Paper or Plastic?

Have you every stepped back while swiping your card to think, "How does this plastic card know all my information?". It probably isn't your first thought that comes to mind, now you're probably thinking about it.

A credit card works because it contains magnets placed in a specific pattern. Another term for this would be a "magstripe". The credit card machine has a coil of wires that case a change in magnetic field when the card is swiped. This process is called electromagnetic induction. The change in magnetic field induces a voltage that creates a current that is used to signal your account information to the machine.

Here are a hew laws that may give you a better understand of the physics behind the swipe.

The Biot-Savart Law
describes how a steady electric current creates a magnetic field.

B is the magnetic field, Mu naut is a constant for the permeability of free space = 4pi * 10^-7, V is the velocity of the moving charge, r-hat is the distance vector from the charge to a point of interest beyond the charge, Q is the magnitude of the charge (measured in coulombs)

In this equation one takes a cross product of velocity and r-hat vectors to get a value for the magnetic field. The process of swiping a card through a reader uses a few simple principles of physics. 

Faraday’s Law of Electromagnetic Induction explains this mechanism.

Faraday's Law

This equation tells us that a changing magnetic field in a given amount of time produces voltage, which in turn can create a current in a pickup coil in the card reader.

Epsilon (E) represents the EMF, otherwise known as the electromotive force – it is equivalent to a potential difference, or, voltage. D(Phi)B represents a changing magnetic flux. A magnetic flux is the measure of the amount of magnetic field passing through a given surface, D(t) is a change in time.

Furthermore, because a potential difference (because of a separation of electric charges on the pickup coil) is created, then, by using Ohm’s Law, you can see that a current is induced in the pickup coil.

Ohm's Law

The current received by the pickup coil goes through signal amplification, and is translated into binary code (the alternating magnetic fields do this) so that the signal could be read by a computer.

V is voltage, which is essentially what a potential difference is, which is what an EMF is. R is the resistance of the material (resistance to movement of electric charges within the material). So in this case we’re concerned with the resistance of the pickup coil. I is the current, which in this situation is induced by the potential difference created in the coil.