Transformers (Oxford AQA IGCSE Physics)
Revision Note
Written by: Dan Mitchell-Garnett
Reviewed by: Caroline Carroll
Structure of a Transformer
A transformer is a device used to change the value of an alternating potential difference or current
This is achieved using the generator effect
A basic transformer consists of:
A soft iron core
A primary coil (turns of wire around one side of the iron core)
A secondary coil (turns of wire around the other side of the iron core)
Iron is used for the core because it is easily magnetised
The structure of a transformer
How a transformer works
An alternating current is supplied to the primary coil
The current is continually changing direction
This means it will produce a changing magnetic field around the primary coil
The iron core is easily magnetised, so the changing magnetic field passes through it
The changing magnetic field of the core is now inside the secondary coil
This changing field cuts through the secondary coil and induces a potential difference
As the magnetic field is continually changing the potential difference induced will be alternating
The alternating potential difference will have the same frequency as the alternating current supplied to the primary coil
If the secondary coil is part of a complete circuit it will cause an alternating current to flow
Step-Up & Step-Down Transformers
A transformer consists of a primary and secondary coil
The primary coil is the first coil
The second coil is the second coil
A step-up transformer increases the potential difference of the power source
A step-up transformer has more turns on the secondary coil than on the primary coil
Step-up transformers are used to increase the potential difference of current before it reaches the distribution lines of The National Grid
A step-down transformer decreases the potential difference of a power source
A step-down transformer has fewer turns on the secondary coil than on the primary coil
Step-down transformers are used to decrease the potential difference to a safe level before it reaches consumers
Examiner Tips and Tricks
The primary coil is defined as being connected to the source and the secondary coil as being connected to the destination (sometimes called the load).
Worked Example
The potential difference of the current flowing into a transformer is 40 V. The potential difference of the current leaving the transformer is <40 V. There are 12 turns on the secondary coil.
How many turns are there on the primary coil?
A. 12
B. <12
C. >12
D. There is not enough information given to know
Answer: C
Step 1: Determine whether the transformer is a step-up or step-down transformer
The potential difference on the primary coil is 40 V
The potential difference on the secondary coil is less than 40 V (<40 V)
The potential difference has decreased
Therefore, it is a step-down transformer
Step 2: Determine the relative number of turns on the primary coil
The secondary coil has 12 turns
A step-down transformer has fewer turns on the secondary coil than on the primary coil
Therefore, there must be more than 12 turns on the primary coil (>12)
The answer is C
Examiner Tips and Tricks
Make sure you are familiar with the greater than and less than symbols.
The widest part of the arrow is always on the side of the largest value.
10 > 5 (10 is greater than 5)
5 < 10 (5 is less than 10)
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