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Schematic diagram: • A diagram of an electric circuit • Straight Lines Only! Schematic Diagrams Symbols: Battery Open Switch Resistor Closed Switch Bulb Types of Electric Circuits Series Circuit 2 types of electric circuits: Series Circuits and Parallel Circuits More than one path possible for electrons to flow A single path for electrons to flow Total resistance = sum of the individual resistances (light bulbs) on the circuit path. If one bulb burns out, the whole strand of lights won’t work! Mini-Lab Make a circuit with two bulbs in series. Done? Have a seat and continue to work on the reading assignment. Mini-Lab Add a switch to your last circuit that can turn both bulbs on and off. Done? Have a seat and continue to work on the reading assignment. 1 Mini-Lab Parallel Circuit Make a circuit with two bulbs in parallel. Done? Have a seat and continue to work on the reading assignment. Mini-Lab Mini-Lab Add a switch to your last circuit that can turn one bulb on and off without affecting the other. Add another switch to your last circuit so that the other bulb can be turned on and off also, without affecting the other. s Done? Have a seat and continue to work on the reading assignment. Done? Have a seat and continue to work on the reading assignment. An Analogy for a Circuit: Voltage, Current and Resistance 2 VOLTAGE (V) is ELECTRICAL POTENTIAL ENERGY and is measured in VOLTS (V) Low Voltage (Low Potential Difference) Voltage can describe the energy stored in batteries: High Voltage (High Potential Difference) Anatomy of a Battery Positive Terminal Zinc Anode (-) Graphite Cathode (+) Chemical Paste (makes the electrons) Negative Terminal About Batteries: How does a battery work? Electrons collect on the negative terminal of the battery. If you connect a wire between the negative and positive terminals, the electrons will flow from the negative to the positive terminal as fast as they can. About Batteries: Where does a battery get its electrons? Inside the battery itself, a chemical reaction produces the electrons. The speed of electron production by this chemical reaction controls how many electrons can flow between the terminals. 3 CURRENT (I) CURRENT (I) ELECTRIC CURRENT is the FLOW of electrons, measured in AMPERES (A) What’s difference between… Current I Voltage V (in Amperes) (in Volts) measures the flow of charge 1 AMPERE of Current Means 1 Coulomb per second 6,250,000,000,000,000,000 electrons per second!! RESISTANCE R you could say that… Volts measure the pressure in the hose. Amps measure how much water comes out of a hose per second. Resistance Think of water flowing through a pipe. A narrow pipe has more resistance because of it slows the flow of water through the pipe. ELECTRICAL RESISTANCE slows down the current and is measured in OHMS () Conductors vs. Insulators •Conductors – material through which electric current flows easily; they have low resistance. Insulators – materials through which electric current cannot move; they are resistors. 4 Ohm’s Law V = IR Examples V Relates: I R Voltage (V) in volts (V) Problem 1: A stove is connected to a 180-V outlet. If the heating element has a resistance of 18 calculate the current flowing through it. V = IR Given: I = V/R V = 180 V I=? R = 18 Current (I) in amperes (A) Formula: I=V R I = 180 V I = 10 A Resistance (R) in ohms () R = V/I Therefore, the current flowing through the stove is 10 A Examples Examples Problem 2: In order to operate a hairdryer, the current required is 2.5 A. What is its resistance if the voltage supplied is 120 V? Formula: Problem 3: If the resistance of fridge is 130 and the current is 0.845 A, what is the voltage? Given: V = 120 V I = 2.5 A R=? R=V I R = 120 V 2.5 A R = 48 Therefore, the resistance of the hairdryer is 48 Question # 1 A light bulb supplied with a voltage of 36 V has a 6 A current flowing through it. Calculate the resistance of the light bulb. Given: V=? I = 8.45 A R = 13.0 Formula: V=IxR V = 8.45 A x 13.0 V = 109.85 V Therefore, the voltage of the fridge is 109.85 V Question #2 A light bulb has a resistance of 3 . What is the current in the light bulb when a voltage of 12 V is supplied? 5 Electric Power Formula P = IV Electric Power (P) P IV Relates: Power = Current x Voltage POWER IS MEASURED IN WATTS Remember, a Watt is defined as a Joule per second 1 W = 1 J/s I V Voltage (V) in volts (V) P=IV Current (I) in amperes (A) V=P/I Power (P) in watts (W) I=P/V Electric Energy (E) Conversions E Pt Electrical Energy = Power x Time WHERE: ELECTRICAL ENERGY IS MEASURED IN kilowatt-hours POWER IS MEASURED IN kilowatts TIME IS MEASURED IN hours P (kW·hr) (kW) (hr) • To convert from Watts (Joules per second) to kilowatts (kW), simply divide by 1000! • To convert from kW to kW·hr, simply multiply kW by the number of hours! NOTE: Example *The kilowatt-hr (kWhr) is a measure of ENERGY, not power! Power = Work / Time, so Power x Time = Work (Energy) Problem 3: Remember our fridge? Given: V = 109.85 I = 8.45 A R = 13.0 This meter measures the amount of electric work done in the circuits, usually over a time period of a month. The work is measured in kWhr. 1. How much power does the fridge use? 2. If it runs 10 hours per day, how many kW·hr will be used per month? (assume 1 month = 30 days) 3. How much will it cost (assume 1 kWhr = 10 cents) 6 END 7