatic electromagnet switch that breaks the circuit if the current exceeds a certain value
• The main circuit breaker can quickly shut off electricity to the whole house, while branch circuit breakers can shut off electricity to specific areas of the house
• Circuit breakers have advantages over fuses as they can be reset, work faster, and are used in mains electricity in homes
• When electricity passes through a component, such as a resistor, some of the electrical energy is turned into heat, increasing its temperature
• Electricity in metals is caused by a flow of electrons, which collide with ions in the metal lattice, transferring energy and increasing temperature
• Electricity in metals is caused by a flow of electrons


  Current
• Metals


  Made up of a lattice of ions
• Electrons pass through the metal lattice

  Collide with ions
• Ions


  Resist the flow of electrons
• Electrons collide with ions in the metal


  Making ions vibrate more
• Electrons lose energy by giving it to ions


  Ions start to vibrate more