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Ohm’s Law Ohm’s Law is the mathematical formula that explains the relationship of Voltage, Amperage and Resistance. Voltage can be explained as the potential difference between to points in a circuit and is represented by the symbol (V). Voltage is also represented by the symbol (E) and can be used interchangeably. (E) was originally used to describe (emf) or Electro Magnetic Force and was used to describe things such as batteries but over time the 2 have become interchangeable. Amperage in a circuit can also be referred to as Current just like a dog can be called a canine these terms are also interchangeable. Current is the amount of electrons that pass a particular point in a circuit at the same time. In fact 1 Ampere is equal to 6.242 × 1018 electrons passing a single point in 1 second. In other words the higher the current the higher the amount of electrons that would be flowing in the circuit. When you are using mathematical formulas to decipher components of ohms law Amperage is symbolized by the Capital letter (I). Now you are wondering why use the letter (I) seeing as Current starts with a (C). The reason is back in the 1700’s most of the scientists that were working on experiments dealing with electricity were either English or French. In France Amperage was referred to as the intensite current (the intensity of the current). So the symbol (I) came from the word intensite. Resistance can be explained as the amount of resistance voltage has to overcome in a circuit for current to flow. The byproduct of resistance is heat we’ll talk about that later. Let’s now look at why copper is used as a carrier for electricity. To find out what makes it so special we have to understand what are some of coppers special features. First it can be drawn into long thin wires (ductility) Second it can be worked or bent into many different shapes without breaking (malleability) Other metals that have the above capabilities are gold, silver, platinum and aluminum As you can guess all the other metals except aluminum are much more expensive and would not be cost effective. Aluminum is not a choice conductor because when electricity passes through aluminum the aluminum tends to vibrates. The other feature of copper that makes it a choice conductor for electricity is its atomic structure. Copper has 29 electrons 28 of which are held tightly by the atoms nucleus and one that is not held very tightly so even slight pressure will cause this one to be in motion. It happens and no I can’t prove it. Just to give you an idea of how many free electrons there would be in a piece of copper 1 cubic inch of copper would have 1.4 × 1024 free electrons. Back to Ohm Law OHM’S LAW Ohms Law is fundamentally based on the following formula: V=IxR This formula can be written different ways depending on what you are trying to find as in: V = IR I = V/R R = V/I Let’s start by looking at a simple series circuit like the circuits you were building with your resistors last week. A series circuit can be defined as a circuit that has only one path. From the negative terminal of the power supply, out through the circuit then back to the positive terminal of the power supply. In other words all devices are connected together like a group of people holding hands in a circle. It can be said that a change in voltage will have a proportional change in current if the resistance stays the same. Let’s look at the next 2 circuits and see how this works. Using the Formula V=IxR R= V = 10V V= 10V I = 2A R= ? V = IR 10 = 2 x R can be simplified to equal 10/2 = R Therefore the resistance of the unknown resistor would be 5 ohms Now let’s see the relationship of Voltage and Current if the resistance in the circuit stays the same I= V = 100V R = 5 ohms V = 100V R = 5 ohms I=? V/R = I 100/5 = I 20 = I Therefore I = 20A So we can see if resistance stays the same in a series circuit Voltage and Current change proportional to each other. If on the other hand we keep the Voltage the same and change the resistance. You will notice that as the resistance increases the Amperage decreases. This works the same as a water tap. As you close the tap (put up more resistance to the water) less water will flow. Likewise if you open the tap more (put less resistance in the path of the water) more water will flow out of the tap. I= V= 100V V = 100V R = 10 ohms I =? V/R = I 100/10 = I R= 10 ohms 10 = I I = 10A Therefore as long as the Voltage stays constant an increase in Resistance will result in a decrease in Current. Now let’s look at what happens if we have more than one resistor in a circuit R1 = 5 ohms R2 = 10 ohms V = 30 Volts R3 = 15 ohms Before we can tackle the above problem we must learn another Formula that works with resistors in a series circuit RTotal = R1 + R2 + R3 + . . . Now using the above formula lets look at how it would work: RTotal = ? R1 = 5 ohms R2 = 10 ohms R3 = 15 ohms RTotal = 5 + 10 +15 RTotal = 30 ohms Now we have to figure out what the Current would be: I = V/R I=? V = 30V R = RTotal = 30 ohms I = 30/30 I=1 Therefore the current in the circuit would be 1A Now we need to know a little more about Current in a series circuit. As current is flowing through a circuit it makes sense that if you want to measure current you have to put a meter in line with the circuit like adding another person into the group of people holding hands in a circle. It is because of this that it can be assumed that the current is the same at all points in a series circuit. Now we have to look at how resistance affects voltage in a series circuit. It is said that Voltage drops over resistors in a series circuit. Also if you remember Voltage is the potential difference between 2 points in a circuit. It is because of this that when you are measuring Voltage you measure between 2 points. So looking at the above circuit diagram if we were to place our meter leads at point 1 and 2 we would be measuring the potential difference over R1 To find this Voltage we would use the formula: V = IR V=? I = 1A we know this from the previous equation R = 5 ohms V=1x5 V = 5V So it can be said that the Voltage drop over R1 is equal to 5 Volts. If we repeat this step measuring between points 2 and 3 then also between points 3 and 4 we will notice the Voltage drops will be: Between 2 and 3 (R2) = 10 Volts Between 3 and 4 (R3) = 15 Volts You will notice that the voltage drops throughout the series circuit will add up to equal the total available Voltage VTotal = V1 + V2 + V3 + . . .