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P2 N) The National Gird
​The National Grid takes energy from where it has been produced (supply) to anywhere that it is needed (demand). Some of the places where electricity has been produced are coal power stations, nuclear power stations, wind turbines, solar panels etc. Some of the demanders of electricity are homes, factories, schools etc.
 
Electricity demand changes throughout the day and producers need to make sure that they produce enough electricity so that everyone has the electricity that they need and there are no blackouts. Electricity producers are able to predict electricity demand at different times during the day and make appropriate generation decisions based on their predictions. For example, there is low electricity demand at night as most people are sleeping, and there is considerably higher electricity demand when people get up in the morning and when they get home from work or school in the evening. Electricity demand can also be influenced by popular sporting events on TV such as the world cup. A lot of power stations run at below maximum capacity, so that if there is a surge in demand for electricity, they can increase the amount of electricity that the power station generates, thus reducing the chances of a blackout.
Transformers
The National Grid uses cables to transport electrical power from where it has been produced to where it is needed. To transport a large quantity of power, you need a high potential difference (voltage) or a high current. This is because power is potential difference multiplied by current.
In order for power to be high, you need to have a very high potential difference or a very high current. If we had a very high current, lots of the energy will be transferred to the thermal energy stores of the cable and the surroundings; the cables heat up. Therefore, the National Grid transports electrical energy by having a very high potential difference (400,000 V) and a low current. By having a high PD and a low current, the wires and the surroundings heat up less, which means that less energy is lost.
 
The national grid uses transformers to change the potential difference and current of the electricity. There are two different types of transformers; step-up transformers and step-down transformers. The diagram below shows the locations of the step-up and step-down transformers with respect to power stations and consumers (households).
Power stations tend to produce electricity with a potential difference of 25,000 V. The electricity produced by a power station passes through a step-up transformer, which increases the potential difference to around 400,000 V; the step-up transformer also decreases the current. The potential difference is increased to reduce the amount of energy lost by the transmission cables and the surroundings heating up.
 
After passing through a step-up transformer, the electricity travels through high voltage transmission lines to where it is needed. The transmission lines are held up by very high pylons.
 
When the electricity has travelled to where it is going to be used, it passes through a second transformer known as a step-down transformer. A step-down transformer reduces the potential difference to a safe level/ the level that is required; step-down transformers also increase the current. If the electricity is going to households, the step-down transformer will bring the potential difference of the electricity down to 230 V.
Transformers in More Detail
The diagram below shows a transformer in more detail.
Transformers have two coils; the primary coil and the secondary coil. The two coils are joined together with an iron core. The electricity for the above transformer is travelling from left to right. The number of coils on both the primary coil and secondary coil determines how the potential difference changes.
 
A step-up transformer increases the potential difference and decreases the current. In order for a step-up transformer to increase the potential difference, there must be more turns on the secondary coil than the primary coil.
 
A step-down transformer decreases the potential difference and increases the current. In order for a step-down transformer to decrease the potential difference, there must be more turns on the primary coil than the secondary coil.  
 
We are able to work out power by multiplying the potential difference by the current.
Transformers are essentially 100% efficient, so the power in the primary coil will be the same as the power in the secondary coil. This gives us the equation below.
In the above equation, p stands for primary and s stands for secondary. We are now going to have a look at a mathematical example.

Example 1

We have a transformer. For the primary coil, the potential difference is 240 V and the current is 0.25 A. The secondary coil has a current of 5 A.

 

a) Find the potential difference in the secondary coil.

b) Is the transformer a step-up or a step-down transformer?

 

Part a

The first part of the question asks us to work out the potential difference in the secondary coil. We are able to do this by using the equation below.

We are told in the question that the PD of the primary coil is 240 V (Vp), the current for the primary coil is 0.25 A (Ip) and the current for the secondary coil is 5 A (Is). We sub these values into the formula and solve to find the potential difference in the secondary coil (Vs).

We want Vs and not 5Vs, so we divide both sides of the equation by 5.

The potential difference across the secondary coil is 12 V.

 

Part b

The second part of the question asks us to say whether the transformer is a step-up or step-down transformer. A step-up transformer increases the potential difference (and decreases the current). A step-down transformer decreases the potential difference (and increases the current). The potential difference in the primary coil was 240 V and the potential difference in the secondary coil was 12 V; the potential difference across the coils has decreased, which means that this transformer is a step-down transformer.