S4.13.7 For the adult
Transformers
A phone charger turns the supply of your home into a few volts. Power lines carry thousands of volts. What changes the voltage, and why do the power lines use so much?
What this chapter teaches
A transformer has two coils on an iron core. An alternating current in the primary makes a changing field that induces a voltage in the secondary, so the voltages are in proportion to the turns: Vp ÷ Vs = Np ÷ Ns. An ideal transformer keeps the power the same. Power lines use a high voltage so that the current is small and the heat lost in the wires is small.
Three questions to ask
There is no one right wording. Listen for the idea.
- Why will a transformer not work on a battery?
- How do you find the output voltage from the number of turns?
- Why do power lines use a very high voltage?
After the chapter, the learner can
- Use the turns ratio to calculate output voltage
- Explain why power lines carry electricity at very high voltages
What they copy into their notebook
A transformer needs AC. Vp ÷ Vs = Np ÷ Ns. Ideal: Vp × Ip = Vs × Is. Power lines: high voltage, small current, little heat (I² × R).