Topic 2 - Electricity

• Electricity is a flow of charges - electrons
• Energy is supplied to electrons by a cell/battery or mains electricity which move through the wires to transfer energy
• Cells/batteries have a store of chemical potential energy
• Electrons have charge which travels from the cell/battery to the lamp where their energy in converted into light
• Charge/current always flows from + to -
• The charge returns to the cell to gain more energy
• We can't count indivitual electrons so we group them into coulombs of charge
• Potential difference is the measure of how much energy is transferred to/by each coulomb of electrons/charge
• Volt = V
• Voltmeters are always connected in parrallel to componants
• Current is the rate of flow of charge (how many coulombs pass every second)
• Current = Amps - A
• Current is measured with a ammeter
• Resistance: the measure of how much a componant/object resists to flow fo current
• Resistance = Ohms - Ω
• P.d and current are directly proportional
• A resistor graph is a ohmic graph - straight line going through the centre of the graph (x=y). It is ohmic
• Ohm's law: V=IR
• Filament lamp graph is an elongated S. Line curves = resistance isn't constant
• Diode graph = straight line and then shoots up as diodes only let current flow in one direction. low resistance in forward direction, high resistance in reverse direction
• Series:

  • Total p.d is shared between all componants
  • Current is the same for all components
  • Total resistance = sum of resistances
• Parallel circuits:

  • p.d for each bran = p.d of cell/battery
  • The current is split between the branches
  • Total resistance = sum of all resistance reciprical's
• Thermistor: its resistance decreases when the temperature increases
• Light Dependent Resistor (LDR): If light intensity increases, resistance decreases

  
• Direct current = only acts in one direction
• Alternating current: results from an alternating p.d at a frequency of 50Hz
• Neutral wire = 0V

  Live wire = 230V

  Earth Wire = 0V
• Direct p.d graph = straight line
• Alternating graph = Wavy
• Earth wire is a safety feature and acts as an escape route for current that would otherwise cause a shock if the appliance was touched
• An earth wire is not needed for a double insulated appliance
• Fuse = a thin metal wire in a tube designed to melt of blow if there is a fault that causes a high current, usually 3A, 5A or 13A
• National Grid:

  Power station --> Step-up transformer --> Cables --> Step-down transformer --> Houses/Buildings
• Power stations produce relatively high current - if this went straight into the grid, a high amount of energy would be lost as heat due to the resistance of the cables
• A step-up transformer increases the potential difference of the supply and decreases the current - reducing the energy lost in the cables
• A step-down transformer reduces the voltage down to a safer and usable 230V for homes and businesses
• If electrons are transferred between insulating materials, they will remain charged - this is static electricity
• Electrons are negatively charged, so if an object gains electrons, it becomes negatively charged. If it loses them it become positively charged
• Opposite charges attract while the same charges repel
• When you touch a Van Der Graaf generator, every part of you becomes positively charged - your hairs stand on end as they are actively repelling each other