The Physics of the N95 Face Mask (2024)

It’s 2022, and by now we’ve all been wearing masks for nearly two years. And unless you are a surgeon or a construction worker who was already wearing them daily, in those two years you’ve probably learned a lot about them—which ones you like best, where to get them, and whether you have any extras stashed in a coat pocket or somewhere in your car.

But do you know what makes the prized N95 mask so special? Let’s find out.

Electric Charges

The fibers in regular cloth or paper face masks filter out particles by physically blocking them—but the fibers in an N95 mask also use a great physics trick. These fibers are electrically charged.

Electric charge is one of the fundamental properties of all particles. Just about everything around you is made of three particles: the proton, the electron and the neutron. (For now, let's ignore muons and neutrinos—both fundamental particles that actually exist—as well as other particles that are theoretically possible.)

Just as every particle has a mass, it also has a charge. The proton has a positive electric charge with a value of 1.6 x 10^-19 coulombs, the unit for measuring electric charge. The electron has the exact opposite charge. That leaves the neutron with zero charge (thus the "neut" part of “neutron”).

The electric charge is a key part of the electrostatic interaction, the force between electric charges. The magnitude of this force depends on the magnitudes of the two charges and the distance between them. We can calculate this force with Coulomb's law. It looks like this:

In this expression, k is a constant with a value of 9 x 10⁹ N×m²/C². The charges are q₁ and q₂ and the distance between them is r. This will give a force in newtons. If the two charges are both the same sign (either both positive or both negative) then this will be a repulsive force. If the two charges are different signs, then the force is attractive.

If everything is made of electrons and protons, shouldn't there be electric forces between everything? Well, sort of. Electrons and protons are super tiny. That means that even a small drop of water will have something like 10²² protons in it. That drop will probably have the same number of electrons. (And no one cares about the neutrons—at least for now.) That makes the overall charge of this drop of water equal to zero coulombs. Even if you have extra electrons in your water, the total charge is going to be small, since the electron charge is puny. Essentially, most of the stuff you can see is electrically neutral with no electric forces.

How Do You Charge Something?

Remember that one time you took a sock out of the clothes dryer and it stuck to your shirt? If that's a static electricity interaction, how did the sock get charged?

To make a sock negatively charged, there's only one way to do it—make sure the sock has more electrons than protons. You are going to need a lot of electrons, maybe something on the order of 10¹³ extra electrons. (To give you an idea of how large this number is, it would be the total number of bills you'd need to give everyone on earth $1,000 in singles.) All those extra electrons would give the sock an overall negative charge of around 1 microcoulomb (1 x 10^-6 C).

If you want to make that same sock positively charged, instead of adding electrons you would remove them. This would leave the sock with more protons than electrons for an overall positive charge. But you can’t just remove protons from most objects willy-nilly. Well, you can, but it might be super bad. Think back to the periodic table of elements. Let's say you start with an object that’s made of carbon, which has six protons in the nucleus. If you removed one of these protons, it would no longer be carbon. It would be boron, which has five protons—and you would have just created a nuclear reaction.