5-MAG-8

Outcomes

• Pupils can begin to understand how transformers function
• Pupils can appreciate the affect of changing the number of coils

Specification References

• 6.17 describe the structure of a transformer, and understand that a transformer changes the size of an alternating voltage by having different numbers of turns on the input and output sides
• 6.18 explain the use of step-up and step-down transformers in the large scale generation and transmission of electrical energy
• 6.19 know and use the relationship between input (primary)and output(secondary) voltages and the turns ratio for a transformer:

<math>V_p/V_s=N_p/N_s</math>

• 6.20 know and use the relationship:

input power = output power

<math> V_p I_p = V_s I_s </math>

for 100% efficiency

Starter

Recap previous work on induction. Pupils need to appreciate the that a moving magnetic field can induce a current in a wire/coil. It's also worth recapping AC from last year. These could both be done as mini white board exercises. Example question could be about a north pole moving into a coil producing a positive current. What happens when magnet is: a) pulled out, b) south pole in first, c) left in the coil. Questions about AC could be comparing mains with batteries or sketching current-time graphs for AC and DC.

Main Body of Lesson

As this is the first lesson on transformers the focus will be on working through the various demonstrations available. We have demountable transformers for this purpose.

1. changing the number of secondary turns- have pupils predict V2 when you double/halve etc. the number of turns on the secondary coil. Can note down I and V for both coils and should get the same power
2. show the effect of not completing the core (leave the top off, less efficient)
3. reduce the number of secondary turns increases current and decreases voltage use this to heat water or melt solder (using the single loop trough)
4. demo the coil with a bulb in water to show that no electrical contact is needed between the coils

Be careful here with the high current single ring; ensure that you hold it by the insulating handle. Also, switch power supply off before changing coils.

Once you've gone through all of these you can move onto what pupils know about electricity transfer using pylons. Most will know about high voltages see if they can make the link between high voltage, low current and lower power loss. They don't need to know <math> P=I^2R </math> but it does help here. Then demo the high voltage transmission cables. First send a low voltage through the wires. This will result in the bulb hardly lighting on the "home" end. This is a nice time to show the heating effect of current using the IR camera. Next, using the 2 transformers step up the voltage, transmit, then step it down.

Care should be taken here due to high voltages and exposed wires. The voltage should be approximately 28V when stepped up which is within the CLEAPSS guidelines. Avoid touching or having pupils touch the wire or the contacts. ERS has manufactured a simplified switch for selecting whether to use the transformers or not. Switch off before throwing both switches.

If you have time you can move onto the transformer equation. The plan is to do this via experiment in the following lesson; however, if you have time have pupils consider what happens when you alter the input voltage and the number of turns on each coil.

Plenary

Ask pupils to summarise how a transformer works in their own words. After a minute or two get them to confer with their neighbour and come up with a refined version. You can either then have pupils share these or come back to them next lesson.

Additional Information

Safety/Hazards

Demountable transformers: These should be handled by pupils only if they meet the normal earthing and insulation requirements for mains-operated equipment as detailed in section 6 (Mains electricity) and if there is no risk of any secondary coil producing over 28 V ac at 5 mA or more. During class activities with coils and C-cores, care should be taken to minimise the risk that hazardous voltages may be generated.