HSC Physics. Module 9.3. Motors and. Generators

Transcription

1 HSC Physics Module 9.3 Motors and Generators

2 9.3 Motors and Generators (30 indicative hours) Contextual Outline Electricity is a convenient and flexible form of energy. It can be generated and distributed with comparative ease, and most importantly, it can be readily converted into other forms of energy such as heat, light, sound or mechanical energy. Electricity is a key element in the development of industrialised nations. There are many examples of electric motors both around the home: refrigerators; washing machines; vacuum cleaners; and in the community: industrial motors and trains. The design of a motor for an electrical appliance requires consideration of whether it will run at a set speed, how much power it must supply, whether it will be powered by AC or DC and what reliability is required. The essentials of an electric motor are the supply of electrical energy to a coil in a magnetic field causing it to rotate. The generation of large quantities of electrical power requires relative motion between a magnetic field and a coil. In the generator, mechanical energy is being converted into electrical energy while the opposite occurs in the electric motor. Once generated, electricity must be distributed over long distances from the power station to cities and towns. Transmission lines carry the electrical energy at a high voltage from the generator and transformers eventually reduce the voltage to that required by the consumer. The electricity produced by most generators is in the form of alternating current. In general AC generators, motors and other electrical equipment are simpler, cheaper and more reliable than their DC counterparts. AC electricity can be easily transformed into higher or lower voltages making it more versatile than DC electricity. Since the frequency of AC electricity can be precisely controlled, it is used in motors that require accurate speed, such as clocks and tape recorders.

3 Domremy HSC Physics Module 9.3 Motors and Generators : Student Notes Concept Map Conductors Magnetic Flux Magnetic Fields van Allen Radiation belt Force Loudspeake The Motor Effect torque Lenz s Law DC Motors Galvanometers AC Motors Commutator Back e.m.f Generators Eddy currents DC Generators AC Generators Transmission lines Transformers History of Generator Social Impact of Technnology Physiological Effects of Electrical Generation & Supply Electricity Sub-stations

4 Motors and Generators Module Plan Module Length: 7 weeks Focus Area Time Concept Resources Practical 1. Motors use 1. discuss the effect, on the Humphrey s Sets 1. (Act 1) solve problems using 45, 47 the effect of magnitude of the force on a F Dyett: k I I 1 2 forces on current-carrying conductor, of Contexts II: pp. l d current-carrying variations in: (Exp 2) perform a first-hand conductors in magnetic fields the strength of the magnetic field in which it is located the magnitude of the current in the conductor the length of the conductor in the external magnetic field the angle between the direction of the external magnetic field and the direction of the length of the investigation to demonstrate the motor effect conductor 2. describe qualitatively and quantitatively the force on long parallel current-carrying conductors: F l k I 1 I 2 d Humphrey s Set 45, 48 Dyett: (Act 3) solve problems and analyse information about the force on current-carrying conductors in magnetic fields using F BI 3. define torque as the turning moment of a force using: F d 4. describe the forces experienced by a currentcarrying loop in a magnetic field and describe the net result of the forces 5. identify that the motor effect is due to the force acting on a current-carrying conductor in a magnetic field 6. describe the main features of a DC electric motor and the role of each feature. 7. identify that the required magnetic fields in DC motors can be produced either by current-carrying coils or permanent magnets Contexts II: pp (Act 3) solve problems and analyse information about simple motors using nbiacos 5. (Act 4) identify data sources, gather and process information to qualitatively describe the application of the motor effect in: the galvanometer the loudspeaker

5 Domremy HSC Physics Module 9.3 Motors and Generators : Student Notes Focus Area Time Concept Resources Practical 1. outline Michael Faraday s Contexts II: pp. discovery of the generation of an electric current by a moving magnet 2. The relative motion between a conductor and magnetic field is used to generate an electrical voltage 3. Generators are used to provide large scale power production. 2. define magnetic field strength B as magnetic flux density 3. describe the concept of magnetic flux in terms of magnetic flux density and surface area 4 describe generated potential difference as the rate of change of magnetic flux through a circuit 5 account for Lenz s Law in terms of conservation of energy and relate it to the production of back emf in motors 6. explain that, in electric motors, back emf opposes the supply emf 7. explain the production of eddy currents in terms of Lenz s Law. 1. identify the main components of a generator 2. compare the structure and function of a generator to an electric motor 3. describe differences between AC and DC generators Humphrey s Set 49 Contexts II: pp (Exp 5) perform an investigation to model the generation of an electric current by moving a magnet in a coil or a coil near a magnet 2. (Exp 6) plan, chose equipment or resources for, and perform a first-hand investigation to predict and verify the effect on a generated electric current when: - the distance between the coil and magnet is varied - the strength of the magnet is varied - the relative motion between the coil and the magnet is varied 3. (Act 7) gather, analyse and present information to explain how induction is used in cooktops in electric ranges 4. (Act 8) gather secondary information to identify how eddy currents have been utilised in electromagnetic braking 1. (Exp 9) plan, chose equipment or resources for, and perform a first-hand investigation to demonstrate the production of an alternating current

6 Focus Area Time Concept Resources Practical 4. discuss the energy losses that occur as energy is fed through transmission lines from the generator to the consumer 2. (Act 10) gather and analyse information to identify how transmission lines are: insulated from supporting structures 4. Transformers allow generated power to be either increased or decreased before it is used 5. assess the effects of the development of AC generators on society and the environment 1. describe the purpose of transformers in electrical circuits 2. compare step-up and stepdown transformers 3. identify the relationship between the ratio of the number of turns in the primary and secondary coils and the ratio of primary to secondary voltage 4. explain why current transformations are related to conservation of energy 5. explain the role of transformers in electricity sub-stations 6. discuss why some electrical appliances in the home that are connected to the mains domestic power supply use a transformer 7. discuss the impact of the development of transformers on society Contexts II: pp Humphrey s Set 50 Contexts II: pp Contexts II: pp protected from lightning strikes 3. (Act 11) gather secondary information to discuss advantages and disadvantages of AC and DC generators and relate these to their use 4. (Act 11) analyse secondary information on the competition between Westinghouse and Edison to supply electricity to cities. 1 (Exp 12) perform an investigation to model the structure of a transformer to demonstrate how secondary voltage is produced 2. (Act 13) solve problems and analyse information about transformers using: v p v s n p n s 3. (Act 14) gather, analyse and use available evidence to discuss how difficulties of heating caused by eddy currents in transformers may be overcome 4. (Act 15) gather and analyse information and use available evidence to assess the need for transformers in the transfer of electrical energy from a power station to its point of use

7 Domremy HSC Physics Module 9.3 Motors and Generators : Student Notes Focus Area Time Concept Resources Practical 1. describe the main features Contexts II: pp. of an AC electric motor Motors are used in industries and the home usually to convert electrical energy into more useful forms of energy 1. (Exp 16) perform an investigation to demonstrate the principle of an AC induction motor 2. (Act 17) gather, process and analyse information to identify some of the energy transfers and transformations involving the conversion of electrical energy into more useful forms in the home and industry

8 HSC Physics C2: Motors and Generators Activity 1: Force per unit length Equation Aim: To solve problems using F l k I 1 I 2 d Do Humphrey s Sets 45, 47 Do Dyett problems HSC Physics C2: Motors and Generators Experiment 2: The Motor Effect Aim: To perform a first-hand investigation to demonstrate the motor effect Materials 1 X ½ inch test tube 1 m fine enamelled wire 1 bar magnet 4 pins polystyrene foam block electrical leads with alligator clips Bunsen burner steel wool Power pack Method 1. Wind the wire around the test tube to form a coil. Thread one end through the coil a few times to hold the coil together: 2. Run the ends of the wire quickly through a flame and polish them with steel wool to provide a shiny surface. 3. Set up as follows: N To of power pack To of power pack 4. The motor will work best on 4V ( B setting) on the power pack. A setting higher than this may cause spot welding. N.B. A second magnet may be necessary, or at least a stronger one. 5. Give the coil slight flicks to entice it to turn. 6. Having made the motor, turn the power pack to its highest setting and observe what happens. Discussion 1. What happens to the motor on the highest setting? 2. How can this experiment be modified to determine the conditions needed for torque.

9 Domremy HSC Physics Module 9.3 Motors and Generators : Student Notes HSC Physics C2: Motors and Generators Activity 3:Conductors & Magnetic Fields Aim: To solve problems and analyse information about the force on current-carrying conductors in magnetic fields using F BI To solve problems and analyse information about simple motors using nbiacos 1. Do Humphrey s Sets 45 and Do Dyett problems HSC Physics C2: Motors and Generators Activity 4: Galvanometers & Loudspeakers Aim: identify data sources, gather and process information to qualitatively describe the application of the motor effect in: the galvanometer the loudspeaker Identify and describe the various parts of a galvanometer. Describe the different types of galvanometers. Identify and describe the various parts of a loudspeaker. Describe the different types of loudspeaker. Evaluate the efficiency and frequency band response of each type of loudspeaker Write a 500 word report on this issue, including relevant diagrams. A bibliography must be included and in-text referencing used.

10 HSC Physics C2: Motors and Generators Experiment 5:Generating Electric Current Aim: To perform an investigation to model the generation of an electric current by moving a magnet in a coil or a coil near a magnet Materials Bar magnet Solenoid Galvanometer Major magnet 2 electrical leads Extra wire for the solenoid. Method 1. Connect the solenoid to the galvanometer using the leads. 2. Push the bar magnet, North pole first, slowly through the middle of the solenoid. Record the galvanometer reading and direction. 3. Repeat step 2 quickly. Record the result. 4. Repeat step 2 slowly using the South pole first. 5. Repeat step 2 quickly using the South pole first. 6. Increase the number of turns of wire on the solenoid by winding more wire onto it. 7. Repeat steps 2 to 5 for the new solenoid. Results Record your results in a suitable fashion. Discussion 1. Is a current generated when the magnet is stationary? 2. What is the relationship between the number of turns of wire in the solenoid and current generated? 3. What is the relationship between the speed of the magnet and current generated? 4. On the molecular level, what is the magnetic field doing to the electrons in the metal?

11 Domremy HSC Physics Module 9.3 Motors and Generators : Student Notes HSC Physics C2: Motors and Generators Experiment 6: Magnetism & Electric Current Aim: To plan, chose equipment or resources for, and perform a first-hand investigation to predict and verify by gathering and analysing information about the generated electric current when: - the distance between the coil and magnet is varied - the strength of the magnet is varied - the relative motion between the coil and the magnet is varied You must devise a method using equipment listed below and/or any other equipment you bring in. Equipment Available Power supplies and leads Bar magnets Ammeters and galvanometers Resistor boxes Any equipment that is reasonable (arrange with your teacher beforehand) You should consider the following points: Does the experiment satisfy the aim above? The safety of the experiment. Any safety notes need to be explicit. Design your own result table. Have you repeated the experiment several times to validate the results and to calculate a mean? Did you show your working? What are some possible sources of error? How could these errors be minimised or eliminated?

12 HSC Physics C2: Motors and Generators Activity 7:Electrical Induction Aim: Gather, analyse and present information to explain how the principle of induction applies to cooktops in electric ranges Compare the different types of electric cooktops available hot element, induction Describe how each operates Compare the efficiency of each. Write a 500 word report on this issue, including relevant diagrams. A bibliography must be included and in-text referencing used. HSC Physics C2: Motors and Generators Activity 8: Eddy Currents Aim: To gather secondary information to identify how eddy currents have been utilised in electromagnetic braking Write a 500 word report on this issue, including relevant diagrams. A bibliography must be included and in-text referencing used.

13 Domremy HSC Physics Module 9.3 Motors and Generators : Student Notes HSC Physics C2: Motors and Generators Experiment 9: Producing Alternating Current Aim: To plan, chose equipment or resources for, and perform a first-hand investigation to demonstrate the production of an alternating current You must devise a method using equipment listed below and/or any other equipment you bring in. Equipment Available Power supplies and leads magnets soft iron core long lengths of wire Any equipment that is reasonable (arrange with your teacher beforehand) You should consider the following points: Does the experiment satisfy the aim above? The safety of the experiment. Any safety notes need to be explicit. Design your own result table. Have you repeated the experiment several times to validate the results and to calculate a mean? Did you show your working? What are some possible sources of error? How could these errors be minimised or eliminated?

14 HSC Physics C2: Motors and Generators Activity 10: Transmission Lines Aim: To gather and analyse information to identify how transmission lines are: insulated from supporting structures protected from lightning strikes Write a 500 word report on this issue, including relevant diagrams. A bibliography must be included and in-text referencing used. HSC Physics C2: Motors and Generators Activity 11: AC vs DC Westinghouse vs Edison Aim: 1. To gather secondary information to compare advantages and disadvantages of AC and DC generators and relate these to their use in industry. 2. analyse secondary information on the competition between Westinghouse and Edison to supply electricity to cities. Write a 500 word report on this issue, including relevant diagrams. A bibliography must be included and in-text referencing used.

15 Domremy HSC Physics Module 9.3 Motors and Generators : Student Notes HSC Physics C2: Motors and Generators Experiment 12: Transformers Aim: To model the structure of a transformer to demonstrate how secondary voltage is produced. Materials Soft iron bar Two sets of long wire Resistor box Power supply and leads AC voltmeter AC ammeter Method 1. Set up the equipment as shown in the diagram below: wire Power supply 6V AC V 50 resistor Soft iron core 20 resistor Primary circuit Secondary circuit 2. Measure the voltage produced by the power supply and the secondary circuit. 3. Measure the current produced by the power supply and the secondary circuit. 4. Remove the soft iron core, intertwine the two coils together, and repeat steps 2 and 3. Discussion 1. What is the purpose of the soft iron core? 2. What is the relationship between the voltage and current in the primary and secondary circuits? 3. Why are the resistors used in the primary and secondary circuits?

16 HSC Physics C2: Motors and Generators Activity 13: Transformers II Aim: To solve problems and analyse information about transformers using 1. Do Humphrey s Set 50 v v p s n n p s I I s p 2. Cross country cables transmit voltages at 330,000 V AC at currents of 100A. What current is available when this transmission is stepped down to 10,000 V? What is the winding ratio for such a transformer? HSC Physics C2: Motors and Generators Activity 14: Heating in Transformers Aim: To gather, analyse and use available evidence to discuss how difficulties of heating caused by eddy currents in transformers may be overcome Write a 500 word report on this issue, including relevant diagrams. A bibliography must be included and in-text referencing used. HSC Physics C2: Motors and Generators Activity 15: Electrical Transmission Aim: To gather and analyse information and use available evidence to assess the need for transformers in the transfer of electrical energy from a power station to its point of use. Write a 500 word report on this issue, including relevant diagrams. A bibliography must be included and in-text referencing used.

17 Domremy HSC Physics Module 9.3 Motors and Generators : Student Notes HSC Physics C2: Motors and Generators Experiment 16: AC Induction Motors Aim: To perform an investigation to demonstrate the principle of an AC induction motor Materials Power tool that uses an AC induction motor. Tools to disassemble the power tool Method The power tool is taken apart and examined. Discussion 1. How is an AC induction motor different to other motors? 2. Why are the majority of motors AC induction type? HSC Physics C2: Motors and Generators Activity 17: AC Induction Motors II Aim: To gather, process and analyse information to identify some of the energy transfers and transformations involving the conversion of electrical energy into more useful forms in the home and industry Write a 500 word report on this issue, including relevant diagrams. A bibliography must be included and in-text referencing used.