Regents Physics Mr. Mellon Based on Chapter 22 and 23

Transcription

1 Name Regents Physics Mr. Mellon Based on Chapter 22 and 23 Essential Questions What is current? How is it measured? What are the relationships for Ohm s Law? What device measures current and how is it set up in a circuit? What device measures potential difference (voltage) and how is it set up in a circuit? What are the factors that affect the resistance of a wire? How does each of them affect the resistance? What is electric power? How it is measured? What are the equations for electrical energy? How is it measured? What is a series circuit? Be able to draw series circuits and calculate different variables. What is a parallel circuit? Be able to draw parallel circuits and calculate different variables. What is the junction rule and how does it connect to conservation of charge? I. Current A. Circuit Concept Water Pump VS. Charge Pump Battery acts as the pump to push the charge (electrons) around the circuit B. Current (definition): Equation (see reference tables): Units: Unit 11: Electricity and Circuits Page 1

2 II. Conditions Necessary for Electric Current What is needed? A between points in the circuit Can be provided by: III. Ohm s Law A. Potential Difference (V): Change in electric potential energy between two positions Provides the electrical Units: B. Resistance: the of charges Units: C. Ohm s Law (See Reference Tables): IV. How to measure current: Device used: How to set it up: Place in with circuit V. How to measure potential difference (voltage): Device used: How to set it up: Place it the device you want to measure Circuit Diagram with an Ammeter and Voltmeter to measure current and voltage in a light bulb (see reference tables): Unit 11: Electricity and Circuits Page 2

3 Whiteboard Problems: coulombs of charge pass a given point in a conductor in 4 s. Calculate the current in this conductor. 2. In a television set, an electron beam with a current of 5 x 10-6 ampere is directed at the screen. Approximately how many electrons are transferred to the screen in 60 seconds? 3. A potential difference of 12 volts is applied across a circuit which has a 4 ohm resistor. What is the magnitude of the current in the circuit? 4. A lamp has a current of 2 milliamperes at 6 volts. What is the resistance of the lamp? VI. Resistivity Resistivity - A characteristic of a material that depends on its electronic structure and temperature Factors that affect the resistance of a conductor 1. : Increase, Resistance) 2. : Increase, Resistance) 3. : Increase, Resistance) SEE REFERENCE TABLES FOR VARIOUS RESISTIVITIES 4. : Increase, Resistance). Resistance of a Wire Equation (see Reference Tables): Example 1: Determine the resistance of a copper wire that has a cross-sectional area of 2 x 10-3 m 2 and 40 m long. Example 2: Determine the resistance of a 15 gauge (radius = 7.25x10-4 m) gold wire that is 5.0 m long. Unit 11: Electricity and Circuits Page 3

4 Whiteboard: 1. A potential difference of 12 volts is applied across a circuit having a 4 ohm resistance. What is the current in the circuit? 2. If the temperature of a metal conductor is reduced, its resistance will increase, decrease, or remain the same. 3. A 60 W light bulb has a resistance of 240 ohms. If the lamp in which the bulb is used is plugged into a 120 volt outlet, what current does the bulb draw? 4. Calculate the resistance of 3 m gold wire with a cross sectional area of 0.03 m Calculate the length of an aluminum wire that has a cross-sectional area of 3 x 10-3 m 2 and a resistance of 20 ohms. 6. An aluminum wire has a resistance of 48 ohms. What is the resistance of a second piece of wire of the same composition, same diameter, and at the same temperature, but with one half the length of the first wire? 7. Sketch a graph that shows the relationship between the resistance of a copper wire (y-axis) of uniform crosssectional area and the wire s length (x-axis) at constant temperature. VII. Electrical Power Mechanics Equation for Power: Units - Electrical Power: Alternate Equations (see reference tables): Example 1: A microwave draws 12.5 amperes of current and resistance of 9.6 ohms. What is the power dissipated by the microwave? Example 2: 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? Unit 11: Electricity and Circuits Page 4

5 VIII. Electrical Energy Equations (see Reference Tables): Units: Example 1: A television set draws 2 A when operated on 120 V. A. How much power does the set use? B. Calculate how much energy is used if the television is on for 2 hours a day. Example 2: An electric dryer consumes 1.0 x 10 6 J of energy when operating at 220 volts for 2 minutes. During operation, how much current does the dryer draw? IX. Series Circuit All parts are connected to provide a for the current Schematic Diagram: Draw a circuit diagram with three resistors set up in series connected to a 12 V battery. Unit 11: Electricity and Circuits Page 5

6 Things to Remember for Series Circuits (See Reference Tables): 1. Current: Remains constant through each device (resistor) Equation: 2. Equivalent Resistance: Total (equivalent) resistance is equal to the sum of all resistors Equation: 3. Voltage Drops: Applied (total) voltage from the power source equals the sum of the voltages across each device (resistor) Equation: 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 Total i. Calculate the total resistance of the circuit. ii. Calculate the total current in the circuit. iii. Calculate the potential difference across each resistor. Unit 11: Electricity and Circuits Page 6

7 X. Parallel Circuit Allows for the current to flow Schematic Diagram: Draw a circuit diagram with three resistors set up in parallel connected to a 12 V battery. Things to Remember for Parallel Circuits (See Reference Tables): 1. Current: Sum of the currents in each of the branches is equal to the total current Equation: 2. Voltage: Voltage remains constant across each device (resistor) Equation: 3. Equivalent Resistance: As more devices (resistors) are added in parallel, the total resistance decreases. Equation: 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. Unit 11: Electricity and Circuits Page 7

8 i. Calculate the equivalent resistance of the circuit. Resistor V I R P ii. What is the potential difference across each resistor? Total 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? XI. Conservation of Charge Like energy, there is a conservation of in circuits Junction Rule: of the currents entering a junction must the of the currents leaving. Unit 11: Electricity and Circuits Page 8

9 Examples: What is the reading on each of the ammeters and direction of current (if not identified)? XII. Combination (Compound) Circuit Steps to calculate total (equivalent) resistance: 1. Break circuit into series and parallel parts 2. Find equivalent resistance of parallel devices 3. Find equivalent resistance of series devices Break the following circuit down to a single resistor: Example Two 60 ohm resistors are connected in parallel. This parallel arrangement is connected in series with a 30 ohm resistor. The combination is then placed across a 120 V battery. A) Draw a diagram of the circuit Unit 11: Electricity and Circuits Page 9

10 B) What is the equivalent resistance of the parallel portion of the circuit? C) What resistor could replace the three original resistors? D) What is the current in the circuit? XIII. AC vs. DC (BONUS) A) DC (Direct Current) (HOW WE REFER TO CURRENT IN THIS UNIT) 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 B) AC (Alternating Current) The direction of the current (charges) reverses (alternates) In the US it does this at a rate of 60/sec or 60 Hz using 120V or 240 V. However, in most of Europe the rate is at 50 Hz using approximately 240V. 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 Unit 11: Electricity and Circuits Page 10

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15 Name Regents Physics Unit 11: Electricity Mr. Mellon SERIES CIRCUIT PRACTICE Remember that in a series circuit: the current in every part of the circuit (is the the same. same, adds up). the voltage supplied by the battery is the TOTAL voltage of the circuit, and the voltage drops across each resistor (is adds the up same, to the adds total up voltage. to) the total voltage. to the calculate total resistance total resistance, (equivalent (add resistance) them). is equal to the sum of each of the resistors Unit 11 Work Packet: pg. 15 5

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19 Name Regents Physics Unit 11: Electricity Mr. Mellon PARALLEL CIRCUIT PRACTICE Remember that in a parallel circuit: the current in the branches of the circuit (is the same or adds up). the voltage drops across each branch (is the same or adds up to) the total voltage. to calculate total resistance, (add or use reciprocals). Unit 11 Work Packet: pg. 919

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