What it shows
A solid aluminium or copper plate swinging between magnet poles stops within a few swings, while a slotted plate swings on, because circulating eddy currents are induced in the solid plate and Lenz’s law makes them oppose the motion. This is how train and roller-coaster magnetic brakes work.
The science
When a conductor moves through a magnetic field, the free electrons in it experience a force and swirl around in closed loops called eddy currents. By Lenz’s law these currents produce their own magnetic field that opposes the change causing them, so the plate feels a drag force proportional to its speed and its kinetic energy turns into heat in the metal. Slots cut across the plate break up the large current loops into tiny ones, so the eddy currents are far weaker and the slotted plate swings almost freely. Counting swings until the plate stops gives a clear, measurable comparison.
Materials
Cost: under Rs 500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two aluminium sheets about 8 cm by 8 cm, 1 to 2 mm thick (an old cooking pot lid or a sheet from a scrap shop); one left solid, one cut into comb slots
- Two strong neodymium block magnets or a stack of discs
- A wooden stand with a crossbar and a long bolt as a pivot
- A ruler, a protractor and a stopwatch or phone
- Glue and a small clamp
Safety
For this project: An adult must cut the slots with a hacksaw and file the edges smooth. Keep the magnets apart from each other when not fitted; they pinch fingers.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Drill or punch a hole in the top edge of each plate and hang the solid plate from the pivot so it swings freely.
Fix the two magnets on wooden blocks facing each other with opposite poles, leaving a gap just wider than the plate at the lowest point of the swing.
Pull the plate aside to 30 degrees, release, and count the number of swings until it is nearly still; repeat three times.
Swap in the slotted plate and count again with the same release angle; record both sets in a table.
Remove the magnets and count the swings of the solid plate to show that without the field it swings for a long time.
Change the gap to 1 cm, 2 cm and 3 cm using card spacers and record how the braking changes with field strength.
Touch the solid plate after many swings with the magnets in place and discuss where the energy has gone.
Present a bar chart of swings to rest for each condition and explain the slotted plate result with a diagram of the current loops.
What you should see
The solid plate stops within 3 to 6 swings between the magnets, the slotted plate takes many times longer, and without magnets both swing for minutes. A wider gap gives weaker braking.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What are eddy currents?
Circulating currents induced in a bulk conductor when the magnetic flux through it changes.
2Why does the slotted plate not stop?
The slots break the current paths into small loops, so the eddy currents and their drag are much smaller.
3Where does the kinetic energy of the plate go?
Into heat in the metal, through the resistance of the eddy current paths.
4Give one practical use of eddy currents.
Magnetic braking in trains, induction cooking, or the damping in a moving-coil galvanometer.
Take it further
Slide a magnet down a sloped aluminium sheet and down a slotted sheet and time both to show the same effect in a different form.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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