Transformers and the National GridIB MYP Physics: Subtopic test

1. 1
   A company in Shenzhen designs a phone charger that contains a transformer. The charger is plugged into a 230 V alternating mains supply and must deliver 5 V to the phone. The primary coil of the transformer has 1150 turns.

   (a)

   What type of transformer is needed inside the charger, and why?

   [1 mark]

   • AA step-up transformer, because the output voltage is higher than the input voltage
   • BA step-down transformer, because the output voltage is lower than the input voltage
   • CA step-down transformer, because it changes alternating current into direct current
   • DA step-up transformer, because the secondary coil has fewer turns than the primary

   (b)

   What is the main function of the soft iron core in the transformer?

   [1 mark]

   • ATo store electrical energy so the output voltage stays constant
   • BTo supply extra current to the secondary coil
   • CTo change alternating current into direct current
   • DTo carry the changing magnetic field from the primary coil through the secondary coil

   (c)

   Calculate the number of turns on the secondary coil of the charger. Assume the transformer is ideal.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student in Lagos builds a model transformer from two coils of insulated wire wound on a soft iron core. She connects the primary coil to a low-voltage alternating supply and a lamp to the secondary coil, and the lamp lights. She then replaces the alternating supply with a battery: the lamp flashes briefly when she connects it, then stays off.

   (a)

   Why does the lamp light only briefly when the battery is connected?

   [1 mark]

   • AThe current in the primary is steady after switch-on, so the magnetic field stops changing and no voltage is induced in the secondary
   • BThe battery voltage is too high for the lamp, so the lamp burns out
   • CDirect current cannot flow through a coil of wire
   • DThe iron core stops being magnetic when it is connected to a battery

   (b)

   The student then treats her model as an ideal transformer. The primary is supplied with 12 V and carries a current of 2.0 A, and the secondary voltage is 6.0 V. What is the current in the secondary coil?

   [1 mark]

   • A1.0 A
   • B2.0 A
   • C4.0 A
   • D8.0 A

   (c)

   Explain why the lamp lights continuously when the primary coil is connected to an alternating supply.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A power station in Poland generates electricity at 25 kV. Transformers raise this to 400 kV for transmission through overhead cables to a city, where further transformers reduce it in stages to 230 V for homes. The transmission cables have a total resistance of 20 Ω.

   (a)

   Outline how transformers are used to carry electricity from the power station to homes.

   [3 marks]

   (b)

   Explain why the electricity is transmitted at 400 kV rather than at 25 kV.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A class in Chile investigates a model transformer. The primary coil has 100 turns and is supplied with 4.0 V alternating voltage. They change the number of turns on the secondary coil, using the same iron core each time, and measure the output voltage with an alternating-voltage meter. For secondary coils of 50, 100, 200, 300 and 400 turns, the output voltages are 2.0 V, 3.9 V, 8.1 V, 11.8 V and 16.2 V. The teacher then links the topic to a proposal for a new 400 kV overhead line that would carry electricity from a wind farm to a city, crossing farmland and passing near a village. Local residents are divided about it.

   (a)

   State a testable hypothesis, with a scientific reason, for how the output voltage depends on the number of turns on the secondary coil. Identify the independent variable, the dependent variable and one control variable, and evaluate whether the results support your hypothesis.

   [6 marks]

   (b)

   Discuss and evaluate whether the 400 kV overhead line should be built.

   [6 marks]

   Total for question 4: 12 marks

End of questions