Electricity and Circuits Unit 11 Electricity and Circuits

Starter If you are given a light bulb, What other equipment would you need to light it up? How would you set up the equipment? http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc 2

I. Current Battery acts as the pump to push the charges (electrons) around the circuit http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc 3

I. Current B. Current (definition): HOW QUICKLY CHARGE MOVES Equation (see ref. tabs.) I = Current Δq = change in charge t = time I = Δq t Units: C/s = Ampere (A) 4

“It’s the volts what jolts, but it’s the mills that kills.” I. Current Note: High currents through your body can cause serious injury or death. Here are a few of the typical consequences of different currents “It’s the volts what jolts, but it’s the mills that kills.” 5

II. Conditions Necessary for Electric Current CHANGE IN POTENTIAL What is needed? A ___________ between ______ points in the circuit 2 Provided by: Battery or chemical cell Most types of 9 V batteries consists of 6 1.5 V cells added in series 6

III. Ohm's Law A. Potential Difference (V): Change in electric potential energy between two positions Units: PROVIDES THE ELECTRICAL “PUSH” Volts (V) B. Resistance: OPPOSES THE FLOW OF CHARGES Units: Ohm (Ω) 7

C. Ohm’s Law (see ref. tabs.) R = Resistance (Ω) V = Potential Difference (V) I = Current (A) R = V I 8

IV. How to Measure Current: Device used: AMMETER How to set it up: Place in series (line) with circuit 9

IV. How to Measure Potential Difference (voltage): Device used: Voltmeter How to set it up: Place it across the device you want to measure 10

VI. Resistivity Resistivity: How well a specific conductor allows a current to move through it Factors that affect the resistance: 1. Area (↑ A, ↓ R) 2. Length (↑ L, ↑ R) 3. Material (resistivity – ↑ ρ, ↑ R) (see ref. tabs) 4. Temperature (↑ T, ↑ R) http://phet.colorado.edu/en/simulation/resistance-in-a-wire 11

VI. Resistivity Equation (see ref. tabs) ↑ 12

VI. Resistivity Example: Determine the resistance of a copper wire that has a cross-sectional area of 2 x 10 -3 m 2 and 40 m long. 13

VII. Electric Power Mechanics’ Equation for Power: P = W t Units: J/s = Watt (W) Electric Power: P = W = IV t Alternate Equations (see ref. tabs.): P = IV = I2R = V2 R 14

VII. Electric Power Example: A microwave draws 12.5 amperes of current and resistance of 9.6 ohms. What is the power dissipated in the resistor? 15

VII. Electric Power Example: A 60 W and 100 W light bulb are plugged into the wall that provides a potential difference of 120 V. Which bulb has the greater resistance? What is the resistance of both? 16

VIII. Electrical Energy Equations: W = Pt = IVt = I2Rt = V2t R Units: Joule (J) 17

VIII. Electrical Energy Example: A television set draws 2 A when operated on 120V. How much power does the set use? Calculate how much energy is used if the television is on for 2 hours a day. 18

VIII. Electrical Energy Example: An electric dryer consumes 1.0 x 106 J of energy when operating at 220 volts for 2 minutes. During operation, how much current does the hair dryer draw? 19

All parts are connected to provide a ______________ for the current IX. Series Circuit All parts are connected to provide a ______________ for the current SINGLE PATH Schematic Diagram: Draw a circuit diagram with three resistors set up in series hooked up to a 12 V battery. http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc 20

IX. Series Circuit Things to Remember (See Ref. Tabs) 1. Current Remains constant through each resistor Equation: I = I1 = I2 = I3 = … 21

IX. Series Circuit Things to Remember (See Ref. Tabs) 2. Equivalent Resistance Total resistance is equal to the SUM of all resistors Equation: Req = R1 + R2 + R3 + … 22

IX. Series Circuit Things to Remember (See Ref. Tabs) 3. Voltage Drops Applied (total) voltage from the power source equals the sum of the voltages across each device (resistor) Equation: V = V1 + V2 + V3 + … 23

IX. Series Circuit Example: Draw a circuit diagram for series circuit that contains a 5 ohm, 10 ohm, and 15 ohm resistor that are connected to a 20 V battery. Include an ammeter to read the total current and three voltmeters to measure the potential difference across each resistor. Resistor V I R P 1 2 3 Total Calculate the total resistance of the circuit.   Calculate the total current in the circuit. Calculate the potential difference across each resistor. 24

X. Parallel Circuit TWO OR MORE PATHS Allows ____________________ for the current to flow TWO OR MORE PATHS Schematic Diagram: Draw a circuit diagram with three resistors set up in parallel connected to a 12 V battery. 25

X. Parallel Circuit Things to Remember (See Ref. Tabs) 1. Current Sum of the currents in each of the branches is equal to total current Equation: I = I1 + I2 + I3 + … 26

X. Parallel Circuit Things to Remember (See Ref. Tabs) 2. Voltage Voltage remains constant across each device Equation: V = V1 = V2 = V3 = … 27

X. Parallel Circuit Things to Remember (See Ref. Tabs) 3. Equivalent Resistance As more resistors are added in parallel, total resistance DECREASES Equation: 1 1 1 1 = + + + … Req R1 R2 R3 28

X. Parallel Circuit Example: Draw a circuit diagram for parallel circuit that contains a 10 ohm, 15 ohm, and 20 ohm resistor that are connected to a 20 V battery. Include ammeters to measure the current through each and voltmeters to measure the potential difference across each resistor. 29

X. Parallel Circuit Calculate the equivalent resistance of the circuit. ii. What is the potential difference across each resistor? iii. Calculate the reading on each of the ammeters. iv. Calculate the total current (the current leaving the source). v. What would happen to the total current if more resistors are added in parallel? Res. V I R P 1 2 3 Total 30

XI. Conservation of Charge Like energy, there is a conservation of __________ in circuits CHARGE Junction Rule: _______ of the currents entering a junction must ________ the _______ of the currents leaving SUM EQUAL SUM 31

XI. Conservation of Charge 32

DC (Direct Current) (video clip/animation) XII. AC vs. DC DC (Direct Current) (video clip/animation) AC (Alternating Current) current (charges) flows in the same direction between the + and - terminals used in many electronic devices (ie – computers) can be produced by batteries, solar cells, fuel cells the direction of the current (charges) reverses (alternates) in the US it does this at a rate of 60/sec or 60 Hz advantage is that power companies save a lot of money transmitting power at very high voltages over long distances they convert AC to high voltages for transmission (above 100,000 V) then use transformers to step down to lower volts 33

XII. AC vs. DC Great Minds – Nikola Tesla (10 min) Thomas Edison (DC – Low Voltage) vs. Nikola Tesla (AC – High Voltage) Video (6 min) The high current problem P = IR2 34

XII. AC vs. DC Sending AC Electricity (How does electricity get to you?) Probably the biggest advantage of AC is that you can use high voltages with small currents to reduce losses when you transmit power. Remember that lost energy increases the more collisions you have, and reducing current decreases the amount of collisions (and reduces heating in the wires). You can send power with DC, but the DC power transmission loses a lot of energy. You would have to put much more effort into sending DC power over the same distance 35