Charges, Electrons, Conductors and Insulators വൈദ്യുതചാർജ് · ചാലകം · ഇൻസുലേറ്റർ
An electric current is a flow of charge. Before we can talk about current, we need to know what charge is, where it comes from, and why it flows easily through a copper wire but not through the PVC covering around it. The chapter's very first lines (page 131) ask three questions from your earlier classes: What kinds of charge do objects get when rubbed? What is the charge of an electron? What is the unit of charge? This page answers all three from the start.
- Simple multiplication and division.
- Nothing else – everything is built here. Powers of ten such as 10−19 are explained again on page 3.
1. Inside an atom
Everything around you – a copper wire, a plastic comb, your hair – is made of tiny atoms (ആറ്റം). Each atom has a heavy centre, the nucleus (ന്യൂക്ലിയസ്), with even tinier electrons (ഇലക്ട്രോൺ) moving around it.
- The nucleus contains protons (പ്രോട്ടോൺ), which carry positive (പോസിറ്റീവ്) charge, and neutrons (ന്യൂട്രോൺ), which carry no charge.
- Each electron carries a negative (നെഗറ്റീവ്) charge, exactly as big as a proton's positive charge.
- A normal atom has as many electrons as protons, so the charges cancel and the atom is neutral (it has no overall charge).
The number of protons decides which element it is (6 protons = carbon). Change the number of electrons: with fewer electrons than protons the atom is positive; with more, it is negative. Protons stay locked in the nucleus – only electrons move from one object to another. (Neutrons are shown in grey for the most common form of each element.)
Positive charge = fewer electrons than protons. Negative charge = more electrons than protons. Objects become charged only by gaining or losing electrons.
2. Charging by rubbing
When two different materials are rubbed together (ഉരസൽ), some electrons are pulled off one and stick to the other. The object that gains electrons becomes negative; the object that loses them becomes positive, by the same amount. No charge is made or destroyed – it is only transferred. This is the answer to the chapter's first question: rubbing gives objects positive and negative charges, because electrons are transferred.
In this cartoon each rub moves one electron. Really, a few rubs move many billions of electrons – but even that is a tiny charge. Watch the totals: whatever one object gains, the other loses, so the two together always add up to zero.
On a dry day, run a plastic comb through dry, oil-free hair a few times and hold it over small bits of paper: they jump up to the comb. In the monsoon the trick works badly, because the damp air slowly lets the charge leak away. (Coconut oil in the hair also spoils it!)
3. Like charges repel, unlike charges attract
Charges push or pull each other even without touching. Two like charges (both + or both −) repel (push apart). Two unlike charges (+ and −) attract (pull together). The closer they are, the stronger the push or pull. A charged object can also attract a neutral one, such as the paper bits above, weakly.
Try (+, +), (−, −), (+, −) and (+, neutral). Then move the threads closer: the swing gets bigger, because the force grows quickly as the distance shrinks.
4. Measuring charge: the coulomb
Charge is measured in coulombs (കൂളോം), symbol C. The charge of one electron is tiny: 1.6 × 10−19 C (negative). So one coulomb is the charge of a huge number of electrons:
That is 6 250 000 000 000 000 000 electrons! The charge you make by rubbing a comb is usually less than a millionth of a coulomb. Yet in a torch bulb, about a quarter of a coulomb flows through the filament every second – which is why we need a steady flow of charge, a current, and not just a few rubbed charges.
Worked example 1 · Electrons in one coulomb
Q. The charge of an electron is 1.6 × 10−19 C. How many electrons together carry a charge of 1 C?
Total charge = number of electrons × charge of one electron, so number = total charge ÷ charge of one electron. Divide 1 by 1.6 first, then deal with the power of ten.
Worked example 2 · A charged comb
Q. A plastic comb rubbed on dry hair gains 5 × 1011 extra electrons. What is the charge on the comb?
Q = n × e. Multiply the numbers (5 × 1.6) and add the powers (11 + (−19)). Extra electrons → the sign is negative.
Worked example 3 · A charged atom
Q. An oxygen atom has 8 protons. It picks up electrons until it has 10 electrons. What is its total charge?
Count the extra electrons (10 − 8). Each carries 1.6 × 10−19 C of negative charge.
5. Conductors and insulators
In a metal such as copper, each atom lets go of one or two of its outer electrons. These free electrons (സ്വതന്ത്ര ഇലക്ട്രോണുകൾ) wander through the whole wire. When a cell is connected, they all drift together – an electric current. A material with many free electrons is a conductor (ചാലകം). In plastic, rubber, glass or dry wood the electrons stay tied to their atoms. These materials have very few free electrons, so almost no current flows: they are insulators (ഇൻസുലേറ്റർ).
The right panel is a cartoon of the inside of the material. In a conductor, the blue free electrons drift from the − side to the + side as soon as the circuit is complete. In an insulator they stay bound to their atoms. Graphite conducts, but less well than a metal, so the bulb glows dimly.
Look at a piece of house wire left over after the electrician's work: a copper core (an excellent conductor) inside a PVC covering (a good insulator). The copper carries the current; the PVC stops it from reaching your hand or the wall. The electrician's tester screwdriver has a plastic handle, and the KSEB lineman wears rubber gloves, for the same reason. The overhead lines on poles are bare aluminium, held on porcelain or glass insulators – which is why you must never touch them or anything touching them.
Pure water is a poor conductor, but tap water, rain water and sweat contain dissolved salts and do conduct. Your body conducts too. At a torch cell's 1.5 V you feel nothing, but at the mains 230 V a current through your body can kill. Page 4 lists the safety rules.