CHAPTER 4 SIMPLE D.C. CIRCUIT - OHM’S LAW

4.1       VOLTS AND AMPS

In Figure 1.4 the electron flow was maintained by a ‘pump’ or generator in an anti-clockwise direction, but, because of a historical mistake, current was deemed to flow in a clockwise direction.

In an equivalent water circuit, the pump delivers a pressure (measured in psi, kgf/cm² or bars), and the water flow is measured in gal/min, or m³/s or other unit.

So for an electric circuit.  Pressure is measured in VOLTS (after Volta, the early Italian experimenter) and current in AMPERES (after an early French pioneer).  Instruments are made which indicate pressures in volts and currents in amperes - all switchboards have voltmeters and ammeters.

On platforms and large installations pressures tend to be very high, involving thousands or tens of thousands of volts.  In those cases the ‘kilovolt’ (equals one thousand volts) is usually taken as the unit of pressure.  Thus on most platforms the main generation pressure is 6.6kV, or 6.6 thousand volts.  For domestic appliances and small services 440 or 250 volts is usual on platforms and 415 or 240 volts ashore.

4.2       CURRENT FLOW - OHM’S LAW

Once the units of pressure and current flow were established, a German experimenter named Georg Simon Ohm discovered a very important relationship between them.

It has already been seen that some materials (mainly metals) allow electrons to move freely (but not all as freely as each other), whereas others do not do so and tend to resist such movement - again some more so than others.

                                                              Figure 4.1 OHM’S LAW (D.C.).

Ohm discovered that, for a given sample of material, the current flowing I (in amperes) was directly proportional to the pressure applied V (in volts).  In other words, for that given sample, the ratio of voltage to current was constant:

V	= const
I

This was true for any one sample, but the constant itself differed from sample to sample.  The ratio is called the ‘resistance’ of that sample, symbol R.  It can be considered as opposition to the flow of electrons - like friction.

Ohm’s Law can then be stated:

	V	=	R
	I
or	V	=	IR

where R is the resistance of the sample and differs from sample to sample.  If V is measured in volts and I in amperes, R is measured in ‘ohms’.

4.3       HEATING

An important result stems from this.  Since the resistance R of a conductor is akin to friction in the mechanical equivalent, it might be expected that loss of energy by heating might result from a current flow.

This indeed is so.  Whenever current is forced by pressure of voltage to flow through a conductor which has resistance (and all conductors do, even metals), heat is generated in that conductor.  The rate of heat generation is proportional to the resistance (in ohms) and to the square of the current (in amperes squared).  That is to say, the heat generated is I²R, and, since it represents a continuing loss of energy, it is expressed in the energy-rate unit ‘watts’ (W).

It is important to remember that current flowing in any conductor, be it cable, generator, motor or transformer, gives rise to heat, which must be conducted away if the temperature is not to rise to a level which can damage the insulation and possibly lead to flashover or breakdown and severe damage, or even danger to life.

To reduce heat generation either the current (I) or the resistance (R) must be reduced (for example, by increasing the cross-section of the conductor).