Domestic use and the national grid

Mains electricity supply in the UK

a frequency of 50 Hz

and 230 V

Compare direct and alternating current

DC:

* continuous and in one direction
* produced by cells and batteries
* has a positive and negative terminal.

\
AC:

* Constantly changing direction
* produced by electrical generators i.e. mains electricity
* has 2 identical terminals

Live wire

Brown.

Carries the alternating potential difference from the supply.

most dangerous; If it touches the appliance without the Earth wire, it can cause electrocution

(230)

\

neutral wire

Blue.

Completes the circuit.

(0 V)

\

Earth wire

green and yellow stripes.

A safety wire to stop the appliance from being live.

only carries a current if there is a fault.

(0 V)

Why is a live wire dangerous even when a switch in the mains circuit is open?

Even if a device is switched **off** but the mains supply is **on**, the live wire can **still** cause an electric shock

what are the dangers of providing any connection between the live wire and earth?

* Because of the large potential difference between the live (230 V) and the earth (0 V), if the two are connected together, a **very large current** can be created
* Therefore, there is a large potential difference between the live wire and the electrician, so, a current would pass through the electrician's body to reach the earth

Energy stores in a heater

in a heater, energy will be transferred to the **thermal** store of the heating element, and then to the **thermal** store of the **surroundings** 

What is the key thing to mention in any questions related to energy transfers with domestic object.

Energy transferred electrically from mains supply.

Step-up transformers

They are used to increase the potential difference (and reduces the current) from the power station to the transmission cables

Step down transformers

the decrease th p.d. (and increase current)to a much lower value for domestic use.

why is the National Grid system an efficient way to transfer energy?

* The **lower** the current, the **less heating** due to resistance there will be in the wires 
* From the diagram, the step-up transformer **increases** the **voltage** and **decreases** the **current** in the cables
* By **decreasing** the current, the energy dissipated to the **thermal** store of the surroundings through the power cables is **reduced**
* Therefore, there is more energy being transferred to homes which **increases** the **efficiency** of the National Grid

why is electricity transmitted through a network of wires?

The current generated by power stations is greater than that which is required for homes and other buildings, and so it must be transmitted through a network of wires that travel across the country