1. 1
   Electricity generated at a remote hydroelectric power station is transmitted to a city many kilometres away along overhead cables. Before the electricity enters the transmission cables, a transformer at the power station raises the potential difference from 25000 V to 400000 V.

   (a)

   Which type of transformer is this?

   [1 mark]

   • AA step-up transformer
   • BA step-down transformer
   • CA switch mode transformer only
   • DNot a transformer, but a generator

   (b)

   Assuming the transformer is 100% efficient, which equation correctly relates the primary and secondary potential differences and currents of the transformer?

   [1 mark]

   • A$V_p + I_p = V_s + I_s$
   • B$V_p \times I_p = V_s \times I_s$
   • C$V_p \times I_s = V_s \times I_p$
   • D$V_p - I_p = V_s - I_s$

   (c)

   If the current in the primary coil of the transformer is 800 A, use the equation identified above to calculate the current in the secondary coil (in the transmission cables), given that the primary pd is 25000 V and the secondary pd is 400000 V.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   When the electricity finally reaches a local substation near the city, a second transformer reduces the potential difference from 400000 V down to 11000 V before it is distributed to homes and businesses.

   (a)

   Which type of transformer is this?

   [1 mark]

   • AAn electromagnet
   • BA step-up transformer
   • CA step-down transformer
   • DAn alternator

   (b)

   Which statement correctly explains why transmitting electricity at a very high voltage (400000 V), rather than at a low voltage, reduces the energy wasted during transmission along the long overhead cables?

   [1 mark]

   • AA higher transmission voltage has no effect on the amount of energy lost as heat in the cables
   • BA higher transmission voltage increases the current in the cables, which reduces heating losses
   • CA higher transmission voltage makes the cables become superconductors with zero resistance
   • DA higher transmission voltage means a lower current is needed to transmit the same power, and lower current reduces heating losses in the cables

   (c)

   Describe the basic structure of a transformer, and explain how an alternating current in the primary coil is able to induce a potential difference in the secondary coil, even though the two coils are not physically connected.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A mobile phone charger contains a small switch mode transformer instead of a traditional iron-core transformer, converting mains electricity into a low pd suitable for charging the phone's battery.

   (a)

   Describe two ways in which a switch mode transformer differs from a traditional transformer of the kind used at a power station substation.

   [3 marks]

   (b)

   Explain why the small size and light weight of a switch mode transformer make it particularly suitable for use in a mobile phone charger, compared with using a traditional iron-core transformer of the same power rating.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A national grid operator is planning to connect a new offshore wind farm to the mainland electricity network. The wind farm's generators produce electricity at 33000 V, which must be transmitted approximately 60 km to the coast through undersea cables before being distributed to towns.

   (a)

   Explain, using the transformer equation and ideas about energy losses, why the operator should use a step-up transformer near the wind farm before transmission, and a step-down transformer once the electricity reaches the coast, rather than transmitting and distributing the electricity at 33000 V throughout.

   [6 marks]

   (b)

   The step-up transformer at the wind farm has 200 turns on its primary coil and 2400 turns on its secondary coil, with a primary potential difference of 33000 V. Calculate the potential difference produced across the secondary coil, showing your working, and explain, using kinetic theory ideas about heating, why reducing the current in the transmission cables (as a result of this voltage increase) reduces the rate at which energy is dissipated in the cables.

   [6 marks]

   Total for question 4: 12 marks

End of questions