Electricity

• SI unit of an electric charge is Coulomb, C
• Definition of Electric Current


  The rate of flow of electric charge passing a point in a circuit

  SI unit: Ampere A
• Direction of flow of charge

  Conventional current flow : from positive to negative terminal

  Electron flow: Negative to positive terminal
• Current i = Charge Q / Time t
• Measuring current

  using an ammeter

  • negligible resistance
  • placed in series with component
• Define potential difference

  Work done to drive a unit charge through the component
• Define electromotive force

  Work done by source drive a unit charge through the whole circuit
• Voltmeter measures the potential difference between two points in a electric circuit

  • very high resistance
  • connected in parallel across component
• SI unit of potential difference :

  volt V
• define resistance

  • measure of how much the component opposes the flow of electric current
  • defined as the ratio of the potential difference across the component to the current flowing through it
• Resistance R = Potential difference V / Current I
• Two kinds of resistors

  Fixed resistors: non-adjustable electrical resistance

  Variable resistor: range of electrical resistance that can be adjusted
• Most electrical appliances are resistors and converts electrical energy to other forms of energy
• Current at every point of a series circuit is equal
• currents from each loop in a parallel circuit add up to total current
• In a series circuit, the emf is split between components
• In a parallel circuit, potential difference across each component is equal to each other and to the emf
• In a series circuit, when a resistor is added,

  • Effective resistance increases
  • current decreases
  • emf no change
• Short circuit


  It is an electrical connection that allows current to

  flow along an unintended path with no or very low

  electrical resistance.
• Short circuits may cause

  • power source to be destroyed
  • overheating and risk of electrical fires
• Comment on the effect on the current and the effective resistance when resistors are combined in parallel .

  The current increases and the effective resistance

  is smaller than the value of the individual

  resistances.
• Uses of electricity

  • heating (using nichrome wire which has high resistance in heating elements)
  • Chemical — separating water, conduction Of electricity in liquid, electrolysis, electroplating
  • magnetic — electromagnet, electric bell
• During electrolysis, Exchange of electrons takes place. Ions are formed (and movement of ions)

  • Bubbles of gas may be formed at electrodes.
  • Deposits of metal may be seen on electrodes.
  • Colour changes of the solution.
• When separating water, H+ ions are attracted to cathode and O- ions are attracted to anode(connected to positive side of power source). Bubbles of gas will be formed and accumulated at the top
• Electroplating

  • metal (anode) connected to positive side
  • positively Charged ions attracted to cathode and coats cathode
  • electrolyte must contain metal ions
• SI unit for energy : Joule J
• Define electrical power

  the amount of electrical energy converted to other forms of energy per unit time
• SI unit for power : Watt W
• Power W = Energy J / time s
• Power rating of an appliance is the amount of

  electrical energy converted to other forms

  of energy per unit time.
• Hazards of electricity

  • electric shock
  • electrical fire
• Hazards arise due to

  • Damaged insulation
  • Overloading (total current drawn too high that leads to overheating)
  • damp conditions (conducting path for current)
• Fuse

  • safety device used to protect equipment

  and wiring from excessive current flow.

  • When a current larger than the fuse rating flows, the fuse wire becomes hot and melts (blown). Hence the circuit will be open.

  • It is always installed on the live wire.
• earth leakage circuit breaker– an electromagnetic switch that breaks (trips) the circuit when there is a current leakage of 30mA or more through the earth wire

  miniature circuit breaker– an electromagnetic switch that breaks (trips) the circuit when there is too much current
• Earth wire

  • yellow-green
  • low resistance conducting path for current to flow to ground should metal case become live
• Which wire is on top

  Earth wire
• Live wire

  • brown
  • connected to fuse
  • high voltage 240V
• si unit of resistance : ohm (Ω)
• More current will go to the loop with less resistance
• In a parallel circuit, don’t add up resistance. Instead, calculate emf and total current