6.5 Electron Theory, Conductors, Insulators & the Effects of Current

6.5 Electron Theory, Conductors, Insulators & the Effects of Current

Unit 202 learning outcome 4 is the largest part of the science test (15 of 40 questions). Alongside Ohm's law and circuit calculations it asks you to describe the basic principles of electron theory, to distinguish good conductors from insulators, and to describe the chemical and thermal effects of electric current.

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The Structure of the Atom

All matter is made of atoms. Each atom has:

• a central nucleus containing protons (positive charge) and neutrons (no charge);
• electrons (negative charge) arranged in shells around the nucleus.

In a normal atom the number of electrons equals the number of protons, so the atom is electrically neutral. An atom that loses an electron becomes a positive ion; one that gains an electron becomes a negative ion. The charge on one electron is about $1.6 \times 10^{-19}$ coulomb.

The electrons in the outer shell are the valence electrons. In metals they are held so loosely that they leave their atoms and wander through the material as free electrons. A copper atom, for example, has 29 electrons with just one in its outer shell, which is why copper is such a good conductor.

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What Is an Electric Current?

When an electromotive force (e.m.f.) is applied across a conductor, the free electrons drift towards the positive terminal. This drift of charge is an electric current.

• Electron flow is from negative to positive.
• Conventional current, used on circuit diagrams and in Fleming's rules, is taken as flowing from positive to negative. The convention was fixed before the electron was discovered.

Current is the rate of flow of charge:

$I = \frac{Q}{t} \qquad \text{or} \qquad Q = I \times t$

where $I$ is in amperes, $Q$ in coulombs and $t$ in seconds. One ampere is one coulomb per second, which is about $6.24 \times 10^{18}$ electrons passing a point every second. The electrons themselves drift slowly, but the effect travels along the circuit almost instantly.

Example: a current of 5 A flowing for 2 minutes transfers $Q = 5 \times 120 = 600$ C of charge.

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Conductors, Insulators and Semiconductors

Carbon (graphite) is a non-metal that conducts, which is why it is used for motor brushes. Pure water is a poor conductor, but water containing dissolved salts and dirt conducts well, which is why damp insulation and wet hands are so dangerous.

How insulation breaks down

Insulation can fail because of excessive voltage (puncture), heat from overload or poor connections, moisture and contamination, mechanical damage, ageing and sunlight, chemical attack, and rodents. Insulation resistance testing (Section 14.3) is designed to find this deterioration before the installation is energised.

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The Effects of Electric Current

An electric current produces three main effects: heating (thermal), magnetic and chemical. (Its effect on the human body is covered in Section 3.2.)

The thermal (heating) effect

As electrons collide with the vibrating ions of a conductor, electrical energy is converted to heat:

$P = I^2 R \qquad \text{and} \qquad W = I^2 R t$

• Useful applications: heaters, kettles, immersion heaters, showers, cookers, incandescent and halogen lamps, fuse elements that melt on overcurrent, the bimetal strips in circuit breaker thermal trips, and soldering irons.
• Unwanted heating: the heat produced in cables is why every cable has a current rating, and loose, high-resistance connections can overheat and start fires. A cable run with a total resistance of 0.15 Ω carrying 20 A converts $20^2 \times 0.15 = 60$ W into heat along its length.

The magnetic effect

A current produces a magnetic field around the conductor. This is the basis of motors, relays, contactors, the magnetic trip in circuit breakers and transformers (Chapter 7).

The chemical effect

When direct current passes through an electrolyte (a liquid that conducts by means of ions, such as a salt or acid solution), it causes chemical changes at the two electrodes: the anode (positive) and the cathode (negative). This is electrolysis. Uses include:

• Electroplating: coating terminals, fixings and accessories with nickel, chromium, zinc or silver for appearance and corrosion protection;
• Refining copper: electrolysis produces the very pure copper used for cables;
• Anodising aluminium to give a hard protective surface;
• Charging secondary cells: the charging current reverses the chemical reaction that produced the energy.

Cells and batteries

A cell converts chemical energy into electrical energy, and a battery is a group of cells.

Hazards include hydrogen gas released when lead-acid batteries are charged (so battery rooms must be ventilated and kept free of sparks), acid burns, and the very high short-circuit current a battery can deliver because of its low internal resistance (Section 6.4).

Electrolytic (galvanic) corrosion

When two dissimilar metals touch in the presence of moisture, they form a small cell and the less noble metal corrodes. Examples on site include:

• aluminium conductors connected directly to copper or brass terminals, which is why bimetallic lugs and connectors are used;
• copper earth clamps on galvanised steel pipe without a suitable clamp, which is why BS 951 clamps are matched to the pipe material;
• steel fixings in contact with copper or aluminium outdoors.

The corrosion increases the resistance of the joint, which can lead to overheating or loss of earth continuity, so terminations must be kept dry and use compatible materials.