Units of measurement and multiple/sub-multiple prefixes. 3a.1 Identify the units of, and abbreviations for Potential Difference (Voltage), Current, Power and Resistance. Note: Prefixes milli, kilo and Mega may be used.
In order to check equipment and
equipment performance radio amateurs need to be able to make
measurements and carry out calculations. The units that are tested in the
Foundation Exam are as follows: Potential Difference This is a measure of the difference
in the number of electrons at any two points in a circuit. Potential
difference (PD) is measured in Volts and is given the symbol V or
sometimes E. For example if you place a voltmeter across a battery you
will be able read off the potential difference between the positive and
negative sides of the battery. Current This is a measure of
the number of electrons flowing through a particular part of a circuit.
It is measured with an Ammeter. The unit of current is the Ampere
(shortened to Amp). It is given the symbol I. For example if you wanted
to know how much current a circuit is using then an ammeter would be
placed in the positive lead between the battery and the piece of
equipment. Power
Electrical Power is a measure of the rate at which electrical
energy is turned to other forms of energy. It is measured in Watts and
given the symbol P. The most common use in radio is measuring the power
output from a transmitter. Resistance
This is a measure of how much current (electrons) flow is restricted by
a material. It is measured in Ohms and given the symbol R.
Components called resistors are made specifically to
provide resistance in a circuit.
Prefixes
Sometimes a measurement is so small or so large that we use a prefix in
front of the unit. For example a circuit may take 0.000001 of an Amp.
This is a millionth of an Amp and the prefix for a millionth is micro.
So we would say that the circuit drew 1 microAmp of current.
The other prefixes that you need to learn for the exam are shown in the table opposite.
Prefix
Meaning
kilo
a thosand
Mega
a million
milli
a thousandths of
Examples
Unit
Symbol
A million Volts
A megavolt
MV
A thousand Volts
A kilovolt
kV
A thousandths of a volt
A millivolt
mV
A million Amps
A megaamp
MA
A thousand Amps
A kiloamp
kA
A thousandths of an Amp
A milliamp
mA
A million Ohms
A mega ohm
MΩ
A thousand Ohms
A kilo ohm
kΩ
A thousandths of an Ohm
A milli ohm
mΩ
A million Watts
A megawatt
MW
A thousand Watts
A kilowatt
kW
A thousandths of a Watt
A milliwatt
mW
Simple circuit theory 3b.1 Recall the relationship between Potential difference (Voltage), Current and Power. (P=V×I, I=P/V, V=P/I) Calculate the unknown quantity given the numerical value of the other two.
The following formulae shows how either power, current or voltage can be calculated if you know the other two:
P=power I=current V=volts
Example 1
Example 2
Example 3
3b.2 Recall that resistance is the opposition to current flow. As outlined above resistance opposes
the flow of current. The higher the resistor value the lower the current flow
is and the lower the resistance the greater current flow is.
3b.3 Recall the relationship between Potential Difference (Voltage), Current and Resistance. (V=I×R, I=V/R, R=V/I) Calculate the unknown quantity given the numerical value of the other two.
In
a circuit with a battery supplying a voltage connected to a resistor
(as shown in the diagram opposite), the following formula applies: NB When two symbols are next to each other it means multiply so IR means I x R By rearranging the values the formula can also be written as I=V/R or R=V/I. If we know two of the values we can calculate the third.
Try the three examples given and then check your answers
Example 1
Example 2
Example 3
3b.4 Recall that a battery
provides Potential Difference (Voltage) at its terminals and that a
circuit is needed to allow current to flow. When a battery is charged up there
will be a potential difference or voltage at each terminal. One will be
labelled + (positive) and the other - (negative). For current to flow
the two terminals of the battery have to be connected to a circuit.The
drawing above shows a resistor connected to the battery. If the
resistor is disconnected no current will flow through the resistor.
3b.5 Recall that the polarity
of a battery is not relevant if a filament bulb is used but that
electronic circuits can be damaged by the wrong polarity. A battery has a positive and a
negative terminal. In some cases it doesn't matter which way round a
device is connected. For example a filament bulb. However in electronic
circuits where there are semiconductors it is important to connect the
positive lead from the circuit to the battery positive and the negative
lead to the negative terminal of the battery. If the circuit is
connected the wrong way round it could damage the semiconductors.
3b.6 Recall what is meant by the abbreviations DC and AC.
DC stands for direct current. In this case the current flows in one direction round a circuit. AC stands for alternating current. In this case the current moves forwards through a circuit and then back.
The number of times it changes direction is called the frequency. For
example if a filament lamp is connected to a 240V mains supply the
current moves through the filament in one direction and then moves back
in the opposite direction. It does this 50 times a second because the
frequency of mains electricity in the UK is 50Hz (50 cycles per second)
3b.7 Identify the circuit symbols shown in Table 1 (at back). Symbols
are used in circuit diagrams and straight lines are used to show
how components are connected together. For the Foundation Exam
you need to
know the components shown opposite.
