Transcript D-3 Notes
Science 9: Unit D – Electrical Principles and Technologies Topic 3: Ohm’s Law and Circuits Resistance Resistance is a number value that tells us how difficult it is for electrons to pass through an object. Almost all objects, even conductors, have resistance. Resistance depends on many factors including the thickness of the object and the material of the object. The thinner the wire the higher the resistance. The longer the wire, the higher the resistance. The higher the temperature of the wire, the higher the resistance. Insulators have much a much higher resistance than Conductors. The job of a resistor is to slow down electrons long enough so that the electrical energy in electrons is transferred into another form of moving, heat, or light energy. Examples of Resistance A light bulb has a thin wire called a filament that acts as a resistor. The wire, made of the metal called tungsten (W) is a conductor which becomes so hot with friction from the slowed down electrons that it glows white hot. The unit for resistance is the ohm which is given the symbol Ω. Resistance of a material is measured with an ohmmeter. Ohm’s Law When dealing with any circuit an easy way to calculate resistance is with the following formula: R=V÷I where R = resistance, V = voltage, and I = current. Ohm’s Law What Ohm’s law tells us is that the higher the resistance the greater the energy it takes in from the electrons (so the greater the voltage needed to make it work). Also Ohm’s law tells us that the greater the resistance of the resistor of a circuit the slower the electrons will be moving overall in the circuit. The resistor acts as a bottleneck in the circuit. So the higher the resistance, the lower the current will be. Keep in mind that: V=I*R and I=V÷R Types of Resistors Resistors can either have a fixed or variable resistance. Fixed Resistors are usually made of a heat conducting material to control the current and voltage levels. Resistors that do not change as other variables change are also called Ohmic Resistors Variable Resistors change their resistance in response to a changing variable such as temperature, light, or voltage. Examples include light dimmers and stove element controls. Rheostats are variable resistors that you can actually change the resistance by turning a dial. They are used in dimmer light switchers; Thermistors are variable resistors that change resistance based on temperature; and varistors are variable resistors that change their resistance depending on the voltage. Circuit Types There are three types of circuits: series, parallel and combined. Most circuits in the house are combined or parallel. An example of series circuits are old fashioned Christmas lights. Series Circuits Series Circuit – The current source and resistors are all connected together on the same path. The voltage from the battery must be shared by the resistors leading to lower energy levels. Also, if one resistor breaks down the circuit becomes broken. See fig. 4.19 on p. 286. Parallel Circuits Parallel Circuit – Each resistor each have their own current path and therefore each is connected directly to the battery. Because of this direct connection the resistors each have the maximum voltage coming to them. Another advantage is that if one resistor breaks down the other resistors can still operate. Most circuits in a building are parallel for this reason. See fig. 4.20 on p. 286. Combined Circuit Combined Circuit – A combination of a series circuit and parallel circuit. At least two resistors are attached directly together on the same path, but there is at least one circuit on its own circuit branch. Problems With Circuits A problem with parallel or combined circuits is that having all the resistors/loads working at the same time will put more of a strain on the current source and may cause it to overheat and catch fire. In a house, circuit breakers and fuses break the circuit if the temperature of the source becomes too high.