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Lok PHYSICS โ Motor and Generators Section I- Motors use the effect of forces on current-carrying conductors in magnetic fields Perform a first-hand investigation to demonstrate the motor effect Aim: To demonstrate the motor effect. Hypothesis: N/A Materials: powerful bar magnets, conducting wire, DC power source, switch, resistor, ammeter (optional) Method: 1. Set up a circuit with the DC power source, wire, switch, resistor and ammeter. 2. Place a section of the circuit between the north and south poles of two bar magnets. 3. Turn on the power source and close the switch. 4. Observe the movement of the wire, in particular its direction relative to the magnetic field and current. 5. Repeat steps 3 and 4 but change the direction of the magnetic field as well as the strength of the current. Results: The wire was observed to move while a current flowed through it in the presence of a magnetic field. The direction of the movement was predicted by the right-hand palm rule (explained later). Conclusion: The motor effect was successfully demonstrated. Information regarding the motor effect is given throughout the rest of the section. Identify that the motor effect is due to the force acting on a current-carrying conductor in a magnetic field As demonstrated in the practical task above, the motor effect is due to the force acting on a currentcarrying conductor in the presence of a magnetic field. The precise relationship can be given by the right-hand palm rule, where the thumb points in direction of (conventional) current, the fingers in the direction of the magnetic field (north to south) and the palm indicates direction of force. Lok Discuss the effect, on the magnitude of the force on a current-carrying conductor, of variations in: - 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 conductor The variables B (magnetic field strength), i (current) and l (length) all relate to the force on the wire linearly. That is, a constant increase in one results in a constant increase in the force. This should be pretty obvious. The only variable that is not linear is the angle (angles are almost always related with ๐ sines and cosines). The force is greatest when the wire is perpendicular to the magnetic field (๐ = 2 ) and equal to zero when the wire is parallel (๐ = 0). Mathematically: ๐น = ๐ต๐๐๐ ๐๐๐ Solve problems and analyse information about the force on current-carrying conductors in magnetic fields using ๐น = ๐ต๐๐๐ ๐๐๐ These calculations should not really be hard, ESPECIALLY IF YOU DO (4U) MATHS. Personally Iโve never seen one ask you to calculate the angle. Make sure your calculator is set to degrees (since HSC physics loves using degrees instead of radians), and remember that the length is in metres, not centimetres. Here is an example: Calculate the force exerted on a wire of length 5cm if it is placed at 30o in a magnetic field of strength 0.01 Teslas and a current of 10 amps is flowing through. ๐น = ๐ต๐๐๐ ๐๐๐ ๐น = 0.01 × 10 × 0.05 × sin 30o ๐น = 0.0025 ๐ Remember to round off correctly in the real exam. I didnโt in this example. Describe qualitatively and quantitatively the force between long parallel current-carrying conductors ๐น ๐ผ1 ๐ผ2 =๐ ๐ ๐ AND Solve problems using ๐น ๐ผ1 ๐ผ2 =๐ ๐ ๐ The force per unit length on two parallel current-carrying conductors is related to the magnitude of the currents and the distance between them. Currents flowing in the same direction attract and currents flowing in opposite directions repel. Mathematically, their relationship is described by the equation: ๐น ๐ผ1 ๐ผ2 =๐ ๐ ๐ Here k is the magnetic force constant 2.0 x 10-7 NA-2. The magnetic force constant is provided on the data sheet. Calculations involving this equation should not be difficult. Remember lengths and distances are in metres. Lok Define torque as the turning moment of a force Torque is a vector that describes the โforceโ on an object around a pivot point. Technically it is not force as in Newtons, and is sometimes described as the โturning moment/strengthโ of a force. Conceptually, torque explains why sitting on the end of a see-saw makes you โheavierโ and why we use spanners to tighten/remove screws instead of our hands. Notice how a longer pivot length increases torque. Mathematically: ๐ = ๐น๐ The small symbol of the Greek letter tau is used for torque. F represents force in Newtons and p is the perpendicular distance. The length must be perpendicular to the direction of the applied force otherwise another form of this equation is needed (not part of HSC). Describe the forces experienced by a current-carrying loop in a magnetic field and describe the net result of the forces AND Solve problems using ๐ = ๐๐ต๐๐ด ๐๐๐ ๐ In a simple coil such as the one above, application of the right-hand palm rule shows that only the length of conductor on the left and right experience a force. In the above diagram, the right hand length experiences a force downwards and the left hand length experiences a force upwards. The net result is a clockwise rotational motion. Mathematically: ๐น = ๐ต๐๐ฟ , for the force on each length of wire ๐ ๐ = 2 (๐น × ) = ๐น๐ 2 ๐ = ๐ต๐๐ฟ๐ ๐ = ๐ต๐๐ด , where A is the area (Lb) ๐ = ๐๐ต๐๐ด cos ๐ , for the general case The reason cosine is used here instead of sine is because the angle measured here is different from the F = Bilsinฮธ one. That angle is rotated in the plane of the coil whereas this is the angle between the planes. Calculations involving this equation shouldnโt be difficult. Lok Describe the main features of a DC electric motor and the role of each feature Magnets Magnets form part of the stator (usually) and are arranged around the coil. Usually curved magnets are used to provide constant torque rather than the straight ones seen in diagrams. The magnets can be either permanent or electromagnetic. The role of the magnets is to provide a (radial) magnetic field surrounding the current-carrying coil so that it experiences a force. Coil and Armature Simple diagrams of motors show only one coil as part of the rotor. However, all working DC motors have many coils of conducting wire wrapped longitudinally around an armature, which is a core of laminated soft iron. As part of the rotor, the coil provides a structure which current can flow through, and the shape of the armature translates this into circular motion. Split-Ring Commutator Applying the right-hand palm rule after the coil has turned the first 90o shows that the force is now in the opposite direction (anticlockwise in the diagram above). Left uncorrected, the coil would merely oscillate around this vertical position and no net rotation would occur. This is corrected with a split-ring commutator, which is a small metal cylinder with two slits parallel to its length. The use of a split-ring commutator reverses the direction of current every half-cycle, making the coil rotate in one direction continuously. This is a noticeable feature of DC motors (AC motors do not use splitring commutators). Conducting Brushes Conducting brushes are pieces of conductor that make contact with the two halves of the commutator, providing current and linking the coils to the rest of the electric circuit. The brushes are usually graphite, which has the added advantage of being a lubricant. Springs hold the brushes tightly against the commutator to minimise sparking (which damages the motor). Axle The axle is a bar of metal (usually hardened steel) that passes through the middle of the armature. The axle provides structural support as the centre of motion for the rotor to rotate around. Work done by the motor is transferred via the axle (for example to the wheels of a vehicle). Identify that the required magnetic fields in DC motors can be produced either by current-carrying coils or permanent magnets As learnt previously, an electric current produces a magnetic field, and when arranged in a coil (solenoid), produces a magnetic field similar to that of a bar magnet. Thus, the required magnetic field in a DC motor can either be from permanent magnets or from a current-carrying coil. Lok Identify data sources, gather and process information to qualitatively describe the applications of the motor effect in the galvanometer and the loudspeaker The Galvanometer A galvanometer is a very sensitive ammeter that detects small levels of current. The main features of a galvanometer, depicted on the right, are a carrying coil, magnets and a restoring spring. When current flows through the circuit and through the galvanometer, the motor effect is induced, producing a force that rotates the needle to one direction. However instead of rotating as a motor would, a spring limits the movement of the needle, producing a reading of current (the force exerted by a spring is proportional to the amount it is rotated by, giving a linear scale). Some galvanometers have the needle zeroed in the centre. The Loudspeaker A loudspeaker usually contains a three-pronged magnet, with the middle projection being the opposite of the other two. A coil of conducting wire is wrapped around the middle, and a variable current applied (not regular DC). The interaction between the currentcarrying coil and the permanent magnetic structure via the motor effect produces varying forces on the magnet, which is transferred to a cardboard (usually) cone. This cone vibrates in accordance to the amplitude and frequency of the current, producing sound waves. current- Lok Section II- The relative motion between a conductor and a magnetic field is used to generate an electrical voltage Outline Michael Faradayโs discovery of the generation of an electric current by a moving magnet Michael Faraday is accredited with the discovery of electromagnetic induction, the phenomenon whereby relative movement between a magnetic field and a conductor generates a current. After Han Oersted showed that a current generated a magnetic field, many attempted to show the converse- that a magnetic field could generate current. Many did not succeed because they failed to realise that it was a moving magnetic field (or conductor) that was required. Faradayโs initial experiments involved an โinduction ringโ, a ring of iron with two separate coils of insulated copper wire wrapped around it. Faraday noticed that when he connected one coil to a DC source, there was momentary current in the other coil. He realised that the initial activation of a magnetic field was the cause of the current in the other coil. From there Faraday performed many more experiments regarding the newly discovered phenomenon of electromagnetic induction. Perform an investigation to model the generation of an electric current by moving a magnet in a coil or a coil near a magnet AND Plan, choose equipment or resources for, and perform a first-hand investigation to predict and verify when: - the distance between the coil and magnet is varied - the strength of the magnet is varied - the relative motion between the coil and magnet is varied Aim: To generate an electric current via electromagnetic induction and investigate factors that may affect the strength of the induced current. Hypothesis: N/A Materials: cardboard tube, length of conducting wire, bar magnets, centre-reading galvanometer Method: 1. Wrap the conducting wire around the cardboard tube to form a coil of about fifty turns. 2. Connect the ends of the wire to the centre-reading galvanometer. 3. Move a bar magnet through the solenoid in one direction. 4. Observe the reading on the galvanometer. 5. Repeat steps 3 and 4 but with varying distances, magnetic strength and relative motion. Results: When the magnet entered the solenoid a current was induced. When it moved inside the solenoid, no current was induced. When it left the solenoid, negative current was induced. A stronger magnet generated a stronger current, greater distance between the solenoid and magnet generated less current, and a greater relative velocity generated a greater current. Conclusion: An electric current was successfully generated via electromagnetic induction, and factors affecting this current were investigated. It was shown that relative motion between a magnet and conductor was required to generate current and that various factors could affect the strength of the current. Lok โ as magnetic flux density Define magnetic field strength ๐ต AND Describe the concept of magnetic flux in terms of magnetic flux density and surface area As with the other forces such as gravity, the magnetic (technically electromagnetic) force can be described using imaginary field lines. The strength of a magnetic field in an area is defined to be the number of arbitrarily designated field lines (flux) passing through a flat surface. Qualitatively, the more field lines, the greater the magnetic field strength. Thus the magnetic field strength can be determined as the number of field lines per unit area, i.e. magnetic flux density. Mathematically: ๐ ๐ด ๐ = ๐ต๐ด ๐ต= The magnetic flux of an area (i.e. the total number of field lines) is given by the product of the flux density (lines per unit area) and the surface area. [NOTE]- Magnetic field strength is actually a vector (as can be seen with the vector notation). The โlines through an areaโ definition gives only the magnitude not the direction, which is what the above formula shows. Describe generated potential difference as the rate of change of magnetic flux through a circuit Previously it was shown that a changing magnetic field was required to induce a current. More specifically, the magnitude of the induced emf (electromotive force) is proportional to the rate of change of magnetic flux. Faradayโs Law describes this relationship: โ๐ โ ๐๐ ๐๐ก Lenzโs Law (explained later) gives the constant of proportionality: โ๐ = โ๐ ๐๐ ๐๐ก Account for Lenzโs Law in terms of conservation of energy and relate it to the production of back emf in motors Lenzโs Law states that the direction of the induced emf is such that the current produced creates a magnetic field opposing the change that produced the emf. Simply put, the magnetic field created opposes the motion of the original magnet. For example, moving a north pole into a solenoid will induce a current such that the new magnetic field produces a north pole closest to the magnet (the repulsion opposes the motion). The direction of the current can be found with the right-hand grip rule (thumb in direction of north pole, fingers curl in direction of current). Lenzโs Law is a necessary consequence of the Law of Conservation of Energy. Because we are generating a current by moving the magnet (i.e. generating energy), kinetic energy of the magnet must be lost to compensate. If this were not so, and the induced magnetic field aided the motion, we would have increased kinetic energy and electrical energy. This is in violation of the Law of Conservation of Energy, and thus the induced magnetic field must oppose the motion, converting kinetic energy into electric energy. Lok Further application of Lenzโs law shows that it applies to electric currents as well as magnetic fields. A current-carrying conductor moving in a magnetic field will induce an emf that opposes the current that created the motion. This is known as back emf, with important consequences in electric motors. Explain that in electric motors, back emf opposes the supply emf In electric motors, a current is produced using an external emf (from the power source). The current in the coil generates a magnetic field which interacts with the existing magnetic field from the stator. This interaction produces the rotational motion of the coil. Once it is moving, however, Lenzโs Law takes effect. A moving conductor in the presence of a magnetic field will induce a current, the back emf, which opposes the original current (the supply emf). The result is that the net current is reduced, with an upper limit to the current in the coil. When the supply emf is equal to the back emf, the motor reaches a steady speed and is self-regulating. Although back emf may sound disadvantageous (after all you are losing current), it has one main significant feature making it desirable. This is that it provides resistance to the coil, preventing a short-circuit. Large amounts of current flowing through the coil can burn out the motor, and as such the back emf provides a self-regulating level of resistance. Motors are most at risk during start-up and if they are jammed, because there is no motion and thus no back emf, but the supply emf is still present. To minimise the risk, a starting resistance is sometimes added during activation, which is taken away after the back emf is significant. Explain the production of eddy currents in terms of Lenzโs Law Previously, Lenzโs Law was only applied to wire conductors. However, it is equally applicable to solid sheets or blocks of conductors, where they induced currents are termed eddy currents. The diagram above shows the formation of a simple eddy current. As a sheet of metal moves in or out of a magnetic field, currents are induced (determined using the right-hand palm rule) which often rotate in a loop. These eddy currents are formed in such a way that they generate a magnetic field to oppose the motion of the conducting metal. Lok Gather, analyse and present information to explain how induction is used in cooktops in electric ranges Induction cooktops use coils beneath a glass-ceramic surface to generate heat for cooking food. Alternating current is supplied to the coils, generating a rapidly oscillating magnetic field which induces eddy currents in the metal cookware above the coils. The eddy currents face resistance from the metal, which converts the energy to heat. The heat form the metal is then transferred to the food, cooking it. Induction heaters have the advantage in that almost all the energy goes into heating the cookware and not the surface of the cooktop, giving them a higher degree of efficiency. Gather secondary information to identify how eddy currents have been utilised in electromagnetic braking Electromagnetic braking involves using eddy currents to slow the motion of a wheel made from a conducting metal (e.g. aluminium). The figure to the right shows a simplified diagram of an electromagnetic braking system. Magnets are brought close to the rotating metal, and the relative motion generates eddy currents. As per Lenzโs Law, these eddy currents generate a magnetic field that is set up to oppose the motion of the original magnetic field. The interaction of the opposing magnetic fields creates a resistive force that quickly slows down the motion of the wheel, bringing it to a halt. Electromagnetic braking has the advantage in that the resistive force is proportional to the rotational velocity of the wheel, making it smoother than traditional braking systems such as disc brakes. Lok Section III- Generators are used to provide large-scale power production Describe the main components of a generator The components of a generator are virtually the same as a motor. An extensive description of the parts is given in the motor dot-point above. Magnets Generators have an array of magnets (permanent or electromagnetic) around the coil to provide the magnetic field. Usually they do not move and form part of the stator. In AC generators they may be part of the rotor. Coil and Armature The rotor contains a central armature and many coils wrapped around it. Rotating the coils in the magnetic field induces the current. In some AC generators they form the stator. Split-Ring Commutator/Slip Rings In DC motors, a split ring commutator is required to ensure that current flows in one direction only by switching the circuit every 180o. In AC motors the current is required to change direction constantly, so no split-ring commutator is required. Instead, slip rings are used. Conducting Brushes The conducting brushes carry current away from the coils. Like most other parts of the generator, they serve the same purpose in a motor. Plan, choose equipment or resources for, and perform a first-hand investigation to demonstrate the production of an alternating current Aim: To produce an alternating current. Hypothesis: N/A Materials: solenoid, bar magnet, centre-reading galvanometer Method: 1. Connect the solenoid to the galvanometer. 2. Move the magnet in and out of the solenoid continuously. 3. Observe the reading on the galvanometer Results: The reading on the galvanometer oscillated. Conclusion: The oscillating current indicated the production of an alternating current. You may have also been shown a larger AC generator that was operated with a handle and lit up a small bulb. Compare the structure and function of a generator to an electric motor Structurally, simple generators and motors are very similar, and most motors can be used as generators and vice versa. Applying a current to the structure of a motor/generator makes it rotate, operating as a motor. Rotating the rotor manually generates an induced emf, making it operate as a generator. The function of a motor is to convert electrical energy to kinetic energy, whereas the function of an electric motor is to convert kinetic energy into electrical energy. Lok Describe the differences between AC and DC generators Structurally, the main difference between AC and DC motors is that DC motors have split-ring commutators while AC motors have slip rings (there are also many differences in auxiliary equipment, but this is not required at the HSC level). DC generators, as their name suggests, are designed to produce a DC output. Previously it was shown that the induced current was proportional to the rate of change of flux: ๐๐ โ๐ = โ๐ ๐๐ก Observing the diagrams above, it can be seen that the flux is zero when the rotor is horizontal and at a maximum when the rotor is vertical. The induced emf is proportional to the rate of change of flux, and this is at a maximum when the rotor is moving about the horizontal position, and zero when the rotor is moving about the vertical position (opposite to the flux). The relationship between the position of the rotor and the size of the current is given by the absolute value of a sinusoidal graph (as shown). This is because the motion is periodic, and the split-ring commutator prevents a negative current. To produce a more constant current, radial magnets and multiple coils at an angle to each other are used. AC motors provide an AC output. Essentially they operate the same as a DC motor but without the split-ring commutator, and thus their current output graph looks like a regular sine/cosine wave. [NOTE]- Mathematically, flux is given as a sine curve, since it is really equivalent to the vertical height of the coil in two-dimensional space (vertical component of circular motion is given by sine). Because the induced current is proportional to the derivative, it is given by a negative cosine curve. When a sine curve is at zero, the corresponding cosine curve is at a maximum and vice-versa. Lok Gather secondary information to discuss advantages/ disadvantages of AC and DC generators and relate these to their use One advantage of AC generators over DC generators is that there is less wear and tear in the rotor. DC generators use split-ring commutators which, lead to constant striking of the carbon brushes against the leading edges of the commutator. The damage accumulates quickly, and thus DC motors require constant maintenance due to the split-ring commutator. In contrast, AC motors use slip rings which have a smooth, continuous surface. This reduces damage and removes the possibility of sparking. Overall, AC motors require less maintenance and are more reliable. In addition, DC motors must induce current in the rotor. For large currents, the rotor must be correspondingly heavier, requiring larger support structures. The high currents also create an opportunity for electrical arcing when the brushes and commutator separate, reducing efficiency and generating radio noise. Because AC generators do not require a split ring commutator, the current can be induced in the stator, with the magnets forming the rotor instead. This makes AC more suited to high-current applications. One advantage of a DC generator is that multiple coils can be arranged as part of the rotor at an angle to each other. This produces multiple currents out of phase, which add up to form a relatively constant current. This is not possible with an AC generator without additional equipment. Thus DC generators can easily supply a steady DC current for smaller devices. Analyse secondary information on the competition between Westinghouse and Edison to supply electricity to cities In the late 19th century, the competition between DC (developed and advocated by Thomas Edison) and AC (developed by Nikola Tesla and advocated by George Westinghouse) was such that it is labelled as the โWar of Currentsโ. Initially Edison had the advantage as his system of DC was well developed and he held a monopoly on electricity distribution. DC worked well as it was simple, but it had to be produced at high current for use in homes and factories, incurring huge energy losses (energy loss is proportional to the square of the current). This meant that many generators had to be built, and placed all around the city, connected with many unsightly power lines. However, Westinghouse realised the potential of AC in that it the voltage/current could easily be altered using a transformer. This gave it a huge advantage over DC, although it was less established than Edisonโs DC system. The ability of AC to be transformed meant it could be converted to high voltage for transportation and then reduced for used in homes and factories. This meant generating plants could be built far away from the busy cities and fewer and smaller power lines were required. The great economic advantages of AC allowed it to compete with Edisonโs DC as the primary source of power for America. Competition was rarely fair, with many attempts by Edison to destroy Westinghouseโs efforts to establish an AC electrical supply system. Edison began a smear campaign against AC, claiming that it was dangerous due to the high voltages involved (which is true). He strongly advocated for the use of AC as the cityโs chosen current for execution, believing that this would prove his point (he tried to have the term โelectrocutedโ known as โWestinghousedโ). However despite his efforts, AC had several important victories, including the contract for the Niagara Falls project and the Chicago World Fair. Eventually AC was chosen over DC due to its many advantages, and we still use AC predominantly today. Lok Discuss the energy losses that occur as energy is fed through transmission lines from the generator to the consumer Although transmission lines for electricity distribution are designed to be low resistance, nevertheless the long distances involved increases the total resistive force of the wire. Mathematically, the power loss in a wire is given by: ๐= ๐ผ2 ๐ Since the power loss is proportional to the square of the current, electricity is transported at the highest reasonable voltage, which results in low current. This minimises the power loss in energy cables, and is the reason AC is used over DC (because AC can easily be transformed to different voltages). A suitable choice of conductor, usually copper or aluminium is also used to minimise energy losses. Further energy losses can be attributed to the ionisation of air by the electron flow, which is minimised by insulation, and inductive energy losses from the formation of eddy currents in transformers, which are minimised by using laminations (explained later). Gather and analyse information to identify how transmission lines are: - insulated from supporting structures - protected from lightning strikes It is essential that transmission lines be sufficiently insulated to prevent arcing and short-circuits, both of which would damage structures, result in massive power losses and be a health hazard to citizens. Insulation from supporting structures is achieved with saucer-shaped ceramic or reinforced glass. These are hung downwards and the power lines wrapped around, preventing electricity from jumping to nearby structures and protecting them from rain and hail. Protection from lightning strikes is achieved by stringing a protective wire high above the normal transmission lines. This line attracts lightning strikes, and the surge of electricity is earthed to prevent damage to the pylon. Lok Assess the effects of the development of AC generators on society and the environment Prior to the establishment of electricity as our main source of energy, cities were extremely dirty, crowded areas. This was because factories and power plants have to be built close to where they were required, meaning cities were dotted with many power plants. Not only was this unsightly, but the pollution produced by the many coal plants in the city made city life extremely unhealthy. The development of AC, which can be transported easily over large distances, is responsible for the cities we see today. In modern times, electricity plants can be situated far away in the countryside, where they are not seen, and far fewer of them are needed. This has led to an improvement in living conditions and a subsequent increase in life-expectancy. Furthermore, the development of electricity as a power source has enabled the technological revolution to occur, resulting in the high-tech society we live in today. Almost every aspect of our lives relates to electronic devices in some way or other, and computers and digital media are slowly finding applications everywhere. The explosion in computer use in recent decades has also greatly increased societyโs technological capabilities, and this would only be possible with the invention of electricity. More evidence showing societyโs dependence on electricity can be seen with the economic losses incurred by blackouts and the presence of back-up generators in hospitals. However, the development of electricity has come at a cost to the natural environment as the demand for energy skyrockets. The need for energy is causing many natural areas to be cleared for power production, and rivers may be dammed for hydroelectric plants. The pollution given off by these plants also negatively impacts on the environment, although it is carefully monitored to minimise serious harm. Overall, it can be concluded that the development of AC generators has had a profound effect on society, allowing for the technological revolution we see today. It has also made cities far cleaner places to be, improving life expectancy. However, this comes at the detriment of the environment, which is negatively affected by the pollution given off by power plants. Lok Section IV- Transformers allow generated voltage to be either increased or decreased before it is used Describe the purpose of transformers in electrical circuits Transformers are used to change the voltage (and current) of a supply of electricity. This is done because many appliances require lower voltages and transportation of electricity requires high voltage to minimise energy losses. Transformers can only operate on AC, not DC. Perform an investigation to model the structure of a transformer to demonstrate how secondary voltage is produced Aim: To model a transformer to demonstrate the production of a secondary voltage. Hypothesis: N/A Materials: Soft iron rod, two lengths of insulated copper wire, DC power source, voltmeter Method: 1. Wrap one length of copper wire around the soft iron core 30 times. 2. Wrap another length of copper wire around the soft iron core 60 times. 3. Connect the coil of 30 turns to the DC power source. 4. Connect the coil of 60 turns to the voltmeter. 5. Turn on the power source and observe the reading. Results: When the power was activated, a voltage was produced for an instant. Conclusion: A secondary voltage was successfully produced in a model of a transformer. In theory the secondary voltage should be twice that of the primary voltage. Compare step-up and step-down transformers Step-up transformer Step-down transformer More turns in the secondary than the primary More turns in the primary than the secondary Higher voltage in the secondary than the primary Lower voltage in the secondary than the primary Used to increase voltage for efficient transmission Used in sub-stations to lower voltage to more useful levels Used for devices such as phone chargers and electronic displays Used in TVs and other high-voltage appliances 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 AND Solve problems and analyse information about transformers using ๐๐ ๐๐ = ๐๐ ๐๐ The relationship is given by: ๐๐ ๐๐ = ๐๐ ๐๐ The voltage is proportional to the turns in both primary and secondary. This is assuming no energy losses. Once again, calculations of this type should not be difficult. Lok Explain why voltage transformations are related to conservation of energy Voltage transformations alter voltage, and by the Law of Conservation of Energy, this must result in a change in current to compensate. Since time is constant (and thus can be neglected), the formula for power/energy is: ๐ = ๐๐ผ To conserve energy, power must be conserved (power is the rate of change of energy). Thus if voltage is increased, current is decreased and vice versa. Conceptually, it can be thought that the increased number of turns โspreads outโ the current, and thus voltage must increase to compensate. Gather and analyse secondary information to discuss the need for transformers in the transfer of electrical energy from a power station to its point of use As mentioned many times before, a high voltage is required for minimising power losses over long distances. However, household appliances do not operate at such high voltages (usually...), so transformers are used to decrease voltage to useful values in populated areas. In NSW, the voltage transformations go as follows: 20 kVโ 500 kV (power plant supply to long-distance transmission voltage) 500 kV โ 132 kV (at district transformer station) 132 kV โ 66 kV (at local transformer station) 66 kV โ 11 kV (at substation) 11 kV โ 240 V (pole transformers for household use) Notice how the voltages decrease as the supply approaches cities and suburbs. Some factories and heavy-industry areas require higher voltages, which they take straight off transformer or substations. Explain the role of transformers in electricity sub-stations Transformers in substations have the role of decreasing voltage from high-voltage transmission to practical voltage for household use. Household appliances cannot operate at 500 kV, and such high voltages would be extremely dangerous in suburbs and cities due to the sparking that could occur. Discuss why some electrical appliances in the home that are connected to the mains domestic power supply use a transformer Many electrical appliances come with a transformer, which usually looks like a small black device. Many of these transformers are built around the electrical plug, for example in mobile phone chargers. Some devices (e.g. game consoles) require larger transformers, and they usually connect the power port from the appliance to the socket. These transformers are required because some appliances do not operate with the 240V supplied to homes. Smaller devices such as mobile phone chargers operate on small voltages around 12V DC, and thus the charger has an in-built transformerrectifier that first reduces voltage then converts it to DC. Some appliances such as microwaves may have multiple transformers. The magnetron requires high voltage while the electronic display operates on low voltage, so two transformers are required. [NOTE]- The power plugs you see that are fat and prevent the use of sockets next to them are most likely transformers (and rectifiers). Lok Discuss the impact of the development of transformers on society The development of transformers has allowed for the efficient transfer of electrical energy over long distances. This means that power plants can be situated far away from population centres, reducing pollution and making cities less crowded. Because electricity can be sent over long distances, rural communities have also benefitted greatly from the development of transformers, and industrial factories no longer need to be situated close to a power plant. This has led to an increase in living standards and health of the population as a whole. Transformers also allow for the use of devices that operate on different voltages, such as microwave ovens and mobile phone chargers. Overall, the development of the transformer has revolutionised modern society, allowing electricity to be used as a necessity rather than a luxury. The rapid improvements in modern technology would not have been possible without transformers. Gather, analyse and use available evidence to discuss how difficulties of heating caused by eddy currents in transformers may be overcome As shown previously, changing magnetic flux through a conductor generates eddy currents that create heat via friction. With the high voltages involved in transformers, it is essential that this heat be minimised, otherwise the heat would reduce its efficiency and cause irreparable damage to the transformer (eddy currents can generate enormous amounts of heat, illustrated by the induction stove). Some ways in which the heat is minimised is listed: ๏ท Laminations (insulation) perpendicular to the plane of the magnetic field in the metal. This limits the size of the eddy currents formed, reducing energy loss by an order of magnitude. ๏ท Cooling fins that extrude from the back of transformers, presenting a large cooling surface area to the air. ๏ท Fans that circulate air. ๏ท A coolant system (usually oil) that removes heat from the transformer and radiates it away into the air. Lok Section V- Motors are used in industries and the home usually to convert electrical energy into more useful forms of energy Describe the main features of an AC electric motor A universal electric motor is essentially the same as a DC motor except for the slip-rings, which augment the split-ring commutator. It can run on AC or DC. The main features are: ๏ท An armature around which many coils are rotated. The force acts on the rotor, making it spin. ๏ท A field structure with magnets, either permanent or electro. This provides the magnetic field. ๏ท Slip rings which augment the split-ring commutator. AC power provides the current switch, so a split-ring commutator is not required for an AC supply. Of more interest is the AC induction motor, which is different to the motors studied previously. As the name suggests, the AC induction motor utilises the principle of electromagnetic induction to rotate a โsquirrel cageโ. The cage is made of a conducting material, and is situated on the inside of the motor. Around the outside is a circular array of many coils, which are activated sequentially in a rotating fashion. This generates a rotating magnetic field, which induces eddy currents in the bars of the cage. These eddy currents form their own magnetic fields which interact with the magnetic fields of the coils, โdraggingโ the rotor around with the rotating external magnetic field. Lok Almost all AC motors is use today are induction motors due to their many advantages. Some are listed below: ๏ท Simplicity of design ๏ท High efficiency and low maintenance required (because there is minimal contact and very few moving parts) ๏ท Ability to self-start (not always the case for AC motors) ๏ท Low costs Most induction motors are single-phase. Heavy-duty motors used in industry tend to be triple-phase. Perform and investigation to demonstrate the principle of an AC induction motor Aim: To demonstrate the principle of an AC induction motor. Hypothesis: N/A Materials: aluminium pie tray, plastic tub, water, bar magnet, string Method: 1. Pour a layer water into the plastic tub. 2. Flatten out the bottom of the pie tray (to minimise friction) and float on the water. 3. Tie a string around the magnet and twist. 4. Hold the magnet over the pie tray with the string and release to spin the magnet. 5. Observe the motion of the pie tray Results: The pie tray rotated in the same direction as the magnet. Conclusion: The pie tray and magnet effectively demonstrated the principles of the AC induction motor. 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 An energy transfer is energy moving from one body to another without changing form. An energy transformation is energy converting from one form to another. Energy transfers include: ๏ท Electricity moving from the power plant to a house ๏ท Electricity moving from the house supply to a mobile phone via a charger Energy transformations include: ๏ท Electrical energy changing to heat energy in a kettle or stove ๏ท Electrical energy changing to light energy in a TV ๏ท Electrical energy changing to kinetic energy in a washing machine ๏ท Electrical energy changing to chemical energy in electroplating (for industry)