P4 - Supplying Electricity Flashcards
power equation
current (squared) x resistance
charge flow equation
current x time
energy transferred equation
charge flow x potential difference
power equation
energy transferred/time
why is the current provided by a cell called direct current (d.c.)
only flows in one direction
what is an alternating current (a.c.)
current that repeatedly reverses direction
what kind of current is supplied by mains electricity
alternating current
what is the frequency and voltage of mains electricity
50Hz, 230V
what colours are the live, neutral, and earth wires in a three-core cable
live=brown, neutral=blue, earth=green and yellow stripes
what is the function of the live wire in a three core cable
carries the alternating potential difference from the supply
what is the function of the neutral wire in a three core cable
completes the circuit
what is the function of the earth wire in a three core cable
safety wire to stop the appliance becoming live
when is there a current in the earth wire
when there is a fault
why is the live wire dangerous
provides a large p.d. that would cause a large current to flow through a person if they touched it
what is the national grid
nationwide network of cables and transformers thta link power stations to customers
what are step-up transformers used for in the national grid
increase yhe p.d. from the power station to the transmission cables
what are step-down transformers used for in the national grid
decrease the p.d. from the transmission cables to the mains supply in building so that it is safe to use
what are the units for power, current, potential difference and resistance
watts, amps, volts and ohms
what two things does energy transfer to an appliance depend on
power of appliance, time it is switched on for
how does having a large potential difference in the transmission cables help to make the nation grid an efficient way to transfer energy
large p.d. means a small current is needed to transfer the same amount of power, small current in the transmission cables means less electrical power is wasted due to heating
