Bredhurst Receiving and Transmitting Society 



Technical Basics Part 2


UNITS Technical Basics 3.a Identify Units of measurement and multiple/submultiple 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. Earlier you were introduced to words such as Voltage (Potential difference) and Current now we have to add a few more. For the moment we are considering that Potential difference and Voltage are one and the same thing, for the purest they are not but for the moment let's accept that they are.
Now we will introduce the following :
Prefixes Prefixes milli, kilo and Mega. I would think that many of you have heard of kilo  it is used in shops for say 1 kilo grams of potatoes and that means 1000 grams of potatoes AND kilo is used in just the same way in electronics thus 1000 ohms is the same as 1k ohms, note the "k" in kilo is small. If we want to go a 1000 times bigger than 1k ohms we used the word Mega. and so 1000000 ohms is the same as 1 Mega ohms or 1M ohms, note the use of the large "M". So that leaves milli, poor little milli. Well that might help you to remember her as she is 1000 times smaller than 1 which if we write it in decimal is 0.001 and so when this is used with say amps and we want 0.001 amps we write it as 1mA, note the small "m" and big "A". 

3.b Simple circuit theory.3b.1 Understand that, in a metallic conductor, an electric current is the flow of electrons. Recall that a conductor allows the electrons to flow easily and an insulator does not. Copper wire is often used as a conductor as it allows the easy flow of electrons. An insulator is any material that does not allow the flow of electrons such as plastic. Understand that metals such as copper and brass are good conductors. Because of there make up copper and brass are good conductors of electrons Plastics, wood, rubber, glass and ceramics are regarded as insulators. Because of their makeup Plastics, wood, rubber, glass and ceramics are regarded as insulators as they resist the flow of electrons. Understand that water is a conductor, and that wet insulators can conduct electricity through the surface water. Water as you know it is a conductor and thus when rain falls on an insulator the surface coating of water allows electrons to flow, which is particularly important when assessing insulators for antennas.


3b.2 Recall the relationship between
Potential Difference ( Voltage ), Current and Power. (P=VxI, I=P/V,
V=P/I)
Calculate the unknown quantity given the numerical value of the other two. Again fortunately for us those clever persons years ago found out that there was a simple relationship between Voltage, Current and Power this formula is : P = V x IPower = Voltage x Current and the relationships to find the third one knowing the other two are more easily shown in what is called a magic triangle.
Just as with the other magic triangle we now have Power at the top and volts and current on the bottom. So if you wish to know the power cover that one and it leaves volts and current exposed and on the same line so they are multiplied together. Wanting to know volts leaves power above current so divide power by current and similarly wanting to know current leaves power above volt so divide power by volts. If there is a lamp in a circuit and the volts measure 6 volts and the current measures 0.5amps what power is being dissipated ? P = V x I P = 6 x 0.5 = 3 watts If there is a resistor in a circuit and power is 4 watts and the input voltage is 2 volts what current is flowing ? P = V x I thus I = P V thus I = 4 2 = 2 amps 3b.3 Recall that resistance is the opposition to current flow. How many of you have played with a garden hose and put your finger over the end and been able to reduce the flow of water whilst say filling a bucket ? What you were in fact doing was providing a resistance or opposition to the flow of water in the hose and which resulted in a much reduced amount of water flow. In electrical circuits a resistance provides an opposition to the flow of electrons and thus reduces the amount of current that can flows in a circuit. Recall the relationship between Potential Difference (Voltage ), Current and Resistance. ( V=IxR, I=V/R, R=V/I ) Calculate the unknown quantity given the numerical value of the other two. Let's start at the beginning. A voltage source, V, drives an electric current, I , through resistor, R, and these three items obey what is know as Ohm's law: V = IR. The law was named after the physicist George Ohm, who published a treatise in 1826 of the relationship between Voltage, Current and Resistance. So the discovery which George Ohm proved by experiment that a precise relationship exists between current, voltage, and resistance still hold true today and is something you need to learn. V = I x RVoltage = Current x Resistance When you are given two of the items the third can be found by manipulation of the formula but this is more easily shown in what is called a magic triangle.
The animation shows how knowing two elements you can work out the third element. Just cover the one you do not know and then if that was volts it leaved current and resistance which being on the same bottom line are multiplied together. If you know current then covering that one leaves volts above a line and resistance below a line so volts are divided by resistance. Likewise knowing resistance leaves volts above a line and current below a line so volts are divided by current. ................................................................... Questions in the exam will not necessarily be calculation as the syllabus states Recall the relationship between Potential Difference (Voltage ), Current and Resistance. so the question may just be a matter of manipulation the equation to state for instance "The formula for voltage is ??" and you would need to know it is V=IR or any of the other combinations eg. I=V/R R=V/I ..................................................................... Another way of using this part of the syllabus is to ask what happens if ? For instance What happens if the Resistance in a circuit is doubled? You would need to think clearly about this the voltage is not changing but the resistance is 2x the original value. to find I (Current) we have V (Volts) divided by R (Resistance) in the first case and then V divided by 2x R if necessary put in your own values say V = 10 and R = 5 then 2x R = 10 working out I = 10 / 5 = 2A and I = 10 / 10 = 1A so the solution to the problem set What happens if the Resistance in a circuit is doubled? Answer "It halves" so even though you have not been given any variables you can introduce them yourself to reach the answer. 

The origin of some of the text on this page is from the RSGB with additions by the web master 

