## The g4egq rae postal course - lesson i

((If a steady current flows through a conductor, the pd. across its ends is proportional to that current
providing the physical conditions of the conductor do not change.))
This means that there is a mathematical relationship between Amps, Ohms and Volts.
This relationship, shown below, is very important.
From Ohm's Law: - Voltage/current is a constant. This "constant" is called resistance.
VOLTAGE = RESISTANCE or V/I = R .
CURRENT
Normally you will know two of the values and you will need to calculate the third. Written in the form R = V/I
we would be able to calculate the resistance if we knew the voltage and the current.
Thus the formula needs to be written in three ways:
The triangle, on the right, can be used an aid to the memory. Just remember that the V goes at the top (The top is "V" shaped) and the other two go underneath. Then cover up the unknown one and it can be worked out from the other two. Remember - always use basic units. In other words, when using formula the voltage must be In VOLTS, the current in AMPS and the resistance in OHMS. Basic units must always be used. For example, lOO mA must be put in the formula as 1/10 Amp or as 0.1 Amps. In electronics it is very common for the values to vary from very large to very small so letters are used as follows: Take care with "M" and "m" as the difference is very great. Example of Ohm's Law in action……. A ten Ohm resistor is connected across a 12 volt car battery. What current will flow through the resistor? Assume that the battery has no internal resistance. From Ohm's Law: I = V/R = 12/10 = 1.2 Amps. Power is measured in WATTS. It is necessary to be able to calculate the power that is dissipated in various components in an electrical circuit. When too much current passes through a resistor the power is given off in the form of heat. In other words the resistor gets hot'. Some resistors are designed to get hot but normal resistors would be damaged. Power is calculated by multiplying the voltage across the resistor by the current flowing through it. In the above circuit the power dissipated in the resistor would be calculated as follows: There are two other formula for power: W = VV/R or W = I2 R Resistors, as well being made in various values of resistance, are also made in various power ratings. Each value of resistance is usually available in the following wattage ratings: 1/8 W; 1/4 W; 1/2 W; I W; 2 W; 5 W; 10 W; 20 Watts etc. Thus, to be on the safe side, a 20 Watt resistor would be used in the above example. Resistors in seriesIf resistors are connected as above, they are said to be in series. If a voltage is connected across A & B then a current will flow. The current through each resistor will be the same but it must be calculated using the total resistance of the circuit. If the total resistance is known and the total voltage is known then the series current can be easily calculated using Ohm's Law. Let us assume that R1 is 10 Ohms, R2 is 20 Ohms and R3 is 30 Ohms. The total resistance is 10 + 20 + 30 = 60 Ohms. If the supply, connected across A & B, is 12 Volts then the series current will be: I = V/R = 12/60 = 0.2 Amps (or 200 mA) Unless the resistors are all the same value the voltage across each will be different. Knowing the series current and the value of each resistor enables the voltage across resistor to be calculated. Voltage across R1 = I x R1 = 0.2 x 10 = 2 Volts Voltage across R2 = l x R2 = 0.2 x 20 = 4 Volts Voltage across R3 = I x R3 = 0.2 x 30 = 6 Volts You will notice that the sum of these Voltages is equal to the supply Voltage. Thus in a series circuit the voltages are different and the current is the same. The two resistors , in the circuit above, are connected in parallel. In this case the voltage across the resistors will be the same but the current through them will differ. In a parallel circuit the current splits. The lower resistance path will carry the greater current. If the two resistors are the same value the current in each will be half of the total supply current. The formula to find the total resistance of resistors in parallel is :For parallel circuits the total resistance will always be less than any of the individual resistors. Parallel Circuit Example: A 30 Ohm resistor and a 60 Ohm resistor are connected in parallel across a 12 V power supply. What current will flow from the supply assuming it has no internal resistance? First it is necessary to find total resistance Ohm's Law can now be used to find the current. Note that the resultant resistance (20 Ohm) was less than either of the resistors. QUESTION : 1 (a) What is the difference between a conductor and an insulator? QUESTION : 2 State Ohm's Law QUESTION : 3 (a) A 5 Ohm resistor, a 10 Ohm resistor and a 35 Ohm resistor are connected in series across a home made 100 Volt power supply unit. What current will flow ? (b) How much heat (in Watts) will be dissipated by the 10 Ohm resistor? QUESTION : 4 The regulations state that a "log" must be kept. What details MUST be written down for each QUESTION : 5 Three resistors are connected in parallel as shown. What is the resultant resistance between terminal "A" and terminal "B" ? QUESTION : 6 (a) A 4 Ohm resistor is then added to the above circuit. What is the total resistance between terminal "A" and terminal "C" ? (b) What power will be dissipated by the whole circuit if 50 Volts is connected across "A" and “C” . QUESTION : 7 What, according to the licence, is the purpose of Amateur Radio? QUESTION: 8 What is 0.03 Amps in milliamps (mA) ? QUESTION : 9 What is 0.00002 Volts In microvolts (uV) ? QUESTION : 10 A torch bulb requires 100 mA to light it brightly. How much electricity (in Coulombs) flows if it is turned on for one and a half minutes ?

Source: http://www.sarts.org.sg/rae/Les1.pdf

### Mrten schultz

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