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.)
 A: 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.
A: 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.
A: 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.
