Unit 2: electricity

• Electrical Charge


  • Circuit must be closed (no open switches)
  • There must be a source of potential difference (battery/cell)
• Electrical Current


  • Flow of electrical charge
  • Greater the rate of flow of charge, greater current
  • Q = It (Charge = Current x Time)
• In a single closed loop, the current has the same value at any point
• Current, potential difference and resistance

  V = IR (Potential Difference = Current x Resistance)
• Resistors


  • If resistance is constant, current is directly proportional to potential difference (linear graph)
  • If resistance changes with current, graph is non-linear (e.g. lamps, diodes, thermistors, LDRs)

  
• How resistance changes


  1. With current (as current increases, electrons have more energy and collide more with atoms, increasing resistance)

  2. With temperature (normal wires - atoms vibrate more when hot, thermistors - resistance decreases in hotter temperatures)

  3. With length (greater length means more resistance)

  4. With light (LDRs - greater light intensity, lower resistance)

  5. With voltage (diodes - allow current in one direction only)
• Series Circuits


  Closed circuit, current follows single path, total resistance is sum of individual resistances
• Alternating Potential Difference


  A continually oscillating current flow, which results in the potential difference across two points continually oscillating between a positive and negative value
• Amperes (Amps)


  The unit of current
• Attraction


  A force pulling two opposite charges together, when they are brought near each other
• Parallel Circuits


  Branched circuit, current splits into multiple paths, total resistance is less than smallest individual resistance
• Coulomb


  The unit of charge
• Diode


  A component that only allows current to flow through in the forward direction. They have very large resistances in the reverse direction
• Direct Potential Difference


  A one-directional current flow
• Earth Wire


  The green and yellow striped safety wire that prevents an appliance from becoming live
• Electric Field Lines


  • Lines representing an electric field, that point in the direction in which a positive charge would experience an electric force
• In a series circuit, potential difference is shared across the whole circuit and current is the same through all parts
• Electric Field


  A region in which a charge will experience a non-contact, electric force. All charged objects have an electric field around them, and this field is stronger the closer you are to the charge
• Electrical Current


  The rate of flow of electrical charge. Its value is the same at any position in a single closed loop
• Electrical Work


  When charge flows in a circuit, electrical work is said to be done
• In a parallel circuit, potential difference is the same across all branches and current is shared between the branches
• Filament Lamp


  A light emitting component consisting of an enclosed metal filament. Its resistance increases as the filament's temperature increases
• Mains electricity in the UK is AC (alternating current) with a frequency of 50 Hz and voltage of 230 V
• Insulation


  The coating around power cables that prevents electrocution and is colour coded to allow for easy identification
• Light Dependent Resistor (LDR)


  A light sensitive component whose resistance increases as its temperature decreases
• Live Wire


  The brown coloured wire that carries the alternating current from the supply in a mains power supply
• Mains Electricity Cabling


  • Live wire (brown, 230 V)
  • Neutral wire (blue, 0 V)
  • Earth wire (green/yellow, 0 V, safety wire)
• Mains Electricity


  An a.c supply, which in the UK has a frequency of 50Hz a value of 230V
• Neutral Wire


  The blue coloured wire that completes the circuit in a mains power supply
• Non-Contact Force


  A force experienced between two separated objects. Examples include gravity, and magnetic and electric forces
• Power


  • Energy transferred per second, directly proportional to current and voltage
  • Power loss is proportional to resistance and square of current
• Energy transferred = Power x Time = Charge x Potential Difference
• Ohmic Conductor


  A conductor whose current flow is directly proportional to the potential difference across it, when held at a constant temperature
• Ohms


  The unit of resistance
• National Grid


  • System of cables and transformers linking power stations to consumers
  • Step-up transformers increase voltage, decrease current (to reduce energy losses)
  • Step-down transformers decrease voltage for consumer safety
• Parallel


  Components connected in parallel have the same potential difference across each component. The total current is equal to the sum of the currents flowing through each component
• Charge


  • Property of all matter, positive and negative charges exist
  • Like charges repel, opposite charges attract
• Potential Difference


  The product of a component's resistance and the magnitude of current flow through it
• Insulators and Conductors


  • Insulators do not conduct electricity, electrons are fixed
  • Conductors can conduct electricity, electrons are delocalised
• Repulsion


  Caused when two like-charges are brought near each other