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Transcript
Resisting the Movement of Charge Recall: Electric current is how fast the charges are moving. Voltage is the pressure that gets the charges (electrons) moving. Resistance (R) is the property of a substance that hinders motion of electric charge and converts electric charge into other forms of energy (such as heat or sound). Resistance is slowing down (sometimes even stopping) the charges. The unit for resistance is the ohm, and it is measure with an ohmmeter. Read page 279. A good conductor of electricity (such as copper wire) has a low resistance, so electric charges flow rapidly. Poor conductors of electricity oppose the movement of charge, and have a high resistance. Therefore the electric current slows down. Demonstration on page 280. Answer questions #1-3 in “What did you find out”. The relationship among current, resistance, and voltage. Pretend an electric circuit is a race track. Each car represents a charge. Each car has a certain amount of gasoline that provides its energy. The gasoline that each car has would represent the voltage. The number of cars that pass by the starting line every second would represent the current. As cars (charges) go up hills (resistors), each one of them uses up gasoline (voltage). Each car needs to get more gasoline (voltage) at the pit stop (battery). Ohms Law – Calculating Resistance Read page 281-282 Ohms Law provides for the relationship between voltage across a resistor (the energy that each charge loses across the resistor), the current through the resistor (the amount of charges that are flowing through the resistor each second), and the resistance of the resistor. R = V/I Where R = resistance (ohm) V = voltage (V) I = current (A) Show triangle. Question: What is the resistance of an electric heater, if a current of 12.5 A runs through it when it is connected to a 120V wall outlet? page 282 #1-4 BLM 4-6 Resistors (read page 283) - it is something that resists the flow of electric charge. - Control current or voltage to suit the specific needs of other electric devices in the circuit. It slows down the current or voltage. One type of resistor is a variable resistor. These don’t have a fixed resistance; (ex. Rheostat). Variable resistors are used in light dimmers, fridge and stove temperature controls. Types of Circuits Series circuits - have only one current path. - All moving charges travel through each component in the circuit Parallel circuits - have several current paths - the total current is divided, with some of the moving charges traveling through each branch, or part of the circuit. - If one path is broken there is still another path for current to come through - Example: your home- lamps, tv, stove, computer, all operate from the same source of electric energy. Classroom analogy of parallel and series circuits Lab on page 287. Advantages of a series circuit - easy to construct - uses less wire - allows batteries to last longer Disadvantages of a series circuit - existing bulbs get dimmer as more bulbs are added - entire circuit stops working if any one bulb burns out Advantages of a parallel circuit - if one bulb burns out, the other bulbs will work - the bulbs don’t burn less brightly when additional bulbs are added Disadvantages of a parallel circuit - requires more wire - batteries don’t last as long due to increased current levels BLM 4-9 House wiring Read page 289. Factors affecting the resistance of a wire 1) length – resistance increases with increasing length 2)cross sectional area (thickness) – increase area = decrease resistance (fig 4.22) 3)temperature – increase temp of wire = increased resistance 4)material BLM 4-10 Page 291 #1-8 Page 292 #1-17