Powering the Cities

Transformers

Electricity is produced at very high power in power stations.

Power stations are usually polluters and are built away from cities. So the electric power needs to be transmitted to the cities via transmission cables. 

High current causes the cables to warm up and wastes some of the power.

If we decrease the current and increase the voltage, we retain the same power and lose less energy in transmission. This way the national grid is more efficient. 

Increasing the voltage is done with a step-up transformer. Transmission voltage is 400’000 V.

High voltage is dangerous! So we use a step-down transformer to lower the voltage to 230 V and use electricity at our homes. Remember that frequency of PD in UK homes is 50 Hz. 

P6.2-1 Energy Resources on Earth

Burning fossil fuels creates CO₂ which causes global warming by trapping the Sun’s energy.

Nuclear waste from nuclear power plants remains radioactive and toxic for decades.

World population is increasing and many countries are using more energy as technology advances. So it is better to use energy more efficiently to leave enough resources for future generations. 

P6.2-2 Power inside the home

Current from a cell or battery (two or more cells) always flows from one direction; so we call it direct current or DC.

Current from the mains in our houses changes direction, usually 50 times in 1 second (frequency = 50 Hz); that’s why we call it alternating current or AC! And its PD is 230 V (we should remember these numbers!)

Plugs in the UK

We should know the function of 3 wires in a plug:

Brown: Live wire: it brings the current to the electrical appliance. This wire can be dangerous as it carries a high PD of 230 V.

Blue: Neutral: takes the current away. Blue is at a Lower PD of 230 V compared to Live.

Green with yellow stripes: Earth wire: used mostly in heating appliances with a metal case. If the current leaks, this wire takes the current to the Earth, rather than the person touching the appliance! Earth is not needed if the device is double insulated! 

A fuse is a safety device made of a metal that melts easy if the current is too high. Each fuse has a rating (usually 13 A in our homes), and if a current higher than the rating flows in it, the fuse acts like opening a switch and protects the electrical devices from the high current.

A circuit-breaker is a fuse that can be reset with a flick of a switch. It is usually found in the fuse box (consumer unit) in a house. 

Good luck with your exams!