Spinning magnetic coins

A simple, clear demonstration of the effect of magnetization!

Safe­ty pre­cau­tions

None.

Equip­ment

  • 0.33L cans;
  • ruler;
  • neodymi­um mag­net;
  • iron coins
  • straw.

Step-by-step in­struc­tions

Con­struct a bridge: sus­pend a ruler be­tween two sup­ports. Set a mag­net on top of the ruler. Sus­pend some coins by the neodymi­um mag­net un­der the “bridge,” form­ing a coin chain. Blow on the low­est coin through a straw. No­tice how fast the coin ro­tates. Mod­i­fy this ex­per­i­ment by con­struct­ing a tow­er of coins bal­anced on a can, at­tract­ed to the mag­net on the ruler above. Now you can spin the whole tow­er!

Process de­scrip­tion

Fer­ro­mag­net­ic ma­te­ri­als, such as iron or nick­el, can be­come mag­ne­tized if placed in an ex­ter­nal mag­net­ic field. We used coins made of steel, an al­loy con­sist­ing of most­ly iron, a small amount of car­bon, and some ad­di­tives. When these coins en­counter an ex­ter­nal mag­net­ic field, they be­come mag­nets them­selves and are there­fore at­tract­ed to each oth­er, able to form a chain or tow­er. In the chain, the strong neodymi­um mag­net at the top coun­ter­acts the force of grav­i­ty; fur­ther, the mag­net al­lows the tow­er to main­tain a po­si­tion of equi­lib­ri­um so that the en­tire stack of coins doesn't tip over. But the mag­net can only sup­port a cer­tain num­ber of coins. Mag­net­ic forces weak­en with in­creas­ing dis­tance. Each ad­di­tion­al coin is mag­ne­tized weak­er than the last, and con­se­quent­ly, the hang­ing chain can't be in­fi­nite. The at­trac­tive forces on the low­est coin are very weak – it’s prac­ti­cal­ly hang­ing in the air. This al­lows it to ro­tate rapid­ly, rel­a­tive­ly un­hin­dered by fric­tion. This prin­ci­ple also ap­plies in the case of the coin tow­er – the mag­net on the ruler at­tracts the top­most coin, which at­tracts the next coin, and so on. The bot­tom coin re­lies min­i­mal­ly on the can sup­port­ing it, and con­se­quent­ly, the fric­tion­al forces there are re­duced. The small con­tact area and the low re­ac­tion force al­low the coin to ro­tate prac­ti­cal­ly freely. Such a seem­ing­ly sim­ple and vis­ual ex­per­i­ment al­lows us to ex­plore many fun­da­men­tal prop­er­ties of mag­nets and fer­ro­mag­net­ic ma­te­ri­als.

Try it