physics electricity

• direct potential difference 


  always the same direction
• alternating potential difference


  changes direction
• voltmeter should be parallel with the resistor
• total resistance in a circuit is the sum of all the resistances in the circuit
• the sum of total resistance if two resistors are parallel is less than the sum of the total resistance of the two resistors
• if the switch is closed in a circuit the potential difference across the resistor becomes 0
• if the switch is closed the potential difference in the lamp will increase because the current increases
• potential difference is the work done per unit charge flowing through a component in a circuit
• the bigger the resistance of a component, the smaller the current that will pass through that component
• adding more resistors in parallel decreases the total resistance because the total current through the resistors is increased and the total potential difference across them is unchanged
• adding more resistors in series increases the total resistance because the current is reduced
• a light-emitting diode (LED) emits light when a current passes through it in a forward direction
• the resistance of a filament lamp increases as the current increases. this is because the resistance of the filament increases as its temperature increases
• diode 


  forward: low resistance

  reverse: high resistance
• step-up transformers increase the potential difference from the power station to the transmission cables
• step-down transformers decrease the potential difference to a much lower value for domestic use
• national grid is a system of cables and transformers linking power stations to consumers
• Uk has a frequency at 50Hz and a potential difference at 230 V
• Each wire in the three-core cable is made of copper, which is a good conductor of electricity
• each wire is coated with plastic which doesn't conduct electricity
• brown wire is the live wire, it carries the alternating potential difference from the supply. It is connected to the fuse in the plug
• blue wire is the neutral wire, it completes the circuit and has a p.d of 0V
• green/yellow wire is the Earth wire, the safety wire where it stops the appliance from becoming live
• appliances with metal cases can be very dangerous, if the live wire becomes loose and touches the metal case the case can become live


  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, preventing someone getting an electric shock from touching the metal case
• if the temperature increases in thermistors the resistance is lower so the current increases
• (LDR) the greater the light intensity, the lower the resistance
• the longer the length, the greater the resistance
• in series circuits the current is the same through all parts of the circuit
• in parallel circuits the current is shared between each of the branches and the total current is equal to the sum of the currents in each branch
• in series the p.d is shared across the whole circuit
• in parallel circuits the p.d is the same across all branches
• advantage of transmitting electricity at a very high potential difference is that less thermal energy is transferred to the surroundings.