‘Transformers’ is one of the trickier topics to teach for GCSE Physics and GCSE Combined Science.
I am not going to dive into the scientific principles underlying electromagnetic induction here (although you could read this post if you wanted to), but just give a brief overview suitable for a GCSE-level understanding of:
- The basic principle of a transformer; and
- How step down and step up transformers work.
One of the PowerPoints I have used for teaching transformers is here. This is best viewed in presenter mode to access the animations.
The basic principle of a transformer
(BTW This can be copied and pasted into a presentation if you wish,)
The primary and secondary coils of a transformer are electrically isolated from each other. There is no charge flow between them.
The coils are also electrically isolated from the core that links them. The material of the core — iron — is chosen not for its electrical properties but rather for its magnetic properties. Iron is roughly 100 times more permeable (or transparent) to magnetic fields than air.
The coils of a transformer are linked, but they are linked magnetically rather than electrically. This is most noticeable when alternating current is supplied to the primary coil (green on the diagram above).
The current flowing in the primary coil sets up a magnetic field as shown by the purple lines on the diagram. Since the current is an alternating current it periodically changes size and direction 50 times per second (in the UK at least; other countries may use different frequencies). This means that the magnetic field also changes size and direction at a frequency of 50 hertz.
The magnetic field lines from the primary coil periodically intersect the secondary coil (red on the diagram). This changes the magnetic flux through the secondary coil and produces an alternating potential difference across its ends. This effect is called electromagnetic induction and was discovered by Michael Faraday in 1831.
Energy is transmitted — magnetically, not electrically — from the primary coil to the secondary coil.
As a matter of fact, a transformer core is carefully engineered so to limit the flow of electrical current. The changing magnetic field can induce circular patterns of current flow (called eddy currents) within the material of the core. These are usually bad news as they heat up the core and make the transformer less efficient. (Eddy currents are good news, however, when they are created in the base of a saucepan on an induction hob.)
Stepping Down
One of the great things about transformers is that they can transform any alternating potential difference. For example, a step down transformer will reduce the potential difference.
(BTW This can be copied and pasted into a presentation if you wish,)
The secondary coil (red) has half the number of turns of the primary coil (green). This halves the amount of electromagnetic induction happening which produces a reduced output voltage: you put in 10 V but get out 5 V.
And why would you want to do this? One reason might be to step down the potential difference to a safer level. The output potential difference can be adjusted by altering the ratio of secondary turns to primary turns.
One other reason might be to boost the current output: for a perfectly efficient transformer (a reasonable assumption as their efficiencies are typically 90% or better) the output power will equal the input power. We can calculate this using the familiar P=VI formula (you can call this the ‘pervy equation’ if you wish to make it more memorable for your students).
Thus: Vp Ip = Vs Is so if Vs is reduced then Is must be increased. This is a consequence of the Principle of Conservation of Energy.
Stepping up
(BTW This can be copied and pasted into a presentation if you wish,)
There are more turns on the secondary coil (red) than the primary (green) for a step up transformer. This means that there is an increased amount of electromagnetic induction at the secondary leading to an increased output potential difference.
Remember that the universe rarely gives us something for nothing as a result of that damned inconvenient Principle of Conservation of Energy. Since Vp Ip = Vs Is so if the output Vs is increased then Is must be reduced.
If the potential difference is stepped up then the current is stepped down, and vice versa.
Last nail in the coffin of the formula triangle…
Although many have tried, you cannot construct a formula triangle to help students with transformer calculations.
Now is your chance to introduce students to a far more sensible and versatile procedure like FIFA (more details on the PowerPoint linked to above)
e=mc2andallthat 
Reblogged this on The Echo Chamber.
Inductance is a good subject to make students think.
At THORN Lighting the question at interview was effectively a look at this property from different angles.
More recently it was: “Which is more enrgy efficient sinusoidal or square wave driven output transistor operations”
Ooh! I’m not even sure I could begin to answer that question! As a wild guess I’d say square….
I answered sinusoidal and my justification was to do with resonance and class c (etc) oscillator circuits. I decided not square due to losses from capacitative discharge. Either way may be right from my hammock. Tonight I will check with the text Lamps & Lighting ….tbc
pg 324 Lamps & Lighting Coaton & Marsden pg 324:
Resonant mode operation. Power losses in switch mode circuits tend to occur in the the magnetic components and the semiconductor switches. Unlike resistance losses in these components increase with switching frequency. At high freqs, typically above 100 kHz, these losses can be excessive, making use of these impractical. Resonant mode operation exploits tuned loads to overcome some of loss mechanisms. The LC time constants of the DC blocking capacitor and the inductor to match the switching frequency so that they act so output is more sinusoidal ….
So I was right ….. 🙂
PS I hired the guy that wrote the above
Finally can I recommend the chapter on Electrical and Electronic Circuits in the above book. There is a BIG step up from what you have described and the real world. The citcuits are sophisticated beyond anything learnt at Uni. When I studied Applied Physics and Electronics at Durham there were only two natural electronc designers, and one was a lawyer. A per-centage of the rest were capable, like myself – bit could not design circuits so good management material.
Nowadays we have LEDs and maybe don’t need the L understanding but as Bigclive.com & Julian Ilett show on YT the buck-boost-.. converter system concept is tricky but a necessary (and rare) skill in this country
Noted. Technical and engineering skill is very much underrated everywhere, I think. It would be very frightening to realise just how few people truly understand significant areas of science and technnology…
But does a square wave or sinusoidal make a difference to resonant energy losses….?
Buy the book ….
Joules Watt wrote an excellent article on this in Wireless WOrld way back – can I find it heck no
Thanks for the awesome gifs and links to your presentations. I teach science at a public high school in Texas and this is exactly the kind of thing I’ve been looking for.
Absolutely delighted that you find them useful 🙂 Thank you for taking the time to comment — I really appreciate it!
Really useful post on a tricky GCSE topic, thanks.
A pleasure! Thanks for taking the time to comment — I really appreciate it 🙂
Brilliant Animations thank you! I’ve added them into my lesson slides. Sorry to be cheeky, any chance of adding the N and S poles to the primary coils field? So they can see the poles alternate. No worries at all if not! Still awesome. What software do you use to make?
Thank you
Really pleased you like them! I never thought of adding N and S, but if you use with Powerpoint you can probably add these yourself as an animation. Making the GIF was quite involved: I captured a PPT animation as a video, then converted it into a GIF using an online GIF maker — my limited IT skills meant this seemed the most direct route to me. Thanks for commenting!