LESSON 13

Names of electrical units, DC and AC, what is a radio
signal, conductors and insulators, electrical components - 1 exam
question

If you have been following the electronic discussions at the bottom of the last three blogs you should have no problem with this lesson.

Electricity is the flow of electrons inside a conductor. The current, which number of electrons that flow past a certain point in a given time, is measured in Amperes. The opposition to the flow of electrons in a conductor is resistance and is measured in Ohms. The force or pressure which pushes the electrons through the conductor is the Electromotive Force also known as voltage and is measured in volts.

When the volts and the amperes are multiplied together the product is the power which is being used by that circuit.

Current is measured with an Ammeter while voltage (EMF) is measured with a Voltmeter. Resistance is measured with an Ohmmeter while power is measured with a Wattmeter.

A standard car battery is said to be 12 volts. It is actually a little higher than that but if asked the voltage of a car battery 12 volts will suffice. A car battery, as with any battery, has a positive and a negative terminal which remains the same at all times. Thus the current always flows in the same direction known as direct current.

If the current was changing back and forth at a given rate it would be called Alternating Current (AC) and the rate of change is measured in Hertz (Hz).

Most conductors today are made from copper because, though copper is not as good of a conductor as silver, it is a very good conductor for the price. Aluminum is also a good conductor but not a good as copper. Mercury is a metal and thus is also a good conductor and is used in some switches.

Wood, glass, and rubber are extremely poor conductors and thus are called insulators.

1. T4A01
Electrical current is measured in which of the following units?
A. Volts
B. Watts
C. Ohms
D. Amperes
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2. T4A02
Electrical Power is measured in which of the following units?
A. Volts
B. Watts
C. Ohms
D. Amperes
~~
3. T4A03
What is the name for the flow of electrons in an electric circuit?
A. Voltage
B. Resistance
C. Capacitance
D. Current
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4. T4A04
What is the name of a current that flows only in one direction?
A. An alternating current
B. A direct current
C. A normal current
D. A smooth current
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5. T4A05
What is the standard unit of frequency?
A. The megacycle
B. The Hertz
C. One thousand cycles per second
D. The electromagnetic force
~~
6. T4A06
How much voltage does an automobile battery usually supply?
A. About 12 volts
B. About 30 volts
C. About 120 volts
D. About 240 volts
~~
7. T4A07
What is the basic unit of resistance?
A. The volt
B. The watt
C. The ampere
D. The ohm
~~
8. T4A08
What is the name of a current that reverses direction on a regular basis?
A. An alternating current
B. A direct current
C. A circular current
D. A vertical current
~~
9. T4A09
Which of the following is a good electrical conductor?
A. Glass
B. Wood
C. Copper
D. Rubber
~~
10. T4A10
Which of the following is a good electrical insulator?
A. Copper
B. Glass
C. Aluminum
D. Mercury
~~
11. T4A11
What is the term used to describe opposition to current flow in ordinary conductors such as wires?
A. Inductance
B. Resistance
C. Counter EMF
D. Magnetism
~~
12. T4A12
What instrument is used to measure the flow of current in an electrical circuit?
A. Frequency meter
B. SWR meter
C. Ammeter
D. Voltmeter
~~
13. T4A13
What instrument is used to measure Electromotive Force (EMF) between two points such as the poles of a battery?
A. Magnetometer
B. Voltmeter
C. Ammeter
D. Ohmmeter
~~

Answers: D B D B B A D A C B B C B

Up to now in our electronic study we have considered what electricity is and electrical current flows. We have seen how AC differs form DC. Then such things as what radio waves are, what frequency is, how fast radio waves travel, and how they are modulated have been revealed.

After the discussion it might be good to explain that when sending an AM signal it is not necessary to send the carrier and anymore then one sideband if a means of reinserting that carrier frequency is provided in the receiver. The carrier is provided by a BFO (beet frequency oscillator).

The carrier can be removed by a balanced modulator which by phasing removes the carrier but allows the sidebands to be established. Then the undesired sideband is removed by filtering most of the time though phasing the undesired sideband out is possible.

An oscillator is a stage in a radio, transmitter or receiver, which generates a radio frequency signal. In a simple transmitter the oscillator will usually be followed by one or more amplifiers which raise the level of a signal. If the output of the amplifier is a faithful reproduction of the input it is said to be linear. This means the whole signal is reproduced in the output so it looks like a larger version of the input signal. That output will also very proportionally to the input. If a 1 watt signal is placed into an amplifier with an amplification factor of 100 then the output will be 100 watts. If that 1 watt signal is reduced to ½ watts the output power will be 50 watts and if the power is raised to 1.5 watts the output power will be 150 watts. While the input and the output look the same the output may be inverted from the input, that is to say as the signal lever goes positive in the input it will go negative in the output. This inversion does not change the fact that it is a linear amplifier.

If the output of the amplifier is 25 watts when the input is ½ watts and then 100 watts when the input is 1 watt and 250 watts output when the input is 1.5 watts the amplifier is non linear.

AM and SSB signals must be amplified by linear amplifiers while FM and CW can be amplified by nonlinear amplifiers.