7.1 Types of charges

Electric charge is a basic property of the particles that make up matter. There are two types of charge, positive and negative: protons in the nucleus carry positive charge and electrons around it carry an equal negative charge, while neutrons carry none. A neutral body has equal numbers of protons and electrons; it becomes charged when it gains electrons (negative) or loses electrons (positive). Electrostatics is the study of charges at rest, and it explains static shocks, the attraction of dust to screens, the behaviour of powders on balances and the working of capacitors, electroscopes and some laboratory instruments. Example: the weighing room of a pharmaceutical quality control laboratory in Mukuru uses an anti-static ioniser beside its analytical balance, because charged plastic weighing boats give unstable readings.

Properties of charge

• Two kinds: Positive and negative.
• Law of charges: Like charges repel; unlike charges attract.
• Unit: The coulomb (C). The charge on one electron is −1.6 × 10⁻¹⁹ C, and on one proton +1.6 × 10⁻¹⁹ C. Since 1 ÷ (1.6 × 10⁻¹⁹) = 6.25 × 10¹⁸, a charge of 1 C is carried by 6.25 × 10¹⁸ electrons.
• Quantised: Any charge is a whole number multiple of the electronic charge e.
• Conserved: Charge is neither created nor destroyed; charging only moves electrons from one body to another.
• Attracts neutral objects: A charged body attracts small uncharged objects such as paper bits, dust and powder by inducing opposite charge on their near side.

Conductors and insulators

• Conductors: Materials such as metals, graphite, the human body, damp earth and ionic solutions, in which charges move freely. A conductor can be charged only if it is insulated from earth.
• Insulators: Materials such as plastics, glass, rubber, dry wood and silk, in which charges cannot move, so charge stays where it is placed.
• Earthing: Connecting a charged conductor to the earth lets charge flow away until it is neutral.

Methods of charging

1. Friction (rubbing): When two different insulators are rubbed together, electrons pass from one to the other. A polythene rod rubbed with wool gains electrons and becomes negative; a cellulose acetate or glass rod rubbed with silk loses electrons and becomes positive. The two materials always gain equal and opposite charges.
2. Contact (conduction): Touching a neutral conductor with a charged body shares the charge, so the conductor gets a charge of the same sign as the charging body.
3. Induction: A charged rod is brought near (not touching) an insulated conductor, drawing opposite charge to the near side and pushing like charge to the far side. The far side is earthed briefly with a finger, the earth connection is removed, and then the rod is taken away. The conductor is left with a charge opposite in sign to that of the rod.

The triboelectric series

Materials can be listed in order of how easily they lose electrons when rubbed. A material higher in the list becomes positive when rubbed with one lower down: glass, human hair, nylon, wool, silk, paper, cotton, rubber, polyester, polythene, PVC, PTFE. Wool rubbed on polythene therefore leaves the wool positive and the polythene negative.

Force between charges (Coulomb's law)

The force between two point charges is directly proportional to the product of the charges and inversely proportional to the square of their separation:

F = kq₁q₂ ÷ r², where k = 1 ÷ (4πε₀) = 8.99 × 10⁹ N m²/C²

Worked example: find the force between charges of 2.0 μC and 3.0 μC placed 0.10 m apart in air.

F = kq₁q₂ ÷ r²

F = (8.99 × 10⁹ × 2.0 × 10⁻⁶ × 3.0 × 10⁻⁶) ÷ 0.10²

F = 0.05394 ÷ 0.01

F = 5.39 N (repulsive, since both charges are positive)

Distribution of charge on a conductor

• On the outer surface: Charge on a conductor lies on its outside surface, none inside.
• Concentrated at points: Charge density is greatest at sharp points and edges. At a sharp point the field can be strong enough to ionise the air and let charge leak away (point action), which is how lightning conductors work.

Static electricity hazards and control in the laboratory

• Sparks and fires: Static sparks can ignite flammable solvent vapours, so solvent drums are earthed and bonded during transfer.
• Weighing errors: Charged containers and powders are attracted or repelled by the balance, giving drifting readings; ionisers, anti-static weighing boats and humidity control reduce this.
• Damage to electronics: Static discharge destroys sensitive chips, so anti-static wrist straps and mats are used when servicing instruments.
• Dust: Charged surfaces attract dust onto optics and sample containers.