Motors and Generators Contents1. Motors use the effect of forces on current-carrying conductors in magnetic fields3Discuss the effect on the magnitude of the force on a current-carrying conductor of variations in:3The strength of the magnetic field in which it is located3The magnitude of the current in the conductor3The length of the conductor in the external magnetic field3The angle between the direction of the external magnetic field and the direction of the length of the conductor3Describe qualitatively and quantitatively the force between long parallel current-carrying conductors:33Define torque as the turning moment of a force using:33Identify that the motor effect is due to the force acting on a current-carrying conductor in a magnetic field3Describe the forces experienced by a current-carrying loop in a magnetic field and describe the net result of the forces3Describe the main features of a DC electric motor and the role of each feature3Identify that the required magnetic fields in DC motors can be produced either by current-carrying coils or permanent magnets32. The relative motion between a conductor and magnetic field is used to generate an electrical voltage4Outline Michael Faradays discovery of the generation of an electric current by a moving magnet4Define magnetic field strength B as magnetic flux density4Describe the concept of magnetic flux in terms of magnetic flux density and surface area4Describe generated potential difference as the rate of change of magnetic flux through a circuit4Account for Lenzs Law in terms of conservation of energy and relate it to the production of back emf in motors4Explain that, in electric motors, back emf opposes the supply emf4Explain the production of eddy currents in terms of Lenzs Law43. Generators are used to provide large scale power production5Describe the main components of a generator5Compare the structure and function of a generator to an electric motor5Describe the differences between AC and DC generators5Discuss the energy losses that occur as energy is fed through transmission lines from the generator to the consumer5Assess the effects of the development of AC generators on society and the environment54. Transformers allow generated voltage to be either increased or decreased before it is used6Describe the purpose of transformers in electrical circuits6Compare step-up and step-down transformers6Identify the relationship between the ratio of the number of turns in the primary and secondary coils and the ratio of primary to secondary voltage6Explain why voltage transformations are related to conservation of energy6Explain the role of transformers in electricity sub-stations6Discuss why some electrical appliances in the home that are connected to the mains domestic power supply use a transformer6Discuss the impact of the development of transformers on society65. Motors are used in industries and the home usually to convert electrical energy into more useful forms of energy6Describe the main features of an AC electric motor6

1. Motors use the effect of forces on current-carrying conductors in magnetic fieldsDiscuss 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 locatedThe force is proportional to the magnetic field strength, B The magnitude of the current in the conductorThe force is proportional to the current, I The length of the conductor in the external magnetic fieldThe force is proportional to the length of the conductor, l The angle between the direction of the external magnetic field and the direction of the length of the conductorThe force is proportional to the component of the field that is perpendicular to the conductor. If the angle between the conductor and the field is , the force is the maximum value multiplied by sin.

Describe qualitatively and quantitatively the force between long parallel current-carrying conductors:

k = 2.0 x 10-7 N A-2

The two conductors have a force pushing it towards each other if the currents are going the same direction right hand palm rule.

Define torque as the turning moment of a force using:Torque is the tendency for an object to rotate around a pivot point the product of the tangential component of a force applied a distance away from the pivot:

: torque (Nm) F: force (N) d: distance (m) If an angle is involved, the force component which is perpendicular to the distance is used or:

Identify that the motor effect is due to the force acting on a current-carrying conductor in a magnetic fieldThe flowing electrons (current) are affected by the magnetic field hence a force acts upon it. The motor effect is the action of this force on a current carrying conductor when in an external magnetic field.

Describe the forces experienced by a current-carrying loop in a magnetic field and describe the net result of the forcesA current carrying loop will experience force due to the motor effect. Opposite ends of the loop have currents going in opposite directions therefore the force applied on one end is in the opposite direction to the other. If the loop is horizontal, the forces are applied tangentially (on one end upwards and the other downwards) and it rotates along an axis.

Describe the main features of a DC electric motor and the role of each feature Stator: the stationary permanent magnets or electromagnets used to produce the external magnetic field Armature: the frame in which the current carrying coil is wound around this is the component which rotates on an axis due to the motor effect Brush: a device used to connect the positive and negative terminals from the power source to the commutator they brush against the commutator as it rotates. Commutator: a split metal ring device used to reverse the direction of the current to allow the armature to continue rotating in one direction. At each half turn, the brushes connect to the other part of the commutator since it is two separate parts allowing the current to flow in the opposite direction in the coil. Each brush retains the same polarity

Identify that the required magnetic fields in DC motors can be produced either by current-carrying coils or permanent magnetsThe magnetic fields used in DC motors can be produced from either permanent magnets or an electromagnet (by wounding coils around a soft iron core). The coils used in the armature can also be used to coil around an iron core to produce an electromagnet e.g.

2. The relative motion between a conductor and magnetic field is used to generate an electrical voltageOutline Michael Faradays discovery of the generation of an electric current by a moving magnetFaraday firstly performed an experiment observing that when a current runs through the primary coil it will induce a temporary current in the secondary coil i.e. the galvanometer needle responded but returned to its original position.

Then by winding coils around an iron ring (iron core), he found the same result but with greater temporary currents induced when there was a current in the primary coil. This led to his conclusion that during the change of magnetic fields from the primary coil, a current is induced in the second coil. A moving magnet can induce a current in a coil; the magnitude of the current is affected by the speed of the magnet, direction and which end of the magnet (north or south).

Define magnetic field strength B as magnetic flux densityThe magnetic field strength, B, can be defined as the magnetic flux density measured in Tesla (T) or Weiber per square metre (Wbm-2) how much magnetic flux is passing through a unit area

Describe the concept of magnetic flux in terms of magnetic flux density and surface areaMagnetic flux B is the amount of magnetic field passing through an area (or threading through a coil) measured in Weiber (Wb). This can be calculated as the product of the component of the strength of the magnetic field flowing perpendicularly through a given area:

B: magnetic flux (Wb) B: magnetic field strength (T or Wbm-2) A: area (m2)

Describe generated potential difference as the rate of change of magnetic flux through a circuitFaradays Law of Induction: The induced emf in a circuit is equal in magnitude to the rate at which the magnetic flux through the circuit is changing with time

: electromagnetic force n: number of turns/coils B: change in magnetic flux (Wb) t: change in time (secs) A change in magnetic flux could mean that there was a change in magnetic field strength and/or the area of the coil. Note that in rotating coils (motors and generators) the magnetic flux is at a maximum when the plane of the coil is perpendicular to the flux direction and is at a minimum when it is parallel.

Account for Lenzs Law in terms of conservation of energy and relate it to the production of back emf in motorsLenzs law: An induced emf always gives rise to a current that creates a magnetic field that opposes the original change in flux through the circuit An induced emf is produced in a way so that a current creates a magnetic field which opposes the change in the incoming magnetic flux or the motion of the magnet. This is because of the conservation of energy. It has to oppose the change in magnetic flux otherwise if not, the magnetic field from the coil will attract the magnet, increasing the magnetic flux and increasing the current produced which increases the magnetic flux further and it keeps going the induced current keeps increasing. This is not possible as energy would be created without doing work. The electrical energy produced is from the work done when moving a magnet towards or further away from the coil thus the induced emf produces a current where the magnetic field opposes the change in flux. In motors, there is a constant change in magnetic flux threading through the coil as it rotates thus inducing an emf (back emf) opposite in direction to the input supply (supply emf), slowing the motor.

Explain that, in electric motors, back emf opposes the supply emfBack emf is induced in a rotating coil opposing the change producing it. Originally, the rotation is caused by the supply emf which induces the back emf and opposes this direction thus the back emf opposes the supply emf. At higher speeds, there will be a higher back emf which means less net emf (net emf = supply emf back emf) meaning there will be a reduction in current going through the armature. If there was a load slowing down the motor, then there will be little back emf thus high currents will flow through the coil and could possibly burn out the motor. A motor rotating rapidly will induce a back emf which opposes the supply emf thus reducing the net emf or current. Explain the production of eddy currents in terms of Lenzs LawEddy currents are produced when there is a change in flux and therefore back emf is induced. The charges flow in a circular path or a loop to create a magnetic field, under Lens Law, opposing the change in flux. If there is an increase in flux, the eddy currents will produce a magnetic field to counteract the increasing flux. If there is a decrease in flux, it will produce a magnetic field to increase back the flux. This can occur when a conductor is entering or leaving a magnetic field or when there is a change in magnetic field. Also, there is a force on the charges opposing the direction of movement (right hand grip rule) of the conductor because the emf opposes the change in flux.

3. Generators are used to provide large scale power productionDescribe the main components of a generator Stator: the stationary parts in a generator, includes magnets or electromagnets providing the magnetic field Rotor: the rotating coil in the generator experiencing changes in flux which induces an emf Slip rings (AC): the two ends of the wire are each connected to a rotating slip ring where brushes are used to transfer current Commutator (DC): a split ring where each end of the wire is connected to each part used to provide a single direction of current into a circuit exactly like the one used in motors

Compare the structure and function of a generator to an electric motorThe structure of a generator is similar to that of a motor however the difference is its functions. A motor converts electrical energy into kinetic energy (e.g. car engine turns the wheels) and a generator converts kinetic energy into electrical energy uses the rotation of the rotor to generate electricity.

Describe the differences between AC and DC generatorsIn a DC generator, emf produced is at a maximum when the magnetic flux is zero since it is the greatest change in flux and emf is at a minimum when the magnetic flux is at a maximum no change in flux. Also, there is only one direction of current in the external circuit hence a split ring commutator is used so that the brushes retain the same polarity even when the current in the coil reverses. In the figure to the right, the current is flowing from d to a because it wants to oppose the increase in magnetic flux. Once the coil reaches maximum flux (half cyle) it then reverses the direction of the current but the split ring commutator keeps the brushes at the same polarity (positive or negative) thus the varying-in-magnitude current in the external circuit always flows in one direction hence direct current (DC)AC generators use slip rings instead of the split ring commutator. Each end of the wire is connected to a different slip ring. Their only function is to simply conduct the current onto the brushes and into the external circuit without the entanglement of the wires. But because of the changing current in the wire, the polarity of the brushes also changes periodically (graph below) hence the direction of current in the external circuit alternates hence alternating current (AC)

In the power industry, three-phase AC generators are used which produces three sets of identical currents but are out of phase by 120o (current waves shown below in the graph). The magnet rotates such that the magnetic flux changes in each stationary coil (stator) thus producing the currents.

AC generatorDC generator

AdvantagesVoltage can be changed using transformers Certain devices utilise DC current (battery recharges and cathode ray tubes)Generally more powerful

DisadvantagesFrequencies of all AC generators in a region must be synced (outputs in phase)Dangerous frequency (50 Hz)Greater insulation requiredUnwanted induction More costly (construction and maintenance) Sparks are produced between the split gapOutput loses more energy during transmission

Discuss the energy losses that occur as energy is fed through transmission lines from the generator to the consumerEnergy is always lost during transmission due to the heat produced when current flows in the wires and since the wires are very long (resistance is higher) energy losses are considerable. The equation describes the energy losses occurring:

(Can be derived from V = IR and P = IV)Transformers are used to minimise energy losses by changing the voltage thus changing the currentWestinghouse VS Thomas Edison There was a competition between AC (Westinghouse) and DC (Edison) transmission for supplying electricity to cities. The AC system was much more efficient and was favoured. In AC the high speed rotating commutators were not a problem and energy losses during transmission can be minimised using transformers. Transmission wires had to be: Protected from lightning An overhead parallel wire is connected to the earth Current from lightning will run through this neutral wire down to earth Insulated from supporting towers Ceramic or porcelain disk stacks are used (insulator and minimises the chance of a spark) Prevents the wire from touching the tower (stops becoming live or short circuiting)

Assess the effects of the development of AC generators on society and the environmentPositiveNegative

Improved standard of living Consumers enjoy the benefits of having electricity Efficient and clean energy Compared to using fuelled devices such as diesel lamp or wood-fire stove Concentration in production AC production can be mass produced in one centre; pollution can be managed Regulation In one centre, data can be recorded easilyMinimised energy loss Generators can be far away without losing too much energy during transmissionDevelopment of industry Improves the speed of technological advances

Pollution Fossil fuels are used; global warming. Discharge of hot water; thermal pollution. Loss of natural habitats Production of large centres requires large plots of lands possibly destroying the animal inhabitants Accidents Electrical shocks can cause serious harm or death. Fires are also possible Labour replacement Labour tasks are replaced by electrical tools and devices thus loss in jobsOverwhelmed industrialisation Increasing industrialisation can cause effects to society and the environment

4. Transformers allow generated voltage to be either increased or decreased before it is usedDescribe the purpose of transformers in electrical circuitsTransformers alter the voltage that passes through them, either increases or decreases. It consists of: Primary coil: input voltage Secondary coil: output voltage; number of coils usually differ from the primary coil Soft iron core: where the coils are wound so flux can flow through the coils

The input voltage in the primary coil produces a changing magnetic flux flowing through the core. The secondary coil experiences the same changing magnetic flux from the input and because the number of coils is different, the voltage output is also different. But it is not 100% efficient; some energy is lost mainly due to heat in the coils and in the soft iron core. Eddy currents will produce heat to the changing magnetic flux and thus some energy is lost. To prevent this lamination is used where insulating material is placed between sheets of iron to increase the resistance to the flow of eddy currents. Compare step-up and step-down transformers Step-up: output voltage is higher; number of coils in secondary are higher than primary Step-down: output voltage is lower; number of coils in secondary are lower Identify the relationship between the ratio of the number of turns in the primary and secondary coils and the ratio of primary to secondary voltageUsing Faradays Law:

Divide one on the other

This means that if the secondary coil has more or less coils than the primary coil the output voltage will be more or less respectively.

Explain why voltage transformations are related to conservation of energyThe conservation of energy states that energy cannot be created or destroyed. Energy is measured as Power (P) and in transformers the input power is the same as the output power. In power remains constant and voltage and current change accordingly.

Explain the role of transformers in electricity sub-stationsFrom it is seen that current affects the power losses in transmission lines by heating it up and dissipating into the air and also increases the resistance. To minimise energy losses as much as possible, the voltage is increased at a step-up transformer so that current decreases but the power is still the same:. This allows an efficient wide distribution of electricity so that users do not have to be located near a power station. A step-down transformer reduces the voltage to a safe and suitable level for use.

Discuss why some electrical appliances in the home that are connected to the mains domestic power supply use a transformerGenerally, appliances run on low DC voltage (3V-12V). However the electricity supplied is 240V so a step-down transformer, as well as a rectifier, is needed to provide suitable power to the appliance. The transformer is usually a power-cube plugged into the socket or is in-built with the appliance.

Discuss the impact of the development of transformers on societyTransformers have allowed the wide distribution of electricity across a wide area resulting in more people having access to higher standards of living and cheaper electricity prices since it is an efficient system. Also the power station can efficiently supply electricity in a large area reducing the fossil fuels burnt and also means that users do not have to be very close to the station. This also means that the pollution is not produced in metropolitan areas but rather in a concentrated and isolated area. However, with the increasing demand for electricity, fossil fuels are being used more and more which contributes to the enhanced global warming effect and reducing our limited supply. It also means that there is a reduction in unskilled jobs or manual labour which leads to unemployment for many people. Thus transformers have had a significant impact on society providing electricity to homes and industries but also have detrimental effects on the environment. 5. Motors are used in industries and the home usually to convert electrical energy into more useful forms of energyDescribe the main features of an AC electric motorAC electric motorSimilar structure to that of a DC motor except uses slip rings commutators. The coil still rotates fully because the current is alternating thus the frequency is one of the factors of the rotational speed. AC universal motor Similar structure to that of a DC motor using a commutator except the wires are wound around the stator making it electromagnets. It can run on both DC and AC because of this feature. It is similar to how a DC motor functions. AC works as well because there is changing magnetic flux along with the direction of current. AC induction motor (three-phase)This motor induces current in the rotor instead of supplying it from a direct source. Because of this there is no contact thus the rotor can rotate freely and there is little wear and tear. The statorThe stator has 6 coils (3 pairs) around iron cores. The magnetic field rotates around because each pair is connected to each phase from the AC supply.

The rotorThis consists of aluminium bars attached to two end rings forming a cylindrical squirrel-cage. Iron laminates are then used to reduce heating due to the induced eddy currents and intensify the magnetic flux.

In the diagram below, the magnetic field is moving to the right or it could be thought as the magnet moving to the left. The rotating magnetic flux induces currents (according to the right hand push rule below). There is a force pushing the wire due to the motor effect and the rotor rotates with the magnetic field.

RIGHT HAND PUSH RULE (inducing currents in wires)1. The thumb points in the direction of movement of positive charges relative to the magnetic field2. The fingers point in the direction of the magnetic field3. The palm of the hand shows the direction of the force on positive charges and consequently the direction of the induced current.

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