4. Transformers allow generated voltage to be either increased or decreased before it is used

1. 4. Transformers allow generated voltage to be either increased or decreased before it is used Caroline Chisholm College Physics

2. Describe the purpose of transformers in electrical circuits Higher Voltage High Voltage Low Voltage Lower Voltage M.Edwards 2003 M.Edwards 2003 SECONDARY SECONDARY PRIMARY PRIMARY e.g. from power station to transmission lines - the higher transmission voltage will mean less current and so less power losses e.g. from power lines to household circuit - the voltage is lowered so that it can be used in household appliances at 240V Caroline Chisholm College Physics Transformers convert A.C. voltage to higher values (step-up) or lower values (step-down) . From a source of fixed e.m.f, other e.m.f.’s can be obtained by varying the number of turns on the primary and secondary coils, depending on the required step-up or step down.

3. Compare step-up and step-down transformers Higher Voltage High Voltage Low Voltage Lower Voltage M.Edwards 2003 M.Edwards 2003 SECONDARY SECONDARY PRIMARY PRIMARY Caroline Chisholm College Physics Transformers consist of two coils wound onto the same soft iron core. The AC current in the primary winding sets up an alternating flux in the core, and the electric field produced by this alternating flux induces an emf in the secondary coil. Since the same flux operates on both coils, the induced emf per turn is the same - - so the more turns, the more emf! A transformer with more turns on the secondary produces a higher emf (voltage) and is called a step-up transformer A transformer with less turns on the secondary coil produces a lower emf (voltage) and is called a step-down transformer 8 turns 4 turns 4 turns 8 turns Identify the relationship between the ratio of the number of turns in the primary and secondary coils and the ratio of primary to secondary voltage In both cases, the ratio of turns is equal to the ratio of voltages

4. Perform an investigation to model the structure of a transformer to demonstrate how secondary voltage is produced Caroline Chisholm College Physics Link to an experiment

5. Caroline Chisholm College Physics Solve problems and analyse information about transformers using: The wires on a telegraph pole carry a potential difference of 11 kV. If the potential difference is stepped down to 240 V for household use, what would be the ratio of primary turns to secondary turns in the transformer? • 1 mark • Vp/Vs = np/ns • Vp/Vs = 11000/240 • Vp:Vs = 46:1

6. Caroline Chisholm College Physics • Winona (a qualified electrician) was creating a step down transformer for her doorbell. She wanted to transform a 240 V line to a 8 V line. • Winona already had a transformer with 2000 turns in the primary coil. Using an appropriate formula she determined the number of turns she needed in the secondary coil. • She then obtained a 300 V voltmeter and although there was a potential difference of 240 V from the voltage source for the primary coil, there was a reading of 5 V across the terminals of the secondary coil. • (a) Determine the number of turns Winona would have put in the secondary coil? • (b) List two reasons why Winona may have obtained a different result than what she expected. • (c) Why did Winona need to install a transformer with the doorbell?

7. Caroline Chisholm College Physics • (a) 1 mark • ns = npvs/vp • ns = 2000 x 8 / 240 • ns = 67 turns • (b) 2 marks • The transformer would not have worked at 100 % efficiency and therefore the potential difference across the secondary terminals would be lower than expected. • The limitation of scale on the 300 V voltmeter would mean that there would be errors in reading the device. • (c) 1 mark • If the supplied potential difference was too large then too much current would be drawn from the mains which would cause the wires in the doorbell to heat up and melt.

8. Question 4Below is a diagram of a transformer.(a) What is the name given to this type of transformer?(b) Explain why this transformer would produce a different output voltage to that which is supplied.(c) Given that the AC power supply has a potential difference of 240 V and the coils in the diagram are proportional to the number of coils in the transformer, determine the output voltage. Caroline Chisholm College Physics

9. 4 (a) 1 mark This transformer is a step-down transformer. (b) 2 marks The AC power supply will produce a constant change in the flux induced in the soft iron core. The magnetic field produced continues around the iron core and therefore there is a change in magnetic flux in the secondary coil also. Because there are less turns of wire in the secondary coil, there is less induced potential difference. (c) 1 mark vs = vpns/np vs = 240x4/8 vs = 120 V Caroline Chisholm College Physics

10. Explain why voltage transformations are related to conservation of energy Caroline Chisholm College Physics

11. Gather, analyse and use available evidence to discuss how difficulties of heating caused by eddy currents in transformers may be overcome Caroline Chisholm College Physics

12. Explain the role of transformers in electricity sub-stations Caroline Chisholm College Physics

13. Gather and analyse secondary information to discuss the need for transformers in the transfer of electrical energy from a power station to its point of use Caroline Chisholm College Physics

14. Discuss why some electrical appliances in the home that are connected to the mains domestic power supply use a transformer Caroline Chisholm College Physics

15. Discuss the impact of the development of transformers on society Caroline Chisholm College Physics

16. A.C. ELECTRIC MOTORS (2 hours)5. Motors are used in industries and the home usually to convert electrical energy into more useful forms of energy Caroline Chisholm College Physics

17. Describe the main features of an AC electric motor Caroline Chisholm College Physics

18. Question 5 Below is a diagram of a shaded-pole induction motor.(a) Describe what the stator is and explain why it is necessary in an AC electric motor.(b) Explain why this motor could be used in a circular saw but not in an electric car. Caroline Chisholm College Physics

19. 5 (a) 2 marks The stator produces an apparently rotating magnetic field. An electric current is induced in the rotor. The rotating magnetic field from the stator then produces a torque due to the induced current. (b) 2 marks An induction motor can only provide a low amount of power. A circular saw does not require much power therefore the induction motor which is efficient and low cost is ideal. The induction motor could not be used in an electric car because it requires too much power. The power is limited by the frequency of the alternating current.. Caroline Chisholm College Physics

20. Perform an investigation to demonstrate the principle of an AC induction motor Caroline Chisholm College Physics

21. Gather, process and analyse information to identify some of the energy transfers and transformations involving the conversion of electrical energy into more useful forms in the home and industry Caroline Chisholm College Physics

22. Question 3 • Below is a diagram of the torque provided by a 3 phase motor. • (a) Determine the maximum total torque generated. • (b) If there were 2500 turns of wire per phase, and the distance from the axis of the rotor to the average wire was 5 cm, determine the magnitude of the maximum force on one turn of an average wire? • (c) Describe the transformations of energy in an induction motor. Caroline Chisholm College Physics

23. Caroline Chisholm College Physics • 3 (a) 1 mark • Adding the torques on the graph, the maximum torque that can be determined is 200 Nm • (b) 1 mark • The maximum torque on any one phase is 100 Nm. The maximum torque on one wire is therefore 100/2500 = 0.04 Nm. •  = Fd • F = /d • F = 0.04/0.05 • F = 0.8 N • (c) 2 marks • Electrical energy is transformed into magnetic potential energy in the stator which is transformed into electrical energy in the armature conductors which is transformed into kinetic energy in the rotor.