Units by which a quantity is measured are given. The corresponding international unit symbol is given in brackets.
| Term |
Definition |
Examples |
Voltage |
The pressure which drives the flow of electricity. Voltage is measured in volts (V). A kilovolt (kV)is one thousand volts. A megavolt (MV) is one million volts |
Typical voltages are
-
Of a torch battery – 1·4 volts, or 1·4 V.
-
Of the electricity supply – 240 V.
-
Of a high voltage transmission line – 275 000 V or 275 kV
-
Of a lightning strike – many MV.
|
High Voltage |
A voltage of more than 1000 volts. High voltage is used in power systems which handle large amounts of power, and/or transmit power long distances. |
High voltages used in Australia can range from 3·3 kV up to 500 kV |
Current |
The amount of electricity which flows in a circuit. Current is measured in amperes (A), although amperes are more commonly referred to as amps (A). One thousand amps is a kiloamp (kA). One thousandth of an amp is a milliamp (mA) |
Typical currents are
|
Power |
Power is strength. Power is measured in watts (W). A kilowatt (kW) is one thousand watts. A megawatt (MW) is one million watts. A thousandth of a watt is a milliwatt (mW). The symbols mW and MW are commonly confused in the press. |
Typical powers are
-
A refrigerator – 250 W
-
A 2 bar radiator – 2 kW
|
Energy |
The amount of work done over time. Energy is measured in watt-hours (W.h) The power used over a period of time. Larger units are the kilowatt-hour = 1000 W.h, the megawatt-hour = 1 000 000 W.h, and the gigawatt-hour = 1 000 000 000 W.h. |
The energy used
|
Resistance |
That which resists the flow of current. Voltage drives the flow of current; resistance resists it or limits it.
The resistance of a conductor depends on the dimensions of the conductor and the properties of the conductor material. A longer conductor has a higher resistance than a shorter conductor of the same cross section and of the same material. For two conductors of the same material and the same length, the smaller one will have the higher resistance.
Resistance is measured in ohms (W). One million ohms is a megohm (MW). One thousandth of an ohm is a milliohm (mW). |
The resistance of copper wire is measured in milliohms.
The resistance of a heater or stove element is measured in ohms.
The resistance of an insulator is measured in megohms.
|
Ohm’s Law |
The current flowing in a circuit is equal to the voltage divided by the circuit resistance, or I = V/R.
Alternative forms of Ohm’s law are:
V = I×R, and R = V/I. |
When a 60 ohm resistor is connected to a 240 volt DC supply, 4 amps of current will flow.
I = V/R = 240 V/60 W = 4 A.
With AC the calculation is more complicated as it must also take into account the inductance and the capacitance of the circuit. |
Voltage Drop |
The voltage appearing between the two ends of a resistance when a current flows in the resistance. Voltage drop is commonly used with reference to a conductor. |
From Ohm’s Law’ V = I×R.
When 500 A DC flows in a conductor with a resistance of 0·05 ohms, a voltage drop of 25 volts occurs between the two ends of the conductor.
As above, calculations for AC are more complicated. |
Direct Current (DC) Power Systems |
Power systems in which the instantaneous voltage remains essentially constant over time. |
A torch battery and a car battery. Many early power systems were DC systems. |
Alternating Current (AC) Power Systems |
Power systems in which the supply voltage alternates, that is it passes through zero volts, many times a second. |
Most commercial power systems are AC systems. Australian power systems are AC systems with a frequency of 50 cycles a second, or 50 hertz (Hz). At 50 hertz the voltage passes through zero 100 times per second. |
Line, Active, or Phase Conductor |
The live supply conductor. |
In Australia, the 240 volt conductor. |
Neutral |
The supply system return conductor |
In Australian AC power systems, for safety, this conductor is connected to the earth; it is ‘earthed’. However it should always be considered alive, as it may at times have dangerous voltages on it. |
Single-phase System |
An AC supply system with two wires. The wires are line, the live wire, and neutral which is normally earthed. |
Most Australian homes have a single-phase supply. |
Three-phase System |
A four wire AC supply system. The system has 3 active or phase conductors and a neutral conductor. |
Used for supply to larger consumers.
In Australia, the voltage between any phase conductor and the neutral is about 240 V, and between any two phase conductors is about 415 V.
In Australia, high voltage power systems are usually three-phase systems with three, phase conductors and no neutral conductor. |
Transformer |
A device used to convert AC supply from one voltage to another. It usually consists of two separate coils of wire (the windings), one for each voltage. These are wound on to an iron former (the core). |
Transformers permit the conveyance of large amounts of power at a high voltage with low losses, and for the power then to be ‘transformed’ down to a voltage such as 240 volts, for use by consumers. AC power plus transformers make possible today’s large electric power systems and cheap electric power. |
Transmission System |
A very high voltage power system used for transmitting large amounts of power over long distances. It is effectively a wholesale system. |
The ‘Grid’ |
Distribution System |
A high voltage power system used for distributing smaller ‘retail’ amounts of power to the vicinity of consumers, plus the 240 volt system directly supplying those consumers. |
|
Distribution Transformer |
A transformer used in a distribution system to ‘transform’ the high voltage power of a distribution system down to the 240 volt power supplied to consumers. |
Commonly these are the familiar pole transformers. Another common type of distribution transformer is the pad-mount or cubicle transformer located by the side of the road or in commercial or industrial premises. |
Generator |
The general term for a rotating machine used to generate electric power. |
|
Dynamo |
A generator used to generate DC electric power. |
These are usually of limited size. |
Alternator |
The common term for a generator used to generate AC electric power. |
Alternators can be quite large. Amongst the largest alternators in Australia are the750 MW alternator at Kogan Creek Power Station on the Darling Downs in Queensland. Each machine can generate enough power to supply more than 200 000 homes. |
Turbo-alternator |
A steam turbine and alternator combination |
This is the usual type of generator in a large coal-fired or nuclear power station.
Increasingly, large alternators driven by gas turbines are being installed. |
Solar generator |
Generates DC electricity from sunlight using solar panels |
Requires complicated equipment to control the voltage; and for AC output, to convert the DC to AC of the required voltage and frequency. |
Instead of writing out the names of the units, often their symbols are used instead. Some of the commonly used electrical units and their symbols are given below. Symbols are not abbreviations.
Many metric units are named after the early scientists and engineers who worked in the particular field. The main electrical units and the origin of their names are as follows.
