TransformersAQA A-Level Physics: Subtopic test

1. 1
   A mobile phone charger contains a step-down transformer. The primary coil has 2300 turns and is connected to the 230 V alternating mains supply. The secondary coil supplies the phone with a potential difference of 5.0 V.

   (a)

   How many turns are on the secondary coil, assuming the transformer is ideal?

   [1 mark]

   • A25
   • B50
   • C460
   • D2300

   (b)

   The charger delivers a current of 1.8 A at 5.0 V while drawing a current of 0.045 A from the mains. What is the efficiency of the transformer?

   [1 mark]

   • A13%
   • B78%
   • C87%
   • D115%

   (c)

   The core of the transformer is made from thin sheets of iron, insulated from each other, rather than from a single solid block of iron. Explain why this reduces the energy wasted in the core.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A technician is testing a laboratory transformer that has 120 turns on its primary coil and 600 turns on its secondary coil. The primary coil is connected to a 12 V alternating supply and a lamp is connected across the secondary coil.

   (a)

   What is the potential difference across the secondary coil if the transformer is ideal?

   [1 mark]

   • A2.4 V
   • B12 V
   • C72 V
   • D60 V

   (b)

   The transformer is ideal. How does the current in the secondary coil compare with the current in the primary coil?

   [1 mark]

   • AOne fifth of the primary current
   • BEqual to the primary current
   • CFive times the primary current
   • DSixty times the primary current

   (c)

   A voltmeter connected across the lamp reads 54 V rather than the value expected from the turns ratio. Suggest two reasons why the secondary p.d. is lower than the ideal value.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A power station generates 50 MW of electrical power. The power is sent to a town through transmission lines of total resistance 8.0 Ω. The power station can either send the power directly at 25 kV or use a step-up transformer to send it at 400 kV.

   (a)

   Calculate the power wasted in the lines if the power is sent directly at 25 kV.

   [3 marks]

   (b)

   Calculate the power wasted in the lines when the power is sent at 400 kV, and explain why electricity is transmitted at high voltage.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A grid substation contains a large step-down transformer. The primary coil is connected to a 132 kV line and the secondary coil supplies a town at 11 kV. The transformer is 98% efficient and the town needs a power of 33 MW.

   (a)

   Explain how eddy currents are produced in the core of a transformer, why they reduce the efficiency, and how the design of the transformer reduces them. Describe two other causes of inefficiency in a transformer.

   [6 marks]

   (b)

   Calculate the input power to the transformer, the current in the primary coil, the current in the secondary coil and the power wasted in the transformer.

   [6 marks]

   Total for question 4: 12 marks

End of questions