These properties of charge are illustrated in the above image, which shows the experimental creation and conservation of electrical charge. Here an invisible high-energy electrically neutral light particle (photon) traveling down from the top of the image scatters off an atom (located near the middle of the figure, where three curves merge at a cusp), leading to the formation of four particles: a positively charged atom (not visible), a negatively charged electron knocked out of the atom corresponding to the nearly straight track continuing downards, and the creation of two new particles, a negatively charged electron and a positively charged antiparticle called a positron. The created positron's charge has exactly the same magnitude e as the created electron's charge but is opposite in sign so that the total charge has not changed (e + (-e) = 0 ). Each of the three moving charged particles (the green arrow heads indicate the direction of motion) leave a wake of tiny bubbles as they plow through super-heated liquid hydrogen in a so-called bubble chamber, rendering their paths visible as continuous lines. An external magnetic field bends the paths of the electron and positron into oppositely oriented spirals whose geometry can be used to deduce that the particles have opposite charges of equal magnitude. The electron descending from the top toward the lower left was moving too fast for the magnetic field to bends it path substantially. The original photograph was heavily edited to show just the given trajectories.
Despite many advances in theoretical and experimental physics, including the development and confirmation of the Standard Model that explains a great diversity of experimental data concerning fundamental particles, many basic questions about electrical charge remain unanswered. We do not know why charge exists in the first place and why it comes in only positive and negative forms. We do not know why the fundamental unit of charge is the magnitude e of an electron, nor why all charges are quantized in integer multiples of e, nor why the charge of a proton (a non-fundamental particle consisting of three quarks) has, within experimental error, the same magnitude of charge as an electron (which appears to be a fundamental point particle according to the best attempts to discover internal structure of the electron). However, we do understand from quantum field theory why charged antiparticles must have exactly the opposite charge of the corresponding charged particle, and why all electrons and positrons must be exactly identical.
Two challenges: in the above figure, can you tell from the above
information which spiral track corresponds to the electron and which
spiral track to the positron? Can you figure out how scientists
deduced that the positron and electron were truly created out of
nothing from the energy of the photon, as opposed to already existing
inside the atom and just being being knocked loose?