# Why do we tap on cardboard to see magnetic field lines?

If we sprinkle iron particles on a cardboard where a bag magnet is kept and tap the board gently then the particles get arranged in a way that they look like field lines. But I am confused why do we have to tap on the board? Why won't it get arranged like that normally?

(Sorry for this stupid question, I have stated studying proper magnetism recently.)

• Analogy: cover a plate (say the bottom of a guitar) with sand/dust. Play a note and watch the particles get bounced into patterns. In this case, they bounce in a random direction but with energy greatest where the plate has max amplitude, and least where there are nodes. The particles end up along nodal lines. Jan 13 at 16:03
• Consider that a compass, for example, only works because the needle is suspended with extremely low friction and can turn even with the very small force that the Earth's magnetic field produces upon it. If you took the needle out of the compass and just dropped it onto the ground, or onto some cardboard, it would not point north - it would stay stuck to whatever surface by friction.
– J...
Jan 14 at 1:20
• @CarlWitthoft not a great analogy, because nodes on a vibrating plate are present only while the plate is actually vibrating, while e.g. a magnet exerts force on tiny pieces of metal continuously. Why is then additional vibration needed for the pieces to move and align along the field lines? This is answered by Ben51. Jan 14 at 6:05
• @user1079505 OK, then -- turn off the magnetic field. Jan 14 at 13:05
• @CarlWitthoft Where's the Earth's power switch? I think we need a reboot. Jan 14 at 15:17

It’s like shaking a measuring cup half full of sugar to make it level out—in both cases there’s an energetically favored configuration you’re trying to reach, but without agitation, friction prevents the grains from moving to that configuration. Each time you tap the cardboard or shake the cup, you give the grains a new opportunity to settle in a new position, and the magnetic/gravitational forces, though not strong enough to overcome friction on their own, determine the end configuration.

• Just to add to this, there are two effects here. One is grains which leave the sheet due to the tap - they are then free to move under the magnetic forces without friction and settle in a new location. And the other for grains which don't move is the "sticktion" effect, where it takes more force to start something moving than to move it more once it's already in motion - in this case the "tap" starts the grains moving and they then have less frictional resistance to continue moving where they want. Also note that if the magnetic field is strong enough, it'll pull them around without a tap. Jan 14 at 13:03
• I'm not sure this completely answers the question. If the particles move towards the lowest energy configuration they will form a clump at the poles of the magnet rather than forming lines. Jan 14 at 14:25
• @ClaraDiazSanchez Yes, I suppose it is not a global minimum energy, but a local one—once nearby iron filings become stuck together in a line, they are stable even if it’s not the absolute minimum energy. Jan 14 at 14:29
• So, instead of tapping we could also use a stronger magnet to overcome friction. Maybe you want to add why that isn't a good idea. Jan 14 at 15:43
• @ClaraDiazSanchez: The tap overcomes static friction, but kinetic friction will prevent the particles from traveling very far before coming to rest again. If you impart too much kinetic energy, or use an overly-strong magnet, then they really will clump at the poles (I think). Jan 14 at 18:49

Imagine rain falling on a landscape in which there are hills and valleys. A small lake may form in a hollow half way up the side of a hill. The water doesn't know that it's half way up the hillside, so it stays where it is. If a storm blows, or if there's an earthquake, the water in the hollow may be shaken out, and descend the hillside to the valley below. By shaking things up, you enable it to find a lower-energy and more stable state, overcoming the small obstacle that previously held it in place.

With the iron filings, the effect is similar. You need to give the filings a bit of a shake to enable them to find their way to the lower-energy state, overcoming the friction that otherwise kept them where they were. Of course, if you shake it too much, they will find an even lower-energy state where they all move close to the poles of the magnet.

There are two main types of friction, static and dynamic friction. Static friction determines the amount of force required for a non-moving object to accelerate, and dynamic friction determines the deceleration of an object that is already in motion. Shaking or vibrating a surface simply has the effect of adding additional forces that aren't biased in any given direction (the filings move left as much as they do to the right). Since the shaking forces cancel out and overcome static friction, the effect of the magnetic fields is visible.

The static "friction coefficient" is generally given by a unitless value μs between 0 and 2, and represents the proportion of the objects weight that must be applied before it begins moving. This value is determined by both materials. For example, when dry the friction coefficient of rubber on concrete it 1.0, meaning that a force equal to the weight of the car is required to move it. When wet, the coefficient becomes 0.3 and the car is much more easily swayed.