Nikki Wylde, a pupil at Wairakei Primary School, asks :-

How does water turn into electricity?

Pat Bodger, an electric power engineer at the University of Canterbury's School of Engineering, responded.

After rain falls in the mountains, it naturally flows downhill back to the sea so we use that energy to make electricity.

To store the water until it is needed dams are built to form natural or manmade lakes. When we want to make electricity, we allow the water to move down again through big pipes called penstocks. At the bottom of these penstocks, the water rushes onto a turbine inside a power station. The turbine has a set of blades like a propeller. The force of the water on the blades makes the turbine rotate.

Connected to the turbine by a shaft are a number of magnets on a support structure known as a rotor. Outside the magnets are a large number of copper bars, all connected in sequence. The action of a magnet passing a bar generates an electric current in the bar. Michael Faraday, a 19th Century scientist, discovered this principle.

To get electricity to a city, we transform the electric voltage to very high levels and transmit it overland in lines. Once there, we bring the voltage down again in stages until it is safe to use. When we connect an appliance to the electrical supply, a current flows. We can use this to make heat or light, or run all the other things that add to our living.

It can take a lot of water to generate electricity. For example, Benmore power station needs about 40 litres of water flowing through a turbine, every minute, just to heat your room. To supply a city like Christchurch, that figure becomes 12 million litres every minute.