Parts of a transformer
Definition
Transformer
A device in which an alternating current in one coil induces an alternating potential difference in a second coil wound on the same iron core.
Definition
Primary coil
The input coil of a transformer, which is connected to the alternating supply.
Definition
Secondary coil
The output coil of a transformer, across which a potential difference is induced.
Definition
Iron core
The soft iron frame that passes through both coils of a transformer and carries the changing magnetic field from one coil to the other.
- A transformer is made of two separate coils of insulated wire wound on the same iron core.
- The primary coil is the input coil, and it is connected to an alternating supply.
- The secondary coil is the output coil, and it is connected to the device or circuit that the transformer supplies.
- The wire of each coil is insulated, so current cannot jump from one turn to the next, and there is no electrical connection between the two coils.
- The core is made of soft iron, which is magnetised very easily and loses its magnetism as soon as the magnetising current falls, so its field follows every change in the current.
- The core forms a closed loop that passes through both coils, so it guides almost all of the magnetic field made by the primary coil through the secondary coil.
How the coils are linked
Definition
Alternating current
A current that repeatedly reverses its direction of flow around a circuit.
Definition
Electromagnetic induction
The production of a potential difference across a conductor when the magnetic field around it changes.
- An alternating current in the primary coil is constantly changing in size and reversing in direction.
- This current turns the primary coil into an electromagnet whose magnetic field also changes in size and reverses in direction.
- The iron core is magnetised by this field, so an alternating magnetic field is set up in the core.
- The core carries this changing magnetic field through the secondary coil.
- A magnetic field that changes around a conductor induces a potential difference across it, so an alternating potential difference is induced across the secondary coil by electromagnetic induction.
- If the secondary coil is part of a complete circuit, this induced potential difference drives an alternating current in the secondary circuit.
- The induced current alternates at the same frequency as the current in the primary coil, because the field in the core changes at that frequency.
- Energy passes from the primary circuit to the secondary circuit through the changing magnetic field in the core, not through any current flowing between the coils.
- In this way an alternating current in one circuit induces a current in a completely separate circuit.
Why direct current does not work
Definition
Direct current
A current in which charge flows around a circuit in one direction only.
- A direct current in the primary coil produces a magnetic field that is steady once the current has settled.
- A steady field in the core does not change, so no potential difference is induced across the secondary coil.
- A transformer connected to a d.c. supply therefore gives no output while the current is steady.
- A brief potential difference is induced in the secondary only at the instant the d.c. supply is switched on or off, because the field in the core changes at those moments.
- This is why transformers are always used with alternating supplies, such as the UK mains supply, which alternates at 50 Hz50\ \text{Hz}50 Hz.
Practical
Linking two circuits
- Aim: to show that an alternating current in one coil induces a current in a separate coil, and that the effect needs a changing magnetic field.
- Apparatus: two coils with the same number of turns, a U-shaped soft-iron core with a straight iron bar (the yoke) that closes it, a clamp, a low-voltage a.c. supply, a low-voltage d.c. supply, a switch, a small filament lamp in a holder, a sensitive centre-zero meter and connecting leads.
- Method:
- Slide one coil onto each limb of the U-shaped core, and clamp the yoke across the top so the iron forms a closed loop through both coils.
- Connect the first coil to the a.c. supply through the switch, and connect the second coil to the lamp only, so no wire links the two circuits.
- Set the supply to a low potential difference, close the switch and look at the lamp.
- Lift the yoke off the core and look at the lamp again, then slide out the core altogether and switch off quickly.
- Rebuild the closed core, swap the a.c. supply for the d.c. supply, and replace the lamp with the centre-zero meter.
- Watch the meter as the switch is closed, while it stays closed, and as it is opened again.
- Results:
- The lamp lights even though no wire connects the two circuits.
- Removing the yoke makes the lamp dimmer, and removing the core makes it go out or almost out, because much less of the changing field reaches the second coil.
- With the d.c. supply, the needle flicks one way at the moment the switch is closed, returns to zero while the current is steady, and flicks the other way when the switch is opened.
- Watch out: without the core, the current in the first coil rises sharply and the coil heats up, so take that observation quickly and switch off.
- Safety: use only a low-voltage supply and never connect the coils to the mains, let the coils cool before handling them, and keep fingers clear when the yoke snaps onto the core.
Common Mistake
- Do not say that current flows through the iron core from one coil to the other, because the core carries a changing magnetic field and the two coils are electrically separate.
- Do not say that a transformer works on direct current, because a steady current gives a steady field and induces nothing in the secondary coil.
Exam technique
Explaining how a transformer works
- Set your answer out as a chain in this order: alternating current in the primary coil, alternating magnetic field, field carried by the iron core, changing field through the secondary coil, alternating potential difference induced, and alternating current if the secondary circuit is complete.
- Describe the field as changing or alternating every time you mention it, because a field that is not described as changing earns no credit.
- If asked why the core is made of iron, say that iron is easily magnetised and carries the magnetic field from the primary coil to the secondary coil.
Self review
- Name the three main parts of a transformer.
- Why must the supply to the primary coil be alternating?
- What links the primary coil to the secondary coil?
- Why is the core of a transformer made of soft iron?
- What is induced in the secondary coil, and what is needed for it to drive a current?
