Conductors, Insulators, and Charging Methods

Conductors are materials through which charge flows easily because they contain free charge carriers (such as the free electrons in metals), insulators are materials in which charges cannot move freely, and an object can be charged by friction, by conduction (contact) or by induction. In brief: metals, graphite and salt solutions conduct; rubber, glass, plastic, dry air and wood insulate. In charging by contact the object acquires the same sign of charge as the charged object that touches it. In charging by induction, the object acquires the opposite charge, without touching, and it must be earthed during the process.

Copper wires carry current while plastic covers protect us, and a charged rod can charge a metal sphere without touching it. This guide explains the difference between conductors and insulators, describes the three charging methods and the electroscope, compares the methods in a table, works through a charge sharing calculation and corrects common mistakes.

What are conductors and insulators?

In a conductor, some electrons are not bound to individual atoms and can move through the material. Metals (copper, aluminum, silver, gold, iron) are the best conductors. Others include graphite, and solutions with ions such as salt water. The human body is also a fair conductor, because of the water and ions in it. In an insulator, the electrons are bound tightly to atoms and cannot move easily: plastics, glass, rubber, dry wood, air (when dry), and pure water. Charge placed on a conductor spreads over its surface; on an insulator it stays where it was put. Semiconductors (silicon, germanium) have a conductivity between the two, which can be controlled by adding impurities and by temperature, and are the basis of electronics. The Earth is a huge conductor which can accept or supply charge, and connecting an object to the ground is called earthing (grounding).

How can an object be charged?

• Friction: rubbing two different insulators together transfers electrons from one to the other, and the two objects acquire opposite charges.
• Conduction (contact): a charged object touches a conductor, and charge flows between them until the potentials are equal. The conductor gets the same sign of charge as the charged object. If the conductors are identical spheres, the charge is shared equally.
• Induction: a charged object is brought near, but does not touch, a neutral conductor. The free charges in the conductor are redistributed: opposite charge gathers on the near side, like charge on the far side. If the conductor is connected to earth while the charged object is nearby, charge of the like sign flows to the ground. Disconnecting the earth connection first, then removing the charged object, leaves the conductor with a net charge of the opposite sign to the charged object. The charged object loses no charge.

An insulator near a charged object is polarized: the molecules are slightly distorted, producing a weak attraction without a net charge.

How does an electroscope work?

A gold-leaf electroscope has a metal rod with a thin gold leaf attached to a metal plate. When a charge is placed on the cap, the rod and the leaf acquire the same charge, so the leaf is repelled and rises. The greater the charge, the larger the angle. A charged rod brought near the cap (without touching) causes the leaf to rise because of induction. If the rod has the same sign as the electroscope, the leaf rises further, and if opposite, it falls. This allows the sign of an unknown charge to be determined. An electroscope can also be discharged by ionized air, for example near a radioactive source.

Charging methods compared

Worked example: charge sharing

1. Contact between identical spheres. A metal sphere with a charge of +6.0 µC touches an identical neutral sphere. The total charge +6.0 µC is shared equally: each sphere has +3.0 µC after separation.
2. Spheres with opposite charges. Spheres with +6.0 µC and −2.0 µC touch. The total is +4.0 µC, so each sphere has +2.0 µC after separation. Charge is conserved: 6.0 − 2.0 = 2.0 + 2.0.
3. Three spheres in turn. Sphere A (+8.0 µC) touches B (neutral): each has +4.0 µC. Then B touches C (neutral): B and C each have +2.0 µC. Final charges: A +4.0 µC, B +2.0 µC, C +2.0 µC, total +8.0 µC.
4. Induction. A negatively charged rod is brought near a neutral metal sphere on an insulating stand. The side near the rod becomes positive and the far side negative. If the sphere is touched by a finger, electrons flow from the sphere to the ground. Removing the finger and then the rod leaves the sphere with a net positive charge. The rod keeps its original charge.

How were these ideas developed?

Stephen Gray discovered in 1729 that electricity can be conducted along a thread, and distinguished conductors from insulators. Benjamin Franklin explained charging with a single “fluid” in the 1740s. The gold-leaf electroscope was invented by Abraham Bennet in 1787. Michael Faraday showed in 1836 that charge on a hollow conductor resides on the outer surface (the “ice pail” experiment, and the Faraday cage). The band theory of solids in the 20th century explained why materials are conductors, semiconductors or insulators.

Which mistakes are common?

Mistake 1: Saying that conduction and induction give the same sign of charge

Conduction gives the same sign; induction gives the opposite sign.

Mistake 2: Thinking that the charging object loses charge in induction

It does not touch the conductor, and its charge remains unchanged.

Mistake 3: Forgetting that insulators can be charged

They hold charge in place. Conductors held in the hand would lose it to the ground.

Mistake 4: Thinking that positive charges move in a metal during induction

Only electrons move. A positive region is a region with a deficit of electrons.