Module 5 - BASIC ELECTRICITY
- Reading Assignment
- Basic Electricity
- Measurements & Instruments
- Ohms Law
- Type of Voltage
- Quiz 5
- Recommended Reading
Read Chapter 3 pages 3-1 to 3-6 in your text before you continue.
If you have questions after reading the assignment, consult the recommended reading.
When dealing with electricity, what we are referring to is the flow of electrons through a conductor. Electrons are negatively charged atomic particles.
A conductor is a material that allows electrons to move with relative freedom within the material and an insulator is a material that has fewer free electrons and therefore is not conducive to conducting electricity.
In electronics and radio, we control the flow of electrons to make things happen. We need to have a basic understanding of how and why we control the flow of electrons so that you can better operate your radio.
There are three characteristics of electricity:
Voltage, also known as Potential or Electromotive Force EMF, measured in volts.
Current - flow of electrons, measured in amperes or amps.
Resistance - opposes current flow, measured in ohms.
All three must be present for electrons to flow.
For current to flow, there must be a path from one side of the source of the current to the other side of the source - this path is called a circuit. There must be a hose (conductive path) through which the water (current) can flow.
Voltage, current and resistance must be present to have current flow. Just like water flowing through a hose, changes in voltage, current and resistance affect each other.
You will notice the voltmeter in the figure just above is accross the source of potential or in parallel. This is how you measure voltage.
Also, notice that the ammeter (used to measure current) has to be in series. If you were to hook up the meter in parallel, you would be causing a second circuit for the current to flow. When we measure current, we only want to measure the current flowing in the circuit.
Lets discuss series circuits: You will notice in Fig.
5-2 light bulbs on the left are in a series arrangement
meaning that there is only one path for current flow,
through one then the other etc. If you were to open or
remove one bulb they would all go out. You could measure
the current by placing a ammeter in series any place in the
loop. The voltage however is divided
among the two. Assuming the bulbs are identical, if you
were to measure the voltage by putting a voltmeter in
across or in parallel with any one of the bulbs you would
find 1/2 of the applied voltage across each
Now drawing your attention to the light bulbs on the right
in Fig. 5-2 are in a parallel arrangement meaning there are
separate paths for the current flow, the current flowing through each
branch individually. To measure the total current flowing
in the circuit, you would have to put the ammeter between
the battery and before the parallel arrangement of the
three bulbs. To measure the current in any one individual
path, you would need to place an ammeter in series with the
individual bulb. If you remove one bulb the other will keep
glowing. The voltage is now the same across each lamp.
Voltage could be measured with a voltmeter in parallel with
(across) the circuit.
Measurements & Instruments
There are several types of meters. An analog meter has a meter with a pointer or needle. A digital meter flashes digits and gives you a direct readout.
When you are attempting to measure volts, amperes & ohms, there are some precautions you need to take with a especially with a VOM (Volt-Ohm-Multimeter). When measuring volts to get a desired reading you must place the leads across the device being measured or depending on the circuit between ground and any point in the circuit.
However, when measuring amperes realize the meter represents almost a complete short. therefore never attempt to measure from any point to ground. It would be like shorting a screwdriver to ground at that point. A spark would ensue, fuses would blow, not a good thing. An ampmeter needs to be placed in series with the component you wish to measure current flow through. Always start at a high scale and come down to keep from pegging the meter if it is an analog meter.
Caution has to be exercised when measuring resistance also. Resistance is measured at a very low level of voltage supplied by the meter itself. Therefore resistance measurements are always performed with the circuit de-energized.
If you were to attempt to measure resistance on a live circuit, you would likely damage the meter.
Likewise if you attempt to measure current to ground, you would likely severely damage the meter.
But wait, are there no safeguards built in to protect the equipment and meter from the bumbling technician? Actually if you are using a DVM (digital voltmeter), there are. Generally you are still in trouble if you try to measure current to ground, because you will blow a fuse, probably in the equipment and the meter both. However, if you attempt to measure resistance (ohms) you might simply hear a beep and nothing bad will happen.
There is another way to determine resistance without turning off the equipment and without removing the component you wish to measure from the circuit. You could deduce it by knowing the current and voltage of the circuit & then using Ohms Law!
Ohms Law is probably one of the easiest things to understand in electronics. Volts (potential) cause Amps (current) to flow in a closed loop with resistance opposing the current flow.
Three different ways to express Ohms Law are by finding one unknown if the other two are known.
Lets take a look at the circular diagram. If you want to to figure 'R' (resistance), place your finger over the 'R' and the formulas is 'E' over 'I'. So, Dive 'E' by 'I' and the answer is in ohms.
If we want to find 'E' (voltage)simply place your finger over the 'E' and the remaining part of the formula reveals itself as I x R. Simply multiply the 'I' times the 'R' for the answer in volts.
All that remains is how to calculate 'I' (current), place your finger over the 'I' and what remains is 'E' over 'R'. So, divide the 'E' by 'R' for the answer in amperes
Power like Ohms Law is covered adequately in the text. Power is consumed and is considered the rate at which work is done. Power is expressed as Voltage (E) times the Current (I) in Watts (P).
There is a circular diagram similar to the one for Ohms Law that makes it easy to calculate power given the voltage and current or figuring any one unknown given the other two values.
If you have the power and voltage and want to find out how many amps are flowing in the circuit, place your finger over the 'I' and the formula is 'P' (watts) divided by 'E' (volts). The answer is in amperes
If you wish to calculate the power, put your finger over the 'P' and the formula is I (current in amps) X 'E' (voltage in volts) and answer is expressed in watts.
When you want to find out the voltage when you have the current and power, place your finger over the 'E' - the formula is the power 'P' (watts) divided by the "I" (current). The answer is in volts.
Practice writing down the diagram and doing practice problems.
Types of Voltage
There are two types of current DC and AC.
DC is Direct Current, the current flows in the same direction all the time. It is said to have the same polarity
AC is Alternating Current, the current regularly reverses polarity
AC and DC Voltage is defined the same way.
Batteries & Solar cells are an example of DC Voltage.
Alternating current is what we have in U.S. households. The common voltage is 115 VAC nominal. 220 VAC is also a common household voltage. The frequency is 60 Hertz (cycles)
Typical voltage in Europe & Asia is 240 VAC 50 Hertz (cycles)
When you travel make certain your chargers and/or power supplies are capable of 50/60 Hz operation and also 110/220 Volt operation.
Again the best way to learn & retain formulas is to do some problems and learn from them. So, lets do some practice.
You can take the quiz as often as you wish.
In this module we covered quite a bit. Measurements, Ohms Law,Power and more. This is a lot to digest. Probably the best thing you can do to become proficient in Ohms Law & Power calculations is to practice the problems. Consider retaking the Quiz a few times. It will pay off come test time!
Review the Term Glossary in Chapter 10 of your Ham Radio License Manual
Practice Exams are available at www.arrl.org/exam-practice and there are links to many other test practice sites also.