P5 Household Electricity Flashcards
what do appliances do (in terms of energy)?
appliances transfer electrical energy into other forms of energy
what type of energy do motors transfer?
kinetic energy
what is power?
the rate at which energy is transferred
what does 1 watt mean?
an energy transfer of 1 J per second
what is usual with the power rating of appliances designed for producing thermal energy?
they have much higher power ratings than ones designed to transfer kinetic energy
what is the equation which links E, P and t?
E = P x t
how do you convert to kilojoules?
divide by 1000
what is the equation that links P, V and I?
P = V x I
what is the equation which links P, I and R?
P = I^2 x R
what is direct current?
the electrons in the circuit only ever travel in one direction
what type of current is the current from a cell?
direct current
what is alternating current?
a current where the electrons are constantly changing direction
what is the advantage of using alternating current?
it is very easy to use a tranformer to increase or decrease the the potential difference
what is the frequency of the mains current?
50 Hertz
what is the potential difference of the mains current?
230 Volts
how do you calculate the frequency of an alternating current from an osciloscope trace?
measure the amount of time from one peak to the next
use the formula:
frequency = 1 / period
why are wires made from copper?
it is a good conductor of electricity
what is the brown wire and what does it do?
It is the LIVE wire
it carries the alternating potential difference from the supply
what is the live wire connected to in the plug?
a fuse
what is the blue wire and what does it do?
the NEUTRAL wire
it completes the circuit with the live wire
it has a potential difference of 0V compared to the live wire
what is the green and yellow striped wire and what does it do?
the EARTH wire
it is a safety wire to stop the appliance becoming live
what would happen if a person touched the live wire?
a current would flow through the person into the earth and they would be electrocuted
this is because the potential difference of the earth is 0V
why are appliances with metal cases dangerous?
if the live wire becomes loose and touches the case, the case can become live
this could gives someone a fatal electric shock
what happens if a metal case of an appliance becomes live and there IS an earth wire?
the earth wire is connected to the ground by a metal rod and connected to the metal case of the appliance.
if the case becomes live, a huge current flows to the earth. this causes the fuse to melt and shut off the current
what is the national grid?
a system of transformers and high voltage cables
what does the national grid connect?
the power stations and the homes
what is the issue with transporting the energy to homes?
energy is always lost in the power cables due to the resistance in the wires
how does distance effect the energy loss in power cables?
it increases
why are step up transformers used?
they increase the potential difference as less energy is lost in the cables when the potential difference is high
what do step down transformers do?
reduce the potential difference before the electricity passes to homes
