electricity

• if we close the switch electrons flow out of the cell and they move around the circuit. the resistance is constant. the resistance does not change if we increase the current.
• the electric current flows from the negative end of the cell to the positive end
• the electrons are carrying energy from the cell, they pass this energy to the components in the circuit such as the lamp. in the lamp the electrical energy is transferred to light energy and thermal energy. when the electrons return to the positive end of the cell they are carrying less energy when they left the negative end.
• we can measure the current in a circuit using an ammeter.
• current is never used up in a circuit. in a series circuit, the current is the same all the way around.
• current in parallel circuits

  the current in the branches add up to the total current leaving the cell.
• the cell has a store of chemical energy. this is transferred to electrical energy which is carried by the electrons passing out of the cell. when electrons pass through components (eg lamp), electrical energy is transferred to other forms of energy. in the case of the lamp, this is light energy and thermal energy.
• for components connected in parallel, the potential difference across each component is the same
• charge flow (C) = current (A) x time (s)
• energy transferred (J) = charge flow (C) x potential difference (V)
• resistance

  as electrons move, they collide with atoms in the metal. electrical energy is transferred into other forms eg thermal. the resistance tells us the potential difference required to drive a current through a component.
• resistance (Ω) = voltage (V) / current (A)
• current through a resistor is directly proportional to the potential difference. the resistance is constant. the resistance does not change if we increase the current. this kind of resistor is an ohmic conductor. the resistance will only stay constant if the temperature is constant.
• the current through a filament lamp is not directly proportional to potential difference. that is because the filament gets hot, which causes the resistance to increase. at high temperatures, the atoms in the filament vibrate more. the electrons in the current now collide more with the atoms. more energy is needed to push the current though the filament.
• diode - the current through the diode flows in one direction only

  because the diode has a very high resistance in the reverse direction
• diodes are extremely useful for controlling the flow of current in circuits.
• an LED give off light when a current flows through, and are an extremely energy efficient source of light
• resistors in series add together, the current has to pass through each resistor in turn, it cannot bypass any resistor
• the total resistance of two resistors in parallel is less than the resistance of the smallest individual resistor, because with two resistors in parallel we have two pathways for current to take. more total current will flow through the circuit
• Light-dependant resistor

  • in dark conditions, the LDR has a high resistance
  • in the light, the resistance of the LDR is very low
  • useful because in a parallel circuit when resistance is high, so is potential difference, so since potential difference is shared the lamp becomes very dim
• thermistor

  • the resistance of a thermistor decreases if the temperature increases
  • under cool conditions the resistance of the thermistor is high, it takes a lot of energy for the current to pass through the thermistor, the potential difference across the thermistor is high
• blender and fan


  electrical energy -> kinetic energy of electric motors
• iron and kettle 


  electrical energy -> thermal energy
• hair dryer, washing machine

  electrical energy -> kinetic energy of electric motors + thermal energy
• appliances which are designed to generate thermal energy have a higher power rating then appliances which are designed to generate kinetic energy
• energy (J) = power (W) x time (s)
• power (W) = potential difference (V) x current (A)
• power (W) = current2 (A) x resistance (Ω)
• the current from a cell is a direct current
• mains electricity in the UK is an alternating current

  so the current is constantly changing direction
• the benefit of alternating current is that it is very easy to use a transformer to Increase or decrease the potential difference
• in the UK, alternating current has a frequency of 50 hertz

  and has a potential difference of around 230 volts
• the wires are made of copper, which is a good conductor of electricity

  the coatings are made of plastic, which does not conduct electricity
• brown wire - live

  carries the alternating potential difference from the supply (230 V) and is connected to a fuse in the plug
• blue wire - neutral

  completes the circuit, potential difference is 0V compared to the live wire
• green/yellow striped wire - earth

  this is a safety wire to stop the appliance from becoming live
• the live wire is extremely dangerous and could easily be fatal if touched
• appliances with a metal case can be very dangerous

  because if the live wire comes loose and touches the metal case, the case can become live

  that could give someone a fatal electric shock if they touch the live case
• the metal case is attached to the earth wire

  the earth wire is connected into the ground with a metal rod

  if the case becomes live, a huge current flows to earth, the fuse melts and shuts off the current

  this prevents anyone from getting an electric shock from touching the case
• the national grid consists of a system of transformers and high-voltage cables