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Bredhurst Receiving and Transmitting Society |
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3. Technical Basics 3b Simple circuit theory 3b.1 Understand that, in a metallic conductor, an electric current is a flow of electrons. In the FLC it was explained that the movement of electrons from an abundant source of the electrons ,such as the negative terminal of a cell or battery, in a conductor around a complete circuit to where there were many fewer electrons, such as the positive terminal of a cell or battery, is called an electric current. The actual direction of the electrons flow is from negative to positive. However due to a mistake made originally by those who discovered the flow of electrons by electronic convention, you must learn and understand is that conventional current or just current is said to flow from positive to negative. Recall that a conductor allows the electrons to flow easily and an insulator does not. Conductor allows the flow of electrons The complete circuit mentioned above has in it a CONDUCTOR a piece of wire, which allows the current (the flow of electrons) to move easily.
The animation shows current being turned on and flowing along a conductor, continuing to flow for a short while and then being turned off. If all materials were conductors then there would be nothing that could separate one conductor from another and the whole of electrical theory would collapse. Insulators prevent the easy flow of electrons Thankfully that is not the case and those items which do not conduct a current or conduct very poorly are called INSULATORS such as plastic, rubber and porcelain.
3b.2 Understand that metals such as copper and brass are good conductors. Good conductors such as copper and brass allow the current to flow easily with very little restriction. Plastics, wood, rubber, glass and ceramics are regarded as insulators. Plastics, wood, rubber, glass and ceramics are regarded as insulators as they prevent the easy flow of current. Understand that water is a conductor, and that wet insulators can conduct electricity through the surface water. Something that you might not have considered until now is that:- "most WATER is a CONDUCTOR"Only very pure water does not conduct. The importance of this is that the human body is mostly made up of water and as it is not pure water it conducts and this is why you can suffer an electric shock if you touch a live wire whilst another part of your body is linked in some way to earth and thus completes the circuit. Water only needs a very little impurity to make it conduct thus rainwater; water puddles and seawater are all conductors even rain is a conductor hence the fact that wet insulator can conduct electricity through surface water. Thus if an insulator, even made of glass or other good insulating material, at the end of an aerial gets wet it too becomes a conductor. Also should a live wire fall onto a wet grassy field the whole of the near area would become live.
3b.3 Understand the relationship between power, potential difference and current. Be able to manipulate the equation P = V x I to find the unknown quantity given the other two. The prefixes milli and kilo may be used. Power in measured in the unit WATT or W and in the FLC you were introduced to a magic triangle this is also needed in the ILC so that you can manipulate the equation P = V x I to find the unknown item given the other two items.
Practice with this until you are certain that you fully understand how to find one unknown from two know items. NOTE: calculations in the exam may use the prefixes milli and kilo. A student from an early course suggested the following to help you remember the order of the letters :- P = Prefers V = Vanilla and I = Ice-cream hence Prefers Vanilla Ice-cream Example :- What power supply would be suitable to run a 50W transceiver from a 12V power supply? from P = V x I we get 50 = 12 x I then I = 50 / 12 result 4.166 Amps The answers given in the exam might be :- A 12 V at 500mA B 12 at 1A C 12 V at 6A D 24V at 4A D is wrong as it is at the wrong voltage, A & B are two low so the answer is C However be aware that the output power actually bares no direct relationship to the input power as sometimes to have an output of 50W RF you need 100W of DC input !!!
3b.4 Understand the relationship between potential difference, current and resistance. Be able to manipulate the equation V = I x R to find the unknown quantity given the other two. The prefixes milli and kilo may be used. In the FLC you were introduced to a magic triangle this is also needed in the ILC so that you can manipulate the equation V = I x R to find the unknown item given the other two items.
Practice with this until you are certain that you fully understand how to find the one unknown from two know items. NOTE: calculations in the exam may use the prefixes milli and kilo. Again student from an early course suggested the following to help you remember the order of the letters :- V = Vanilla I = Ice-cream R = Ready hence Vanilla Ice-cream Ready
Understand circuits comprising series and parallel connections of resistor and cells. Calculate the combined resistance of two or three resistors in series. Calculate the combined resistance of two or three equal resistors in parallel. SERIES RESISTORS Resistance is the opposition to current flow. The bigger the resistance the smaller the current that can flow. Before we look at the topic in electronic terms let's consider a large number of people all lined up in one long straight line.
The effort that you would need, as an electron!!, to push through would be quite a bit but not impossible. Now what would happen if the crowd broke into two lines you would now have to push your way through the first and then the second line and you would find it more difficult and you would have used up more energy.
So if the crowd broke into three lines you would now have to push your way through all three and that would be more difficult than the other two previous examples and even more energy would have been used up.
The same is the case with resistors.
When there is just one resistor in a circuit as shown below then the amount of current flowing for a know voltage through a known resistor can be calculated based on the formula V = I x R and would be shown on the amp meter or ammeter (a ammeter in series with the circuit to measure current).
which re-arranged give you the formula shown below. I = V / R
When we have two resistors in series, as shown below,
the current has to flow through both and finds it more difficult. In fact the value of the total resistance is simply to add up the values of the individual resistors.
"SERIES Resistor TOTAL" = R1 + R2
Even though the two resistors are separated by the meter they are still connected in series. So to find the current flowing in a circuit, through the meter, with two resistors in series you first work out the value of the total resistance and from that calculate the current from I = V / R where R = R1 + R2So when we have three resistors all lined up one behind the other the task of the current passing is still greater and less flows. As with the two resistors mentioned above the total resistance is simply to add up the values of the individual resistors.
"SERIES Resistor TOTAL" = R1 + R2 + R3
Current flow Above the three resistors are "in SERIES" as the current has to flow though each as there is no other way. also it does not matter where in the circuit the meter is to measure the current.
So to find the current flowing in a circuit with three resistors in series you first work out the value of the total resistance and from that calculate the current from I = V / R where R = R1 + R2 + R3Voltage across each resistorIf the voltage is measured across each resistor then the sum of these individual values will be the same as that across the battery as the set of resistors could be replaced by a single resistor equal to the total value of the individual resistors (as they are in series)
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PARALLEL RESISTORS |
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The other way that resistors can be coupled together is for each end of one resistor to be linked to the same end of the next resistor and the same with the other end.
We now have a different situation to the "SERIES" as the electrical path, as we call it, has two routes to chose from. If each of the resistors is of equal value then it is as difficult for the current to pass through each resistor and therefore the current divides equally between the resistors. If they are not of similar value ....... well that is for a later training course so let's just think in terms of similar values R1 = R2 Value of Two EQUAL resistors in parallel = Value of one resistor divided by 2 "Parallel Resistance Total" = R1 / 2
R1 = R2 = R3 Value of Three EQUAL resistors in parallel = Value of one resistor divided by 3 "Parallel Resistance Total R" = R1 / 3 This may look strange but if you think of it this way .... there are in the first example two paths that the current can take therefore with equal values of resistor the resistance is halved as there are two paths
and with three resistors of equal value the resistance is a third as there are three paths!!
Find the current in a circuit with resistors in parallel
You must first find the actual total amount of resistance and from that calculate the current flowing in the circuit. The meter measure the total amount in the circuit and MUST be placed to make this reading outside the resistor network else it would only read the current passing through the single resistor with which it is in series. Practical example - Make a DUMMY LOAD. Let's take a practical example. If you want to make a dummy load then how many carbon resistors will you need and of what value to make a 40W dummy load ? There would be several ways of going about this :- 50 0.8W 1 ohms resistors in series. That would make a fairly bulky dummy load and probably only 1W resistors available so the unit would be over rated. So look at it another way. 2W resistor of the carbon type are available and thus 20 resistors would give us the power capability we require and assuming we are using them in PARALLEL, what value of resistor is needed ? From the equation above :- Resulting value = Value of one resistor / Number of resistors rearranging the equation we get :- Value of one resistor = Resulting value x Number of resistors Value of one resistor = 50 x 20 = 1000 ohms or 1k ohm So to make a simple dummy load we need 20 1k 2w resistors linked in parallel.
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Students are not understanding that when faced with
a question of a current in a simple circuit that you must first determine
the value of the total resistance whether that is resistors in series (as
outlined above) or resistors in parallel as outlined below. It is only then
that you can go forward to calculate the current flowing for a particular
applied voltage but the use of Ohm's law. so let's look in a bit more details
about this type of question.
