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Transcript

Do Now (10/14/13): What is Ohm’s Law? What do you know about electric circuits? In your own words, what is electric current? ELECTRICAL CIRCUITS All you need to be an inventor is a good imagination and a pile of junk. -Thomas Edison Review:Ohm’s Law I=V/R Georg Simon Ohm (1787-1854) I = Current (Amperes) (amps) V = Voltage (Volts) R = Resistance (ohms) How you should be thinking about electric circuits: Voltage: what pushes the current through the circuit (in this picture it would be equivalent to gravity) How you should be thinking about electric circuits: Resistance: friction that impedes flow of current through the circuit (rocks in the river) How you should be thinking about electric circuits: Current: the actual “substance” that is flowing through the wires of the circuit (electrons!) Would This Work? Would This Work? Would This Work? The Central Concept: Closed Circuit Series and Parallel Circuits Circuits Can either be series or parallel. 20.1 Series and Parallel Circuits In series circuits, current can only take one path. The amount of current is the same at all points in a series circuit. Series Current only takes one path for electrons Current flows through every part of the circuit Lights in a Series Series If you add a resistor (like another light): Total resistance goes UP since all the current has must go through each resistor. Adding Resistors to Series: Current in the circuit will go DOWN (lights will dim) If you remove a light bulb or one burns out—all go out! Current in Series Current is the same at all points Use Ohm’s Law to find current using resistance and voltage Calculating Current Example: A 6 V battery is connected to Voltage in Series Voltage is reduced by each resistance – voltage drop Calculating Voltage Example A 6 V battery is connected to a series circuit. If the current is 2 A, what is the voltage across a 2Ω resistor? Resistance in Series Add up all resistors to get total Total resistance will go up because all of the current must go through each resistor. Adding resistances in series Each resistance in a series circuit adds to the total (or equivalent) resistance of the circuit. Rtotal = R1 + R2 + R3... Total resistance (ohms) Individual resistances Calculating Resistance Example: A series circuit contains a 12Ω, 10 Ω and 8Ω resistor. What is the equivalent resistance? Circuit Symbols Exit Question: Draw a series circuit with the following: A 12 V battery Two 3Ω resistors One 5Ω resistor What is the one variable that will not change throughout the circuit? Do Now (10/15/13): Draw a series circuit diagram containing the following: A 2Ω resistor A 1Ω resistor A 6V battery Which variable does not change throughout a series circuit? 20.1 Total resistance in a series circuit Light bulbs, resistors, motors, and heaters usually have much greater resistance than wires and batteries. 20.1 Calculate current How much current flows in a circuit with a 1.5volt battery and three 1 ohm resistances (bulbs) in series? 20.1 Voltage in a series circuit Each separate resistance creates a voltage drop as the current passes through. As current flows along a series circuit, each type of resistor transforms some of the electrical energy into another form of energy Ohm’s law is used to calculate the voltage drop across each resistor. Sample Problem #1 1. 2. 3. Draw a series circuit with two 1.5 V batteries, 3 resistors, and a current of 0.5 A. What is the total voltage of the circuit? What is the resistance of each resistor? What is the voltage drop across each resistor? Label on your Do Now (10/24/13): Draw an example of what you a parallel circuit would look like. Extra Credit Opportunity Open House – Sunday, October 28, 12 – 3 PM 5 service learning hours! Free pizza!! Extra credit!!! Parallel Circuits Has at least one point where current divides More than one path for current to flow Paths are also known as branches Lights in Parallel Parallel: If you add a resistor: Total resistance goes down Total current goes up when you add another path Removing a Light Bulb If you remove a light bulb or one burns out, the others stay on because the circuit is still closed. Current in Parallel Current flows into a branching point, the same total current must flow out again Current depends on resistance in each branch Calculating Current Example: Voltage in Parallel Voltage is the same across each branch – because each branch is on the same wire Calculating Voltage Example: Resistance in Parallel Calculate current in each branch based on resistance in each branch by using Ohm’s Law Calculating Resistance Example: Practice problem #2 1. 2. Draw a parallel circuit with two resistors (one on each branch) and a 12 V battery. What is the voltage through each resistor? What is the current flowing through each branch? Toll Road—Circuit Analogy Toll Booth Explanation Adding toll booths in series increases resistance and slows the current flow. Adding toll booths in parallel lowers resistance and increases the current flow. Batteries in Series and Parallel: In series—The voltage is increased. In parallel—No change in voltage; these batteries will last longer! One More FINAL Thing: Two Types of Current: DC—Direct Current— produced by solar cells and chemical cells (batteries) Current only flows in one direction. nd 2 type of current: AC—Alternating Current Current flows back and forth (alternates) Found in homes Generators produce AC current Practice: Work on the paper “Parallel Circuits.” Be prepared for an exit question! Exit Question: Draw a parallel circuit containing 3 branches: Each branch should have one resister (10Ω, 2Ω, 5Ω) The battery is 6 V What is the voltage in each branch?