1. Science A Physics – P1 Science A Physics – P1 Topic 2b : Transmission of Electricity Topic 2b : Transmission of Electricity

2. Transformers

3. Michael Faraday 1791 - 1867 Michael Faraday In 1831, Michael Faraday wrapped two insulated coils of wire around a large iron ring, attached to a chair. When he passed a current through one coil, a momentary current was induced in the other coil. In this experiment, Faraday had developed the first transformer.

4. Transformers are made by wrapping two coils of wire around a soft iron core. An alternating current can then be passed through the primary coil. Transformers do not work with direct current. Transformers The secondary coil is positioned near to the primary coil, so that it can pick up the changing magnetic field in the iron core. Transformers transfer energy continuously from the primary coil to the secondary coil.

5. Transformers As the changing field cuts through the wires of the secondary coil, a voltage is induced across the secondary coil. This means that when the secondary coil is connected to a complete circuit, an alternating current flows through the secondary coil, and a similar current flows through the primary coil. Both the size and direction of this induced voltage changes, as the voltage applied to the primary coil changes.

6. A step-up transformer changes a low voltage to a high voltage. Step-up Transformers Step-up transformers have a greater number of turns on the secondary coil than on the primary coil. Step-up transformers increase the voltage across the secondary coil, compared to that across the primary coil.

7. All types of power stations use step-up transformers to substantially increase the voltage before it is distributed. Step-up transformers Step-Up Transformers

8. Step-down Transformers A step-down transformer changes a high voltage to a low voltage. Step-down transformers have fewer turns on the secondary coil than on the primary coil. Step-down transformers decrease the voltage across the secondary coil, compared to that across the primary coil.

9. It is the relative number of turns on the two coils that determines whether the voltage induced in the second coil is greater, or less than, the voltage in the primary coil. Two Types of Transformer A step-down transformer A step-up transformer

10. A transformer, such as this one, is used as part of the National Grid, which delivers electricity to homes and industries. These transformers work using the alternating current produced by power stations. Transformers - Uses

11. Why Does the National Grid use High Voltages? On the National Grid, turn up the voltage to see what happens to the energy lost as heat. The power remains constant throughout.

12. Why Does the National Grid use High Voltages?

13. There are three factors to consider:  Resistance (R)  Voltage (V)  Current (I) The greater the resistance to the flow of electricity, the more it is converted into heat. P = I 2 x R The formula for the power (P) loss due to resistance is: Heat is the form of energy by which power is lost.

14. If you know any two of Power (P), Current (I) or Resistance (R), you can use the formula triangle to find the remaining value... P = I 2 x R R = P I2I2 I 2 = P R Power, Current & Resistance

15. Why Does the National Grid use High Voltages? To transmit large amounts of electrical power around the National Grid, you either use a high voltage or a high current. In both cases the power output from the power station remains constant - this is explained by the power equation: P = I x V  When the current is high, the voltage is low.  When the voltage is high, the current is low. If electricity was transmitted from the power station with a voltage of 25,000 V the current would be 8,000A. As power loss due to resistance is calculated by P = I 2 x R, this would cause the cables in the National Grid to overheat. For a constant power...

16. If you know any two of Power (P), Current (I) or Voltage (V), you can use the formula triangle to find the remaining value... P = I x V V = P I I = P V Power, Current & Voltage

17. When the transmission voltage from the power station is increased from 25,000 V to 400,000 V (increased by 16 times), the current is reduced by the same factor. 8,000 A = 500 A 16 This makes the current in the National Grid: The use of a high voltage and a low current allows electricity to be transmitted around the National Grid with minimal power loss. Why Does the National Grid use High Voltages? Although this is still a relatively high current, the use of thick wires reduces the resistance, so the heating effect is reduced.

18. Two 100W light bulbs from Europe and America are compared. What happens to the current if we connect these lamps to the correct power source? (i) a 230V supply, (ii) a 120V supply? i)100W using 230V ii) 100W using 120V

19. A typical lighting circuit in a British house is designed to take up to 5 A. How many 100 W light bulbs could you run from it? 1 lamp take 0.43A

20. Safety and the National Grid

21. For safety reasons, the high voltage cables on electricity pylons are hung from porcelain insulators. Safety and the National Grid

22. At power stations and substations, remote switchgear is used to switch on and off the flow of electricity. This helps to protect the workers from the danger of electrocution. Safety and the National Grid

23. Overhead Power Lines Overhead power lines are very dangerous. You do not have to touch them to be electrocuted, as the electricity can jump through the air. For your safety and that of others, here are some simple rules for you to follow...

24. Overhead Power Lines  DO NOT try to retrieve anything stuck in power lines.  DO NOT climb a pylon or touch power lines.  DO NOT climb trees which have power lines passing close by them.  DO NOT go fishing or fly kites near power lines.