Preparing and introducing
What the activity is for
Use this set-up to introduce the motor effect: a motor's spinning comes about from the interaction between the current in a conductor in a magnetic field. There's no spinning here, but the geometry is very clear.
What to prepare
- an eclipse major magnet
- some card and Blu-tack (to cover the magnet, as shown in the photograph)
- a strip of aluminium foil approximately 1.5 cm by 15.0 cm
- 2 crocodile clips
- 2 leads
- 2 bulldog clips
- 8 100 g masses (or similar objects) to elevate the magnet by about 1 cm from bench
- a standard laboratory power pack (vary the p.d. between around 1 V and 4 V)
- a signal generator
If the current is too large then the foil will get hot and the force may be too large and tear the foil. The power supply may suffer if it cannot supply a sufficiently large current. Care is needed, and the potential difference must remain modest.
What happens during this activity
Introduce the very strong magnet. It is shaped in such a way that the north and south poles are at each end of the
horseshoe: between the two poles there is a magnetic field that's uniform (the strength and direction of the field is the same in that space). Remind students that magnetic field lines run from north to south. You might put an arrow, or a piece of card with field lines drawn on it, across the flat front edge of the horseshoe pointing from north to south to show the magnetic field.
Introducing the apparatus and switching on the supply
Teacher: Between the two poles of the magnet there is a thin strip of aluminium foil, which is a good electrical conductor. Two bulldog clips (one on each side of the magnet) connect it to the power supply, which drives an alternating current through the foil, changing direction 50 times a second.
Now switch on the supply.
Teacher: What do you hear?
Teacher: The buzz is caused by the foil: moving in; then out; then in; then out. There must be a resultant force on the foil causing it to move that wasn't there before. The direction of that resultant force must be constantly changing to move the foil backwards then forwards and so on.
This is the motor effect in action. Now for some details on the forces and fields – adapt this dialogue to suit your class (you may need to seek more elucidation to get to these answers).
Teacher: In what direction is the wire moving relative to the direction of the current?
Sally: The direction of the current and the direction of the back-and-forth motion are at right angles.
Teacher: In what direction is the wire moving relative to the direction of the magnetic field?
Elmira: The direction of the magnetic field and the direction of the motion are at right angles.
Now place another arrow in the direction of the back and forth movement (and so in the direction of the resultant force which is causing the wire to move). Notice that every one of the three arrows is at right angles to every other arrow.
Bringing things to a close – and a possible extension
Summarise the discussion:
Teacher: A current-carrying conductor, at right angles to the field lines of a uniform magnetic field, has a magnetic force acting on it. The direction of this force is at right-angles to both the current direction and the field direction. This is called the motor effect.
Why does the strip change direction constantly?
This is because the direction of the current is constantly changing (remember: the supply drives an alternating current). The current alternates – charge moves: up the strip; then down; then up; then down. Each time the current changes direction (up or down) the direction of the resultant force changes (out or in). There is a vibration. This leads to a sound, if of the right frequency.
It's clear that the direction of the current affects the direction of the resultant force. By reversing the direction of the current only, the resultant force direction is reversed/changed.
Increase the size of the current in the strip (increase the applied potential difference a little here) and the size of the resultant force will increase. What do you hear?
A final (optional) step:
Now connect up the signal generator (take care not to draw too much current from this – keep the amplitude small). Now you can vary the frequency and hear different sounds. But the principle is the same.
So now you know how to get movement: arrange a current-carrying conductor at right angles to a magnetic field. There is some more cunning engineering to be done to convert this to a spinning motion.