10.06.24  010
1
Theory
Electrical engineering
Basic concepts in electrical engineering
We cannot imagine our present society without electricity. We only have to look around to list a number of applications such as: radio, television, vacuumcleaner, coffeemaker etc. The number of applications in which electricity isused is still growing; for a case in point, we only have to think of computers.The development of 'electricity' has taken place mainly over the past 100 years.A big advantage of electricity is that it can be generated in a central location andconveyed with relatively small losses. To keep these losses low, electricalenergy must be conveyed at a high voltage. For large distances, voltages of up to380 kV (380,000 Volts) are used nowadays.On the consumers' side, these high voltages are not suitable for things such as to power machines. These voltages will have to be reduced. This can be doneeasily by means of transformers, but for this purpose only alternating current can be used. Direct voltages cannot be converted to a lower or higher voltage bymeans of a transformer.Apart from the conveyance of energy, electricity can also be used for the transfer of information: radio, television and the telephone are examples of this. Here,the objective is to minimise any distortion of the data in transfer.In contrast to the conveyance of electricity as a source of energy, in which onlyone frequency is used (50 or 60 Hertz depending on the country), with electricityas a data carrier, a wide range of frequencies is used. These versatileapplications of electricity are based on a number of laws. A number of theselaws and concepts from electrical engineering are examined in the followinglessons.
Contents of the lesson
1
Electrical quantities: current, resistance and voltage2
Ohm's law3
Specific resistance, line resistance4
Voltage loss across line resistance
5
Electrical power, unit of power
The copyright in this material is vested in Shell Global Solutions International B.V., The Hague, The Netherlands and Shell Netherlands Raffinaderij B.V. All rightsreserved. Neither the whole or any part of this document may be reproduced, stored in any retrieval system or transmitted in any form by any means (electronic,mechanical, reprographic, recording or otherwise) without the prior written consent of the copyright owner.
Theory / 10.06.24  010
2
Lesson
1. Electrical quantities: current, resistance andvoltage
1.1 Electric current, unit of current
An electric current in a conductor is a flow of electrons due to a potentialdifference. This potential difference can be supplied by a battery, a generator or the mains.By the current
I
we mean the charge
Q
passing through a cross section per unitof time.The unit of current is the ampere. This is the current in the case of a charge of 1 coulomb per second passing a cross section, so 1 A = 1 C/s (1 coulomb is theelectric charge of 6.3 * 10
18
electrons).Other indications of current are:1
µ
A (microampere) = 0.000001 A or 1 * 10
6
A1 mA (milliampere) = 0.001 A or 1 * 10
3
A1 kA (kiloampere) = 1000 A or 1 * 10
3
ACurrent can be measured with an ammeter.The symbol of an ammeter in a circuit is a circle containing a capital A (seefigure 1).
5575010001
Figure 1Symbol of ammeter
1.2 Resistance, unit of resistance
A good conductor conducts an electric current easily; in other words, a goodconductor offers little resistance.The resistance of a certain conductor depends not only on the material of theconductor but also on the dimensions of the conductor.The unit of resistance is the ohm (
Ω
). This is the resistance of a column of mercury with a height of 106.3 cm and a cross section of 1 mm
2
at 0
°
C.A column of mercury twice as long has a resistance of 2
Ω
. A column of mercury with a height of 106.3 cm and a cross section of 3 mm
2
has a resistanceof 1/3
Ω
.
current

current

ampere

resistance

ohm
Theory / 10.06.24  010
3
In engineering, resistors are produced varying from very small values to verylarge ones, for instance 0.1
Ω
to 10 M
Ω
(megaohm). (M = mega = 1,000,000= 10
6
).The symbol of a resistor
R
(
R
= resistance) in a circuit is a rectangle containing acapital R, if necessary with an index, for instance
R
1
, R
2
,
etc. (see figure 2).
5578010002
Figure 2Symbol of resistance
1.3 Voltage, unit of voltage
An electric current is caused by a voltage. The higher the voltage, the bigger theelectric current through a conductor at a certain resistance.This can be compared with the flow of a river. A river flows as a result of adifference in altitude. This could be referred to as the 'potential difference' of theriver flow. The bigger the difference in altitude, the larger the flow of the river through a certain river bed. A wide, deep river bed has a low flow resistance. Anarrow, shallow river bed has a high flow resistance.The unit of voltage is the volt (V). This is the voltage that must be applied acrossa resistance of 1
Ω
to obtain a current of 1 A in that wire.Voltages can be measured with a voltmeter. The symbol of a voltmeter in acircuit is a circle containing a capital V (see figure 3).
5578010003
Figure 3Symbol of voltmeter
The symbol of a (direct) voltage supply in a circuit is a long line (+ pole) and ashort line ( pole) at right angles to the conductor (see figure 4).
5578010004
Figure 4Symbol of direct voltage supply

voltage

volt
Theory / 10.06.24  010
4
2. Ohm's law
From the above, it is clear that the current
I
depends on:
the voltage
V;

the resistance
R.
The current flows from a point with a high voltage (+) to a point with a lower voltage (). This can again be compared with the flow of a river, whichinvariably flows from a point at a higher altitude to a point at a lower altitude(see figure 5).
5578010005
Figure 5 Electrical circuit with one voltage supply and one resistor
Measurements show that the current
I
is directly proportional to the voltage
V
and inversely proportional to the resistance
R,
i.e.:
if
V
becomes twice as big,
I
also becomes twice as big;
if
R
becomes twice as big,
I
becomes twice as small.The equation is:
RV I
=
Then:
V = I * R
or
I V R
=
In which:
I =
current in ampere (A)
V =
voltage in volt (V)
R =
resistance in ohm (
Ω
)This is Ohm's law. Once two of the three quantities in the equation are know, thethird quantity can be calculated.
Ohm's law