2. Electricty

• Quantity


  • Quantity
  • Unit
  • Symbol
• Current


  Amps (A)
• Voltage


  Volts (V)
• Resistance


  Ohms (Ω)
• Energy


  Joules (J)
• Power


  Watts (W)
• Time


  Seconds (S)
• A.C. - Alternating current


  Alternate voltage causes the current to change direction
• D.C. - Direct Current


  Direct Voltage cause the current to move in a fixed direction
• An increase in voltage


  Causes an increase in current
• An increase in resistance


  Causes a decrease in current
• Main supply


  A.C. - Alternating current, constantly changing direction
• Cells and battery supply


  D.C. - Direct current, flowing in the same direction
• Current-Voltage Graphs


  • Wire - The current through a wire is proportional to the voltage (Looks like y=x)
  • Different Resistors - The current through resistor is proportional to voltage. Different resistors have different resistances (Linear graphs with different gradients)
  • Metal Filament Lamp - As the temperature of the metal filament increases the resistance increases causing a curve (Like y=x^3)
  • Diode - Current will only flow through a diode in one direction (Graph starts straight vertical line with a sudden change going up)
• Light emitting diodes


  Emit light when a current flows through them in a forward direction. Used for numbers in digital clocks, traffic lights and remote controls. They do not have a filament that can burn out.
• Light dependent resistors


  A type of resistor which changes resistance depending on how much light falls on it, this can be used in burglar detectors.
• Thermistor


  A temperature dependent resistor, in hot conditions the resistance drops and in cool conditions the resistance goes up. They make useful temperature detectors.
• Series circuits


  1. Different components are connected to a line between +ve and -ve ends of the power supply (except the voltmeters which are always connected in parallel)

  2. If you remove one component the circuit becomes broken and they all stop working

  3. More cells increase the voltage

  4. The current is the same everywhere (Current = Voltage/Resistance)

  5. The total voltage is shared between components and depends on the resistance

  6. The total resistance is the sum of the resistance of each component in the circuit
• Parallel circuits


  1. Each component is separate from the +ve and -ve ends of the power supply (Except ammeters which are always connected in series)

  2. If you remove or disconnect one component it hardly affects the others

  3. The potential difference is the same across all branches

  4. Current is shared between branches, total current is all components current added

  5. The higher resistance in a branch the lower the current, if two branches are identical the same current will flow through both components

  6. Total resistance decreases if you add a second resistor/branch in parallel
• When an electrical charge goes through a change in voltage

  Energy is transferred
• Electrical Safety Features


  • Insulation - Covered wire
  • Double insulation - Two layers of covered insulation
  • Earthing, When a fuse cuts off live wire if blown
  • Fuse, thin piece of wire which melts when reaches a certain temperature
  • Circuit Breaker, Breaks connection if too much current flows on one side
• Appliances


  • Must be earthed or insulated which means three wires called the live, neutral and earth are in a plug
  • The live and neutral wires are the only ones neutral are usually needed but the earth stops people getting hurt
  • The live wire alternates between high +ve and +ve voltage of around 230V
  • The neutral wire is always 0V
  • The earth wire and fuse are for safety
• Double insulated wire


  Metal cannot be seen and only plastics are there, an earth wire is not needed
• Earth wire


  Works if there is a surge in current/fault, blows the fuse which cuts off the live supply and isolates the whole appliance, making it impossible to get electric shock or cause a fire
• Circuit breakers


  • Used in some circuits and protect the circuit from damage if too much current flows
  • When circuit breakers detect a surge in the current they break the switch by opening a switch
  • One common type is Residual Current Circuit Breaker, they work with small current surges and prevent more electrical shocks
• Fuses


  • A thin piece of wire which melt at a certain temperature and break the circuit
  • Common fuses are 3A 5A and 13A and you always use a fuse slightly higher than what your appliance you need it for runs out
• Energy and Power


  1. When there is an electrical current in a resistor there is an energy transfer which heats the resistor, which happens when electrons collide with the ions, this gives ions energy which causes them to vibrate and heat up

  2. This can cause components to melt causing the circuit to work, which is why fuse are used to protect circuits

  3. The heating effect can have advantages such as heats something such as toasters which have collided with high resistance causing a temperature increase to cause a glow which gives off infrared radiation which cooks the bread
• Static electricity


  Charges which are not free to move, causing them to build up in one place and often ends with a spark of shock when they finally move
• Opposite electric charges


  Are attracted to each other, same electric charges repel each other
• Electrical conductors


  Materials that allow charge to flow
• Electrical insulators


  Materials that don't conduct charge very well so a current can't flow, such as plastic and rubber
• Static charge


  1. A charge which builds up in one place and is not free to move, common on insulators

  2. A common cause of static electricity is friction, when two insulating materials are rubbed together electrons will be scraped off and dumped on one another

  3. As charge builds up, so does voltage
• Ammeter measures the current in amps, it must be put on the main circuit (not parallel like a voltmeter)
• Voltage is what drives the current round the circuit, and can be known as electrical pressure/ potential difference.
• Voltmeter measures voltage in volts across the components, it must be placed parallel around a component (Not around a variable resistor or battery)
• Resistance is anything in the circuit which slows the flow down, adding more components adds to the overall resistance.