Physics · Ch 10 — Electrostatics
Electric Charges
Electric Charges
Opposite electric charges were already known to the ancient Greeks around 600 BC, who observed that rubbing amber against fur produces equal and opposite charges on the two materials. Today it is understood that electric charge is a basic, intrinsic property of the elementary particles that make up all matter: the proton (positively charged), the electron (negatively charged), and the neutron (electrically neutral, carrying no charge at all). Every atom's nucleus is built from protons and neutrons, so the nucleus itself is positively charged; negatively charged electrons surround the nucleus in numbers that exactly match the number of protons, which is why ordinary matter is, overall, electrically neutral.
When certain dissimilar substances -- fur and amber, or a comb and dry hair -- are rubbed together, electrons are transferred from one surface to the other. The substance that GAINS electrons becomes negatively charged, while the substance that LOSES an equal number of electrons is left with an equal and opposite positive charge. This way of producing charge is called charging BY CONDUCTION, since charge is physically transferred (conducted) from one body to the other by direct contact/friction. Charge can also be produced through other physical processes besides friction -- for example, chemical reactions inside electrochemical cells, convection currents inside storm clouds, or diffusion across living cell membranes. …
What this figure shows. Four linked panels showing the standard induction-charging procedure on an insulated metal conductor mounted on a stand: (a) the conductor sits alone, uninsulated from a positively charged rod nearby, initially neutral; (b) the positive rod is held close to one end (not touching); the conductor is simultaneously connected to Earth by a wire, and electrons flow IN from Earth to neutralize/cancel the positive charge that induction would otherwise leave on the far end, so the conductor as a whole gains a net negative charge concentrated near the rod while the earthing wire is shown carrying electrons inward; (c) the earthing wire is disconnected while the charged rod is still held in place, so the negative charge that flowed in stays trapped on the conductor, held there by the continuing attraction of the nearby positive rod; (d) the charged rod is finally taken away, and the trapped negative charge -- no longer held in place by any external attraction -- spreads out and redistributes itself uniformly over the whole surface of the now permanently negatively-charged conductor. Together the four panels show …
Worked out. A classic charge-detecting instrument: a metal disc sits atop a narrow metal rod, with a thin gold leaf fixed to the rod's lower end; the whole disc-rod-leaf assembly is electrically insulated from the outer case, and a glass front lets the leaf's motion be observed while blocking air draughts that would disturb it. Placing any charge on the top disc spreads it down the rod to the leaf, and the leaf diverges/moves away from a fixed reference plate because it and the plate now carry like charges that repel -- a larger charge on the disc produces a larger leaf separation, making the instrument a qualitative charge detector. Touching (earthing) the disc lets the leaf fall back to neutral. Two distinct charging methods are described: (a) BY CONTACT -- touching a charged rod directly to the disc transfers some charge across, giving the leaf the SAME sign as the rod, but is not very effective; (b) BY INDUCTION -- bringing a charged rod close to the disc without touching it, earthing the electroscope, then removing the ea …