OCR GCSE Physics A (J249) — Magnetism and magnetic fields
Plug a charger into the wall, and it somehow steps the deadly 230 V mains supply down to a safe 5 V for your phone. A transformer achieves this without the two electrical circuits ever physically touching.
Transformers are devices that use electromagnetic induction to change the size of an alternating potential difference. They consist of a primary coil (the input), a secondary coil (the output), and a soft iron core.
The coils are electrically insulated from each other and from the core. This means no electrical current flows directly between the two sides. Instead, all energy is transferred entirely via magnetism.
The operation of a transformer relies on the generator effect. For OCR exams, you must be able to explain this causal chain step-by-step:
Transformers will absolutely not work with Direct Current (DC). DC creates a static, unchanging magnetic field. Without a changing magnetic field, no field lines are "cut" by the secondary coil, meaning no potential difference can be induced.
The core is made of iron because iron is a magnetically soft material. This means it can be easily magnetised and demagnetised, allowing it to rapidly follow the changing magnetic field of the primary coil.
The core's job is to link, focus, and guide the magnetic field lines, ensuring that almost all the magnetic flux from the primary coil successfully cuts through the secondary coil. To improve efficiency, the core is often laminated (made of thin iron layers separated by an insulator) to reduce wasted heat energy.
The amount the potential difference changes depends on the number of turns (loops of wire) on each coil.
The ratio of the potential differences across the coils is exactly equal to the ratio of the number of turns on the coils. We calculate this using the following equation:
Where:
A step-down transformer has 400 turns on the primary coil and 20 turns on the secondary coil. If the input potential difference is 240 V, calculate the output potential difference.
Step 1: Identify the known values.
Step 2: State the formula and substitute the values.
Step 3: Rearrange and calculate the final answer.
Energy cannot be created or destroyed. In calculations, OCR questions typically assume a transformer is 100% efficient. This means the electrical power input to the primary coil equals the electrical power output from the secondary coil.
Because Power = Potential Difference Current (), we can write this as:
Where:
This equation shows that if a step-up transformer increases the voltage, it must decrease the current proportionally to keep the power exactly the same.
A transformer increases an input voltage of 12 V to an output of 120 V. If the current in the primary coil is 5 A, calculate the current in the secondary coil. Assume the transformer is 100% efficient.
Step 1: Identify the known values.
Step 2: State the formula and substitute the values.
Step 3: Rearrange and calculate the final answer.
Students often mistakenly state that 'current flows through the core'. Remember that the core only carries the magnetic field, while the two coils are completely electrically insulated from each other.
In 6-mark 'Explain' questions on how transformers work, examiners specifically look for the three-step causal chain: alternating current in the primary coil → changing magnetic field in the core → induced potential difference in the secondary coil.
Always use the word 'induce' or 'induced' when describing what happens in the secondary coil; examiners will frequently penalise words like 'creates', 'makes', or 'produces' for the voltage.
If an exam question shows a battery (DC supply) connected to a primary coil, the secondary potential difference will be 0 V, because only a changing magnetic field can induce a voltage.
Transformer
A device that uses electromagnetic induction to change the size of an alternating potential difference.
Electromagnetic induction
The process of inducing a potential difference in a conductor by exposing it to a changing magnetic field.
Primary coil
The input coil of a transformer, connected to the initial alternating potential difference source.
Secondary coil
The output coil of a transformer, where the alternating potential difference is induced.
Soft iron core
A block of iron used in a transformer to link, focus, and direct the magnetic field lines between the primary and secondary coils.
Alternating current (AC)
An electric current that repeatedly reverses its direction of flow.
Magnetically soft
A material, such as iron, that can be easily magnetised and demagnetised without permanently retaining its magnetism.
Step-up transformer
A transformer that increases the potential difference by having more turns on the secondary coil than on the primary coil.
Step-down transformer
A transformer that decreases the potential difference by having fewer turns on the secondary coil than on the primary coil.
Put your knowledge into practice — try past paper questions for Physics A
Transformer
A device that uses electromagnetic induction to change the size of an alternating potential difference.
Electromagnetic induction
The process of inducing a potential difference in a conductor by exposing it to a changing magnetic field.
Primary coil
The input coil of a transformer, connected to the initial alternating potential difference source.
Secondary coil
The output coil of a transformer, where the alternating potential difference is induced.
Soft iron core
A block of iron used in a transformer to link, focus, and direct the magnetic field lines between the primary and secondary coils.
Alternating current (AC)
An electric current that repeatedly reverses its direction of flow.
Magnetically soft
A material, such as iron, that can be easily magnetised and demagnetised without permanently retaining its magnetism.
Step-up transformer
A transformer that increases the potential difference by having more turns on the secondary coil than on the primary coil.
Step-down transformer
A transformer that decreases the potential difference by having fewer turns on the secondary coil than on the primary coil.
