AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY
Exam Practice: Transformers
Transformers · Magnetism and electromagnetism · Lesson 8 of 8 · 16 marks · 17 minutes
|
Name |
|
Date |
|
Instructions
• Answer all the questions.
• Write your answers in the spaces provided.
• The marks for each question are shown in brackets - use this as a guide to how much to write.
• The answers are on separate pages at the back. Attempt every question before you look at them.
• Answer all questions. Use the mark allocation as a guide to how much to write.
Question 1 CALCULATE (2 marks)
A transformer has 200 turns on its primary coil and 1000 turns on its secondary coil. The input potential difference is 12 V. Calculate the output potential difference. Use the equation: Vp ÷ Vs = np ÷ ns
|
|
|
|
|
|
(Total for Question 1 = 2 marks)
Question 2 EXPLAIN (4 marks)
Explain how a transformer changes the potential difference of an alternating supply.
|
|
|
|
|
|
|
|
|
|
|
|
(Total for Question 2 = 4 marks)
Question 3 CALCULATE (3 marks)
A transformer is 100% efficient. The primary p.d. is 230 V and the primary current is 2.0 A. The secondary p.d. is 11.5 V. Calculate the secondary current.
|
|
|
|
|
|
|
|
(Total for Question 3 = 3 marks)
Question 4 EXPLAIN (3 marks)
Explain why a transformer does not work with a direct current.
|
|
|
|
|
|
|
|
(Total for Question 4 = 3 marks)
Question 5 EXPLAIN (4 marks)
Explain why electricity is transmitted in the National Grid at a very high potential difference.
|
|
|
|
|
|
|
|
|
|
|
|
(Total for Question 5 = 4 marks)
TOTAL FOR PAPER = 16 MARKS
|
|
Answers and mark scheme
Check your answer only once you have written one.
Question 1 (2 marks)
V_s = 12 × 1000 ÷ 200 = 60 V
• Correct rearrangement and substitution 1 mark
• 60 V 1 mark
Question 2 (4 marks)
An alternating current in the primary coil produces a changing magnetic field in the iron core. This changing field passes through the secondary coil and induces an alternating potential difference in it. The ratio of the turns determines the ratio of the potential differences.
• a.c. in the primary makes a changing magnetic field 1 mark
• Field carried by the iron core to the secondary 1 mark
• Induced p.d. in the secondary (generator effect) 1 mark
• Turns ratio sets the p.d. ratio 1 mark
Question 3 (3 marks)
I_s = 230 × 2.0 ÷ 11.5 = 40 A
• Vs Is = Vp Ip 1 mark
• Correct substitution 1 mark
• 40 A 1 mark
Question 4 (3 marks)
A direct current gives a steady magnetic field in the core. No change in field passes through the secondary coil, so no potential difference is induced.
• d.c. produces a constant field 1 mark
• No change in field in the secondary 1 mark
• No p.d. induced 1 mark
Question 5 (4 marks)
A step-up transformer increases the potential difference and so, for the same power, the current is lower. A lower current means less energy is wasted as heat in the cables, so the transmission is more efficient. Step-down transformers then reduce the p.d. for use in homes.
• Step-up transformer raises p.d. 1 mark
• Current is lower for the same power 1 mark
• Less energy wasted as heat 1 mark
• Step-down near homes for safety 1 mark