P2 The National Grid (page 189) Flashcards
What is the National Grid?
it is a giant web/system of wires (cables) and transformers that covers the whole of Britain. Getting electricity from power stations to homes everywhere.
(whoever you pay for your electricity, its the national grid that gets it to you).
The National Grid transfers electrical Power from where?
from power stations anywhere on the grid (they supply to anywhere else on the grid where it’s needed (the demand) - e.g. homes and industry.
Does electricity usage change throughout the day?
Yes. Power stations have to produce enough electricity for everyone to have it when they need it.
Can Electricity Production be predicted when the most electricity will be used? Explain your answer
Yes. Demand increases when people get up in the morning, come home from school or work and when it starts to get dark or cold outside.
Popular events like a sporting final being shown on TV could also cause a peak in demand.
Why don’t we seem to run out of electricity, when demand is high using electricity?
Power stations often run at well below their maximum power output, so there’s spare capacity to cope with a high demand, even if there’s an unexpected shut-down of another station.
Lots of smaller power stations that can start up quickly are also kept in standby just in case.
To transmit the huge amount of power needed, what do you need?
you need either high potential difference or a high current (as P = VI, from page 188).
Remember that power is the energy transferred in a given time, so a higher power means more energy transferred.
The problem with a high current is that you lose loads of energy, why?
as the wires heat up and energy is transferred to the thermal energy store of the surroundings.
Is it cheaper to boost the pd up really high (400 000 V) and keep the current as low as possible?
Yes.
Why is the national grid an efficient way of transferring energy?
For a given power, increasing the pd, decreases the current, which decreases the energy lost by heating the wires and the surroundings. (this makes the national grid an efficient way of transferring energy).
Potential Difference is Changed by what?
by a Transformer
To get the potential difference to 400 000 V for efficient transmission what do we use?
we use transformers (and big pylons with hugh insulators).
Transformers all have two coils, a primary coil and a secondary coil, joined with an iron core.
How is Potential difference increased?
by using a step-up transformer. They have more turns on the secondary coil than the primary coil. As the pd is increased by the transformer, the current is decreased.
How is the Potential difference reduced?
it is then reduced by the local consumer using a step-down transformer (the current is therefore increased by this transformer). They have more turns on the primary coil than the secondary.
For formula is used for the power of a primary coil?
Power = pd x current.
Are Transformers efficient?
Yes, nearly 100% efficient, so the power in primary coil = power in secondary coil.
This means:
pd across current in pd across current in
primary coil (v) x primary coil (A) = secondary coil (v) x secondary coil (A)
VpIp = VsIs
(see diagram on page 189).
