1. 1
   An electronics hobbyist uses a small transformer to convert the 230 V mains supply down to 9 V to power a model railway. The transformer has a primary coil of 2000 turns and a secondary coil of fewer turns, both wound onto a soft iron core.

   (a)

   What type of current must be supplied to the primary coil for the transformer to work?

   [1 mark]

   • AAlternating current (a.c.)
   • BDirect current (d.c.) only
   • CNo current is needed
   • DA constant, unchanging current

   (b)

   The hobbyist's transformer steps the voltage down from 230 V to 9 V. What is this type of transformer called?

   [1 mark]

   • AA step-up transformer
   • BA step-down transformer
   • CA generator
   • DA rectifier

   (c)

   The secondary coil has 78 turns. Calculate the number of turns on the primary coil that would be needed if, instead, the transformer were required to step 230 V down to 9 V using this secondary coil, showing your working.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A national grid company transmits electrical power from a power station to a distant city along overhead cables. A transformer at the power station steps the voltage up from 25 kV to 275 kV before transmission, and another transformer near the city steps the voltage back down for use in homes.

   (a)

   What is the main reason for transmitting electrical power at such a high voltage?

   [1 mark]

   • ATo reduce the resistance of the transmission cables
   • BTo make the electricity flow faster along the cables
   • CTo reduce the current in the cables and so reduce power losses due to heating
   • DTo allow the current to become direct current

   (b)

   For a transformer operating at 100% efficiency, which equation correctly relates the primary current $I_p$, primary voltage $V_p$, secondary current $I_s$ and secondary voltage $V_s$?

   [1 mark]

   • A$I_pV_s = I_sV_p$
   • B$I_p + V_p = I_s + V_s$
   • C$I_p / V_p = I_s / V_s$
   • D$I_pV_p = I_sV_s$

   (c)

   The power station transformer supplies a power of 4.4 MW to the transmission cables at 275 kV. Assuming the transformer is 100% efficient, calculate the current flowing in the transmission cables, showing your working, and explain briefly why transmitting at 275 kV rather than 25 kV reduces the energy wasted as heat in the cables.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A museum has an old induction demonstration set consisting of two separate coils of insulated wire wound onto a common soft iron core, one coil connected to an a.c. supply and the other connected to a lamp.

   (a)

   Describe the construction of this simple transformer and explain, in terms of the changing magnetic field, how an e.m.f. is induced in the secondary coil when the a.c. supply is switched on.

   [3 marks]

   (b)

   A curator accidentally connects the primary coil of the demonstration set to a d.c. battery instead of the a.c. supply. Explain what would be observed at the lamp connected to the secondary coil, both at the moment the battery is first connected and after it has been connected for some time, giving reasons for each observation.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An electricity distribution company plans to upgrade the transformers used to supply a rural town, which currently experiences noticeable voltage drops and energy losses along its long overhead transmission cables.

   (a)

   Describe the construction and principle of operation of a simple iron-cored transformer, and explain, using appropriate equations, why increasing the transmission voltage between the power station and the town reduces the power lost in the cables as heat.

   [6 marks]

   (b)

   The company is deciding between two cable options for the upgrade: thinner, cheaper cables with higher resistance, or thicker, more expensive cables with lower resistance, given that the transmission voltage is also being increased. Discuss how the choice of transmission voltage and the choice of cable resistance together affect the power lost as heat in the cables, and explain which combination would minimise energy losses, referring to relevant equations in your answer.

   [6 marks]

   Total for question 4: 12 marks

End of questions