Resistance is the opposition to current flow in various degrees. The practical unit of resistance is called the ohm. A resistor on one ohm is physically very large but provides only a small resistance to current flow. A resistor of one million ohm's is physically small but presents a high resistance to current flow. A resistance that develops 0.24 calorie of heat when one ampere of current flows through it for one second has one ohm of resistance. The unit of resistance is often represented by the Greek letter omega. Resistors are often made of thin layers of carbon or lengths of small copper wire. They can also be thin deposited layers of metallic material. An image of a few resistor types is shown below.
Resistors can be connected in series (end to end), or in parallel (across one another), or in a combination of series and parallel. If you connect two, 1/4 watt, 100 ohm resistors across one another (i.e. in parallel) then the total resistance in ohms is one half of one of the resistors. In this example the resistance would be 50 ohms. The wattage doubles as the current is now split between the two resistors. The combination can now handle up to one half a watt safely. If the two resistors were connected end-to-end (i.e. in series) the resistances add and in this case would be 200 ohms. The wattage in this series case stays the same, 1/4 watt. This information is handy to know as it is easy to calculate in your head and will allow you to devise additional resister values from a limited resistor bench stock.
What is electrical current? Electrical current, represented by the letter "I" in formulas, and it is the flow or rate of electric charge. This flowing electric charge is typically carried by moving electrons in a metallic conductor or electronic components such as resistors or transistors as an example. The unit of electrical current is the ampere, named after a french mathematician, Andre Marie Ampere. What is electrical voltage? Electrical voltage is represented by the letter "V" in formulas and it is the electrical pressure a moving charge is under. In the case of a static charge, one that is not moving, then voltage is the potential difference or pressure of the charge. The relationship between current (I), resistance (R), and voltage (V) is represented by the formulas developed in Ohm's law. We will study that in section 5 below.
RESISTORS AND RESISTOR CIRCUITS-
Resistors can be connected in series (end to end), or in parallel (across one another), or in a combination of series and parallel. If you connect two, 1/4 watt, 100 ohm resistors across one another (i.e. in parallel) then the total resistance in ohms is one half of one of the resistors. In this example the resistance would be 50 ohms. The wattage doubles as the current is now split between the two resistors. The combination can now handle up to one half a watt safely. If the two resistors were connected end-to-end (i.e. in series) the resistances add and in this case would be 200 ohms. The wattage in this series case stays the same, 1/4 watt. This information is handy to know as it is easy to calculate in your head and will allow you to devise additional resister values from a limited resistor bench stock.RESISTORS IN SERIES: Connecting resistors in a string one pigtail to another is called connecting them in series. When connected this way the resistance of one resistor adds to the next in line. For example a 100 ohm resistor in series with a 500 ohm resistor is the same as having a 600 ohm resistor. The wattage capability stays the same, in other words if the resistors are all 1/4 watt the string is 1/4 watt.
Resistance in series resistance simply adds: R = R1 + R2. This can be extended for more resistors: R = R1 + R2 + R3 + R4 + ...
RESISTORS IN PARALLEL: When resistors are connected in parallel (parallel; meaning they are tied across one another) their combined resistance is less than any of the individual resistances. There is a special equation for the combined resistance of two resistors R1 and R2:
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For more than two resistors connected in parallel a more difficult equation must be used. This adds up the reciprocal ("one over") of each resistance to give the reciprocal of the combined resistance, R:
The simpler equation for two resistors in parallel is much easier to use!Note that the combined resistance in parallel will always be less than any of the individual resistances.Resistor values are measured in ohms. A thousand ohms is written as 1k to eliminate all the zeros. The k represents three zeros. A million ohms is represented by 1M. Therefore; 1000 ohms = 1k ohm and 1000k ohms = 1M ohm. Since resistors are so small their value is marked by a color code.RESISTOR COLOR CODES - Resistors use color coded stripes to indicate their value in ohms. 0=Black, 1=Brown, 2=Red, 3=Orange, 4=Yellow, 5=Green, 6=Blue, 7=Purple, 8=Gray, 9=White.
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