A demonstration that needs to be carefully choreographed
What the activity is for
This is an interactive teacher demonstration that can be used to explore and to get students talking about various aspects of electromagnetic induction in the context of a simple transformer.
What to prepare
- a mounted 2.5 V filament bulb
- a low-voltage AC power supply, 0–12 V
- 2 (120 + 120)-turn coils
- an iron retort stand
- connecting wires
What happens during this activity
This demonstration might be carried out after electromagnetic induction has been introduced. It then provides a way into thinking and talking about transformers. The main aim is to get students talking about what they can see happening. The demonstration can be carried out in a series of steps.
Start simple – with a single glowing bulb (we used a 2.5 volt bulb) connected to the AC low-voltage output of a power supply. Switch on to about 3.0 volt and the bulb lights nice and bright. No surprise there!
Now set the challenge:
Teacher: How can we light the bulb without connecting it up to this kind of power supply or to a battery?
Using coils to make a link
Connect the bulb to a 240-turn coil of wire.
Teacher: How might connecting the bulb to a coil of wire help in making it light up?
Place the second 240-turn coil at the bottom of an iron retort stand and connect it to the alternating supply still set at about 3.0 volt. Now place the coil connected to the bulb over the retort stand. Nothing happens… until the second coil (with bulb) is slid down the vertical stand.
Concluding the demonstration
The bulb lights, getting brighter the further the
secondary coil is moved down the iron rod.
simple demonstration is based on some fascinating physics and provides an excellent opportunity for students to talk through quite a complicated, multi–step explanation based on electromagnetic induction. The first point you might make:
Teacher: Look! The bulb is not connected to the supply and yet it lights! The first coil is connected to the supply but there's no connection to the second coil. What on Earth's going on here? Who can explain?
The explanation to move towards is as follows:
The changing potential difference from the supply drives a changing electric current round the lower coil or
The changing electric current in the lower coil produces a changing magnetic field. When the current is zero, the field is zero; when the current is a maximum, the magnetic field is at maximum strength.
The changing magnetic field is
carried by the retort stand
core and links the
secondary (upper) coil to the
primary (lower) coil.
The changing magnetic field linking the upper coil induces a changing voltage across this secondary coil.
The changing voltage across the upper coil drives a changing current in that coil and in the bulb, making it light.
Further questions to ask, and some (smart) students' answers:
Teacher: So why does the bulb get dimmer as the upper coil is moved up the retort stand?
Bill: There must be a weakening of the magnetic field as you go higher.
Teacher: What happens if the bulb is connected to 120 turns rather than across all 240? Try it!
Emily: The bulb goes dimmer. This is because a smaller voltage is induced across the secondary coil as there are a smaller number of terms. We have a simple step–down transformer!