P2

• electrical charge is measured in coulombs (C)
• a current of 1 ampere = 1 coulomb of charge flowing per second
• the size of the electric current is the rate of flow of electrical charge
• charge flow = current x time

  Q = I x T
•  A current has the same value at any point in a single closed loop.
• The current (I) through a component depends on both the resistance (R) of the component and the potential difference (V) across the component.
• The greater the resistance of the component the smaller the current for a given potential difference (pd) across the component.
• potential difference = current × resistance
• a potential difference of 1 volt tells us that 1 joule of energy is transferred for each coulomb of charge that is moving through the circuit
• voltmeter is used to measure the potential difference
• • if we close the switch in the circuit, then the electrons flow out of the cell and move around the circuit
  • this is called electric 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 e.g. lamp
  • in the lamp, the electrical energy transferred to light and thermal energy
  • when the electrons to the positive end of the cell, they are carrying less energy than when they left the negative end
• series circuit has no branches and so the current flows in one path
• • an electric current is a flow of electrical charge around a circuit
  • unit of electric current is ampere (A) shortened to amps
• current is measured by an ammeter
• current is never used up in a circuit
• in a series circuit, the current is the same all the way around
• • a parallel circuit has branches and some of the current passes through both of the branches
  • the current in the branches adds up to the total current leaving the cell
• the cell contains chemical energy which is transferred to electrical energy and is carried by the current
• when the electrons pass through a component, the electrical energy is transferred to other forms of energy
• • an electric current is a flow of electrons through a conductor
  • as they move, electrons collide with atoms in the metal
  • electrical energy is transferred to other forms
• the resistance tells us the potential difference required to drive a current through a component
• resistance = potential difference / current
• The current through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference across the resistor. This means that the resistance remains constant as the current changes.
• The resistance of components such as lamps, diodes, thermistors and LDRs is not constant; it changes with the current through the component. The resistance of a filament lamp increases as the temperature of the filament increases.
• The current through a diode flows in one direction only. The diode has a very high resistance in the reverse direction.
• The resistance of a thermistor decreases as the temperature increases.
• The resistance of an LDR decreases as light intensity increases.
• the current from a cell only moves in one direction and this is called direct current
• mains electricity in the UK is an alternating current which means the current constantly changes direction
• the benefit of using alternating current is that it is very easy to use a transformer to increase or decrease the potential difference
• • in the Uk, AC has a frequency of 50 hertz
  • AC has a potential difference of around 230 volts
• Most electrical appliances are connected to the mains using three core cable.
• the wires are made of copper which is a good conductor of electricity however the coatings are made of plastic which does not conduct electricity
• The insulation covering each wire is colour coded for easy identification
• • The brown wire is the live wire
  • The live wire carries the alternating potential difference from the supply.
  • potential difference of around 230V
  • live wire is connected to a fuse in the plug
• • the blue wire is the neutral wire
  • this completes the circuit
  • potential difference in around 0V compared to the live wire
• • the green / yellow wire is the earth wire
  • this is a safety wire to stop the appliance becoming live.
• • the live wire is very dangerous and could easily be fatal if touched
  • if someone touched the live wire, a current would flow through the person into the earth and they would be electrocuted
• • appliances with a metal case can be dangerous because if the live wire comes loose and touches the metal case, the case can become live and that could give someone a fatal electric shock if they touch the live case
  • the metal case is attached to the earth
  • the earth wire is connected into the ground with a metal rod
  • so if the case does become live, a huge current flows to Earth.
  • the fuse melts and shuts off the current
  • this prevents anyone from getting a shock from touching the case
•  power = potential difference × current