Category Archives: HSC Physics – Motors and Generators

AC Motors and Energy Transformations

Describe the main features of an AC motor

The AC induction generator used in large-scale power stations has a very similar structure to an AC induction motor. However, in an AC induction generator, the rotor is an electromagnet powered by a separate DC circuit, and the stator consists of 6 coils. A source of torque is used to rotate the electromagnet at 50 revolutions per second, which causes AC electricity to be generated in the field coils.

There are three types of AC motors- standard AC motors, universal motors and AC induction motors, and they each work differently. A standard AC motor is essentially identical to an AC generator, with a stator providing a magnetic field, a rotor that current is passed through, and slip rings connecting the rotor to a circuit. In addition, an AC motor usually has a fan to keep the rotor cool, a ferromagnetic core in the rotor to strengthen the magnetic field and it runs at 50 revolutions per second, the same as the frequency of AC power oscillation (50Hz). A universal motor is similar to a DC motor. It can operate on an AC or DC supply. Power is fed in, and runs through electromagnetic stators before entering a commutator. Each brush is connected to a wire that comprises one of the field coils, and is also connected to one end of a circuit. With a DC source, the commutator switches the current and the motor operates. With an AC source, although the direction of current being fed into the commutator is varying, the same variations are fed into the field coils, with the net effect that AC oscillation is cancelled out and the motor runs.

AC induction motors are entirely different. Induction motors have a rotor that is not connected to a power source- instead changing flux is used to induce a current in the rotor. This means that there is very low friction as the rotor is not actually in contact with the rest of the motor, and it also means there is very little wear and tear. AC induction motors have a more complicated stator with several field coil pairs. There are a total of 6 field coils, and each opposite pair is fed one phase of triple-phase AC power. This sets up a rotating magnetic field inside the stator. The rotor of an induction motor is generally similar to a squirrel cage (the type that allows pets to run endlessly), with two end rings and aluminium or copper bars linking the end rings to form a cylindrical shape. This cylinder is encased in a laminated iron armature so that the magnetic field passing through the rotor cage is intensified. As the field rotates, it induces current in the bars of the squirrel cage. This creates a force in the same direction as the rotation of the magnetic field, from Lenz’s Law. The squirrel cage then rotates, ‘chasing’ the changing magnetic field.

Remember- AC motors have a stator, rotor and slip rings. They also use an iron core and usually a fan. A universal motor uses a commutator and has a magnetic field generated using field coils. AC induction motors have a stator with 3 pairs of field coils (for a total of 6), and a “squirrel cage” rotor.

Perform an investigation to demonstrate the principle of an AC induction motor

An AC induction motor relies on the principle that a moving magnetic field induces a current in the rotor with a direction that, according to Lenz’s law, causes the rotor to spin in the same direction as the magnetic field. We demonstrated this principle by using a thin aluminium disk suspended by a string from a clamp on a retort stand so that the disk was free to rotate. To demonstrate the principle of an AC induction motor, we moved a strong ceramic magnet in circles around the circumference of the disk. The induced eddy currents caused the disk to rotate in the same direction as the magnet, thereby demonstrating the principle.

Remember- An aluminium disk was suspended by a string and rotated by moving a magnet.

Gather, process and analyse information to identify some of the energy trans- fers and transformations involving the conversion of electrical energy into more useful forms in the home and industry

Electricity is simply an easy way to transmit energy from point to point which enables energy to be collected and transmitted on a large scale. The advantage of electricity is not only that it is relatively easy to transport, but also that it is easy to convert it into other forms. In light bulbs, electrical energy is converted into light energy. In the home, it is also converted into heat in devices such as heaters and toasters, and into sound through speakers. In the industry electricity is most often converted into kinetic energy which drives machinery used in the production of goods. So generally electricity is converted into kinetic energy or electromagnetic radiation in the house and in industry.

Remember- All electrical devices convert electrical energy into other forms.

Transformers

Describe the purpose of transformers in electrical circuits

A transformer is designed to change the voltage of electricity. Its purpose is to either step-up (raise) or step-down (lower) the voltage that is fed into it.

Remember- Transformers change the voltage of electricity.

Compare step-up and step-down transformers

Step-up and step-down transformers are almost identical. Both have an identical structure, with primary and secondary coils and an iron core. In both transformers, the number of turns in each of the coil varies, with one coil having more turns than the other. In a step-up transformer, the secondary coil has more turns than the primary coil. This results in a higher voltage output. In a step-down transformer, the secondary coil has less turns than the primary coil. This results in a lower voltage output. So essentially the difference between a step-up and a step-down transformer is whether the primary coil has more or less turns than the secondary coil.

Remember- Step-up transformers increase voltage, and step-down transformers decrease voltage.

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

The ratio between the ratio of the number of turns in the primary and secondary coils and the ratio of primary to secondary voltage is identical, according to the formula \frac{V_p}{V_s} = \frac{n_p}{n_s}

Gather, analyse and use available evidence to discuss how difficulties of heating caused by eddy currents in transformers may be overcome

Heating due to eddy currents isn’t the only form of energy loss in transformers. Current in the coils causes them to heat up, increasing resistance. This heating is countered by the use of coolant to keep the coils conducting efficiently.
The founding principle of the transformer is the induction of current in the secondary coil because the secondary coil experiences changing flux. However, the iron core of the transformer also experiences changing flux, which induces eddy currents in the core, heating it up. There are two ways in which this can be addressed:
Firstly, instead of iron, ferrites, complex oxides of iron and other metals can be used. Ferrites are good at transmitting flux but poor at conducting electricity, so eddy currents and heating are minimised.

Alternatively, the iron core can be sliced into thin layers and then put back together with insulation between each layer. This process, known as lamination, breaks up large eddy currents and minimises them because currents can only form in each of the lamina. This means smaller eddy currents and therefore less heating. The laminations must not be in the same plane as the coils- instead they must “slice” this plane as thinly as possible to minimise eddy current formation.

 

figure 1

Remember- Laminations cut the plane of the coils to break up eddy currents.

Perform an investigation to model the structure of a transformer to demon- strate how secondary voltage is produced

In this experiment, we had a primary coil producing a changing magnetic field which was used to induce a current in a secondary coil. The setup consisted of a hollow coil that slid into the middle of a larger hollow coil. An iron core consisting of a solid iron rod fitted into the middle of the smaller coil. An AC power supply was connected to the primary coil, and a galvanometer was connected to the secondary coil. When we passed AC current through the large coil, the galvanometer detected current in the secondary coil, showing that induction was taking place. The iron core intensified the induction- when we removed the core the induced current dropped greatly in strength. This is because the iron core directs the magnetic field from the primary coil into the secondary coil, thereby increasing efficiency.

figure 2
Remember- The primary coil induces a current in a secondary coil, the voltage is changed because they each have a different number of turns, and the iron core intensifies the effect.

Explain the role of transformers in electricity substations

Transformers are used in substations to step-up and step-down electrical energy for long distance transmission. At the generator, a step-up transformer at a substation raises the output voltage from 23kV to 330kV. This minimises losses during long distance transmission by reducing the current flowing through transmission wires. In substations located in urban areas, step-down transformers are used to reduce the voltage for transmission within cities or suburbs. So transformers convert voltages in substations to reduce losses when transmitting electricity.

Remember- Transformers are used in substations to change the voltage of electricity to minimise transmission losses.

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

Power losses in the transmission of electricity are largely caused by heating in transmission wires. The energy consumed is equal to I^2R(since the energy lost is the same as the power “used” by the wire, from P = I^2R), so it can be seen that power loss is dependant on the current flowing through the wire, as well as the wire’s resistance. This heating is a huge problem, because it results in less energy reaching the point of use. However, transformers can be used to raise the voltage of electricity, and thereby reduce current. This dramatically reduces the power consumed by transmission wires, and thereby reduces wasted energy. Using transformers in the transfer of electrical energy from a power station to its point of use provides massive efficiency gains, reducing the fuel consumed by a power plant and reducing the price of electricity.

Also, different devices require different voltages- computers and incandescent lights require much lower voltages than TV’s and fluorescent lamps (which can require up to 10000V). Transformers are required to ensure each device is supplied with an appropriate voltage. However, high voltage power lines are subject to arcing and so need to be separated, as do substations which can be extremely dangerous for people nearby. Although transformers have dramatically increased the efficiency of electricity transmission, they have also produced safety concerns and have required specialised infrastructure to keep people safe.

Remember- Transformers are required for electricity transmission to reduce otherwise prohibitive losses.

Explain why voltage transformations are related to conservation of energy

According to conservation of energy, energy cannot be created nor destroyed, only transformed. Electrical energy is expressed as P , measured in watts. P = IV . Conservation of energy means that the energy in the secondary coil must equal the energy in the primary coil, so that P_p = P_s. This means I_p V_p = I_s V_s. Therefore, when voltage is changed in a transformer, the current then must also change so that P remains constant, according to conservation of energy. So when voltage is stepped up, current is reduced, and vice versa.

Remember- Voltage transformations are related to conservation of energy because the total power on either side of a transformer is the same.

Discuss why some electrical appliances in the home that are connected to the mains domestic power supply use a transformer

Many electrical appliances are designed to run on low DC voltages. This is particularly true of any device which uses a battery, because batteries are only capable of providing low DC voltages. The reason batteries are used is to provide portability- so that devices such as laptops and mobile phones can be moved around. Also, some circuits particularly those in computers, only function at low DC voltages- otherwise they would overheat and burn out. In these electrical appliances, a transformer is required, in addition with a rectifier, to convert household mains domestic power (240V AC) into low voltage DC for use in the appliance.

Remember- Home devices with transformers usually have low-voltage chips, or can run on batteries as well.

Discuss the impact of the development of transformers on society

The direct impact of transformers was to make AC power a viable solution. By allowing large scale generation from outside urban areas, the uptake of electricity was rapid. Therefore the impacts of transformers on society are the same as the impact of AC power, because the key impact of transformers was to provide efficient AC power distribution.
Transformers have had a significant impact upon society. The main reason AC power was successful over DC power was because the voltage could be changed to minimise transmission losses and dramatically slash losses in the electricity grid. These voltages changes were only made possible by the development of the transformer. It has resulted in the wide uptake of electricity and it helped lower the cost of electricity, making it accessible to almost everyone in economic terms. However, the widespread introduction of electricity made many unskilled jobs redundant and increased unemployment levels, which was detrimental to many people. Also, widespread demand for electricity led to large scale use of fossil fuels such as coal to power the generators, which has resulted in a great deal of atmospheric pollution in the form of sulfur and nitrogen oxides, as well as increased carbon dioxide levels which contribute to global warming. So transformers have had a huge impact on society, from bringing electricity into the reach of the broad public to indirectly causing environmental damage and social problems.

Remember- Transformers have led to the large scale uptake of electricity.

Generators and Transmission

Describe the main components of a generator

All generators consist of two parts- a stator and a rotor. The stator consists of magnets, either permanent magnets or electromagnets that are arranged around the centre of the motor, such that the magnetic field produced immerses the rotor. The rotor is the rotating part of the motor and is attached to a spinning energy source, for example a windmill or a petrol engine. The rotor consists of a coil that rotates in a magnetic field, so that it experiences changing flux and therefore has an induced emf. AC generators have slip rings, where a ring is connected to each end of the rotor wire. The rings rotate with the rotor, and current is passed to a circuit from the rotor by brushes that are in contact with the slip rings. A DC generator has a commutator identical to that found in a motor, so that the direction of emf is always the same. The commutator is a ring split into two parts, each part connected to one end of the rotor wire and in contact with brushes that connect the rotor to the circuit via the commutator.

Remember- Generators consist of a stator and a rotor. They also have either slip rings or a commutator.

Describe the differences between AC and DC generators

Bear in mind that the DC generator does not produce a constant current. The current in a DC generator also fluctuates because the rate of change of flux is not always constant. The key point is that DC always flows in the same direction.

AC generators produce alternating current that switches direction periodically, while DC generators produce a current that, while varying in magnitude, always flows in the same direction. This stems from the fact that AC generators use slip rings with brushes to transfer electricity from the rotor to a circuit, while a DC generator uses a split-ring commutator to reverse current direction every half-turn.

Remember- DC generators produce current that always flows in the same direction, and they use a commutator instead of slip rings

Compare the structure and function of a generator to an electric motor

In terms of structure, standard AC motors and generators are identical, and DC motors and generators are identical. They consist of the same parts connected in the same way. The difference between them lies in function. The role of a motor is to turn electrical energy into kinetic energy, and the role of a generator is to convert kinetic energy into electrical energy. In a motor, electrical energy is fed in, in a generator, electrical current is extracted. A motor is connected to an object that the operators wants to rotate (such as wheels), while a generator is connected to a rotating object (such as a steam turbine). Essentially, motors and generators have the same structure but function in opposite directions.

Remember- Motors and generators have the same structure, but perform opposite tasks.

Gather secondary information to discuss advantages/disadvantages of AC and DC generators and relate these to their use

AC and DC generators produce electricity that is very different, and so each has its own advantages and disadvantages.

AC generators have several advantages, including the ability for the output voltage to be changed easily, as well as greater durability because slip rings encounter less wear and tear than a commutator. On the other hand, the output from an AC generator needs to be constantly shielded so that energy is not lost to the environment by induction (as AC current is always fluctuating, and therefore causes changing flux around the wire). Also, wires carrying high voltages are subject to arcing, and so are more dangerous to their surroundings. Stronger insulation is required compared to DC generated power.

DC generators have the advantage of producing electricity that doesn’t induce emf in its surroundings, so less insulation and separation is required, as well as the fact that DC cable insulation can be lighter and therefore cheaper. However, the commutator in a DC generator is subject to wear and tear and is more prone to breakage. The commutator also undergoes sparking as it rotates, leading to further power loss and additional wear and tear. Since DC generators tend to produce low-voltage high- current electricity, a great deal of energy is wasted in power lines as heat.

Further, it is extremely difficult to change the voltage of DC power, which means it is difficult to reduce losses in transmission. These advantages and disadvantages mean that AC generators are used for large scale power generation where power is transmitted long distances, while DC generators are used for small-scale applications such as generators in vehicles.

Remember- AC can have its voltage transformed and can be transmitted efficiently, but it can also cause unwanted induction. It is hard to change DC voltage, and DC often has high current, but it is useful in small applications because induction is less of a problem.

Discuss the energy losses that occur as energy is fed through transmission lines from the generator to the consumer

Don’t forget to include relevant formulae in your answer to this dotpoint.

Moving electrical charge through a conductor wastes energy, because some of the electrical energy is converted to heat. The resistance of an object is essentially its capacity to convert electrical energy into undesired forms such as heat. This is because electrons collide with other atoms causing them to vibrate, resulting in heat, and in a loss of electrical energy. This is particularly problematic for power lines, because large amounts of electricity are passed through them for long distances. A portion of electricity passed through a power line is converted to heat, and this is how electricity is lost through transmission. The cost of lost energy can be very significant.

However, the formula P = I2R shows that the amount of energy lost (power consumed by the wire) is related to how much current is passing through the wire. By reducing the current, energy losses between the generator and the consumer can be minimised. This is done through the use of transformers, because transformers can be used to alter the voltage to current ratio without changing the total amount of energy.

From the generator, the voltage is stepped up by a transformer. According to P = IV , since the total power is constant, I must decrease when V increases. This means that the step up transformer produces electricity with low current, and this minimises heat losses through transmission lines. At the consumer’s end, a step down transformer lowers the voltage so that it is appropriate for use by the consumer.

Further, there are additional losses in the transformer, largely due to induction producing eddy currents in the iron core. Not only is the induction of eddy currents inefficient because emf has been used to produce them, but the resulting eddy currents heat the iron core and therefore the transformer coils, increasing their resistance. These are dealt with firstly by laminating the iron core, and by using cooling fans to keep the transformer cool. In this way, transmission losses are minimised between the generator and the consumer.

figure 21

Remember- Resistance in the wire causes heating, transformers are used to minimise current and thereby minimise energy loss.

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

Make sure you cover the key points where AC had effects- transmission, economies of scale, the location of the generators closer to fuel and pollution moved outside the city.

AC generation and its ability to have its voltage changed by transformers has revolutionised society and had an environmental impact. The major problem with DC power was that it could not effectively be transmitted- as DC power could not have its voltage switched, large volumes of current had to be passed through wires, resulting in huge energy losses. This meant that DC generators would have to be close to consumers, and there would have to be many generators as the electricity could not be transmitted long distances. However, AC power can be transmitted long distances by altering its voltage. This meant that electricity generation could be shifted outside urban areas where consumers were located and instead located close to the natural resources required to run the generator, such as coal. This helped to bring down the price of electricity. Further, AC generation could be carried out on a large scale and then distributed over long distances to many people. This meant that economies of scale could be achieved, resulting in dramatically cheaper electricity as well as increased efficiency. This placed electricity within the reach of the majority of the population, rather than the rich, privileged minority who would have been the only people capable of affording electricity under a DC grid. Therefore, the uptake of electricity was rapid and widespread, dramatically changing society with its labour-saving benefits (although in some cases making unskilled jobs redundant).

In terms of the environment, by placing the generator away from the city, pollution levels in urban areas were reduced. Further, switching to electricity reduced the need to burn fuels in the home, further reducing pollution levels. So while coal was still burnt, and likely in greater quantities than before the uptake of AC generation, pollution was shifted away from urban areas and out into the rural areas where the generators are located, resulting in a cleaner urban environment. However, the pollution resulting from large-scale electricity generation is a significant contributor to global warming. Overall, AC generators dramatically changed society because they made the labour-saving benefits of electricity available to the majority of the population. While AC generators have resulted in a cleaner urban environment, they are still a large source of pollution and have a significant impact on global warming.

Remember- AC made electricity available to almost all the population, not just the rich people. However, it generated more pollution as electricity consumption rose. However, this pollution was not in cities but in other areas.

Analyse secondary information on the competition between Westinghouse and Edison to supply electricity to cities

Westinghouse and Edison were in direct competition to supply electricity to cities. Edison, who had invented appliances for DC power, planned out a DC power grid and advocated DC as a solution for powering cities. Westinghouse on the other hand owned the rights to the transformer and advocated AC power. The main problem with DC power was its inability to be transmitted- the furthest it could be sent at the time was 14km and that was with 38% of the energy lost to heat. Westinghouse’s AC grid used transformers to slash losses to merely 1% by using high voltages unattainable with DC power to optimise transmission. Further than this, because of its inability to be transported, DC electricity would have to be generated by multiple generators throughout the city. This would have resulted in infrastructure difficulties in bringing fuel into cities, high levels of pollution, and more costly electricity because economies of scale could not be realised. Because AC power could be transported, it could be generated near the source of fuel on a large scale, resulting in much cheaper electricity. Edison temporarily achieved a propaganda victory by claiming AC power lines were unsafe, but the lines were kept out of reach and substations fenced off, and eventually the high efficiency and economies of scale made AC the victor.

Remember- AC could have its voltage changed, which made it easy to transport efficiently and gen- erate on a large scale.

Gather and analyse information to identify how transmission lines are insulated from supporting structures and protected from lightning strikes

Power lines have two protective devices- insulation from supporting towers and protection against lightning strikes. In dry air, sparks can jump around 33cm from a 330kV source. This means that wires need to be held at least that far away from the supporting towers it is strung from. This is achieved by using disk-shaped ceramic insulators. The disks are stacked on top of each other, so that if it rains some of the disks remain and therefore do not conduct. Also, the disk shape means that current has a longer distance to traverse (since the current must go around the disks, instead of in a straight line), increasing safety. In terms of lightning, on power lines there is another single line strung at the very top of power poles, above the conducting wires. This wire is known as a shield conductor. In the event of a lightning strike, lightning hits points as high as possible, and so the shield conductor at the top will be hit instead of the lower conducting lines. The shield conductor is periodically earthed by having a connection to a wire that runs from the top of a power pole right down to the ground (known as an earth wire), so that lightning can travel from the sky to the ground via shield conductors rather than power lines. For high-voltage towers, the tower itself is taller than the height of the wires, so lighting will strike the top of the tower then travel down to the ground via the metal structure, thereby not interfering with the power lines.

Remember- Stacked ceramic disks isolate wires from towers, and a shield conductor above the trans- mission wires protects against lightning strikes.

Induction and Electricity Generation

Outline Michael Faraday’s discovery of the generation of an electric current by a moving magnet

Following O¨ersted’s discovery that moving charge produced a magnetic field, Faraday wondered if the reverse was true- if a moving magnetic field could produce electromotive force (EMF). Faraday initially assembled a crude transformer, running a current through a primary coil and checking for current in a secondary coil that was immersed in the magnetic field produced by the primary coil. Faraday found that when the primary coil was connected and disconnected to a battery, the galvanometer needle moved slightly, but the effect was only momentary. Faraday then took an iron ring and wound a wire around it. When he moved a magnet in and out of the ring, a galvanometer showed a constantly changing current. From this, Faraday concluded that a moving magnet could be used to generate electric current.

figure 18

Remember- Faraday built a DC transformer, but it did not work as transformers require AC current for operation. He then moved a magnet in and out of a wire loop and detected a current.

Perform an investigation to model the generation of an electric current by moving a magnet in a coil or a coil near a magnet (including ”Plan, choose equipment or resources for, and perform a first hand investigation to demonstrate the production of an alternating current”)

Note that this experiment addresses two dot points- creating an electric current by moving a magnet, and producing an alternating current.

In this experiment, we moved a magnet into a hollow coil. The coil was a length of thin wire wrapped around a cardboard cylinder, with around 200 turns. The coil was connected to a galvanometer so that current passing through the coil would be detected. We attached a strong ceramic magnet to the end of a small wooden stick, and then moved the magnet in and out of the coil repeatedly. This causes the coil to experience changing flux, and therefore a current was induced, which was detected by the galvanometer. In this way we modelled the generation of an electric current. Further, because we moved the coil in and out, in both directions, the current produced was an alternating current. This was shown by the needle on the galvanometer oscillating between positive and negative readings, showing the current changing direction.

Plan, choose 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, and the relative motion between the coil and the magnet is varied

This experiment used the same setup as that described previously, with a ceramic magnet on a stick being moved in and out of a coil attached to a galvanometer. To test the effect of distance, we moved the magnet with a constant amplitude and period, while changing the centre of motion. When the centre of motion was at the very edge of the coil, so that the magnet passed in and out of the coil, current flow was maximised. When the centre of motion was the middle of the coil, some current was produced but it was diminished, because the coil experienced less change in flux (since the magnet was inside the coil the whole time). When the centre of motion was outside the coil, induced current also dropped. Using a stronger magnet induced a larger current, and when we altered the relative motion between the coil and the magnet by changing the speed of the magnet, we found that the induced current was larger when the magnet was moved faster.

figure 19

Remember- Current generation is maximised with the coil and magnet close together, with a strong magnet, and with a fast relative movement between the coil and magnet.

Define magnetic field strength B as magnetic flux density

Magnetic Flux Density is another name for magnetic field strength, which is measured in Teslas or in webers per square metre. It refers to how much magnetic flux is passing through a unit area.

Describe the concept of magnetic flux in terms of magnetic flux density and surface area

 

Magnetic flux density can be thought of as a measure of how many magnetic field lines pass through a square metre. Magnetic flux is a measure of how many magnetic field lines pass through a different area. It is found by multiplying the magnetic flux density by the area in question. So essentially, magnetic flux is a measure of the amount of magnetic field passing through an area.

figure 20

Describe generated potential difference as the rate of change of magnetic flux through a circuit

According to Faraday, the voltage, the potential difference, or the emf (which all mean the same thing) is dependant on how fast magnetic flux is changed, that is, how quickly the amount of magnetic field passing through an area changes. According to Faraday’s law, ǫ = −∆Φb , where ∆Φb

is the change in magnetic flux over time t. So the generated potential difference is essentially the rate of change of flux. The negative sign accounts for the direction of the induced emf, as explained in the next dotpoint.

Account for Lenz’s Law in terms of conservation of energy and relate it to the production of back emf in motors

This is an important dotpoint that needs to be understood correctly. Make sure you thoroughly study this section and that you are able to clearly justify the existence of back emf. Also, make sure you practice writing explanations of Lenz’s Law, because it is a concept that many students find difficult to understand.

Lenz’s Law states that an induced emf always produces a magnetic field that opposes the change that produced it. This essentially means that when an induced emf is generated by moving a magnet, the induced emf will produce a magnetic field that exerts a force on the magnet in the opposite direction to the original movement of the magnet.

It comes about because of the law of conservation of energy. When a magnet is moved into a coil, the resulting current can flow in one of two directions. Each direction produces a magnetic field through the coil. This magnetic field can either attract the magnet further into the coil, or repel the magnet.

If the magnet was attracted, it would be sucked into the coil. This further movement would result in further change in flux, which would induce more emf. Effectively, the magnet would be sucked through the coil, generating energy.

However, this violates conservation of energy. If the magnet was sucked in, there would be energy output in the form of emf and charge movement, but there would be no energy input, no work done. This means that energy is being created, and according to conservation of energy, energy cannot be created nor destroyed.

So if the magnetic field induced by the magnet cannot attract it further into the coil, it must repel the magnet. In this case, in order to continue energy production the magnet must be forced into the coil- this kinetic energy input is then converted into emf, obeying conservation of energy. This gives rise to the production of back emf in motors.

As shown before, moving charge in a magnetic field experiences force. This force causes the motor to turn, and the charge movement is as a result of the supply emf. However, by Faraday’s law, changing flux induces emf. The rotating coils of a motor experience changing flux, and this causes a current to be induced in them.

According to Lenz’s law, the direction of emf must oppose the original change that produced it. In this case, it means that if the motor is spinning clockwise, the induced emf applies a force in an anticlockwise direction, slowing the motor.

Now the movement of the motor is the result of supply emf, and the supply emf causes the motor to spin in a particular direction, either clockwise or anticlockwise. As the induced emf causes torque in the opposite direction, the direction of emf must be opposite to the supply emf, hence the induced emf in a motor is referred to as back emf.

Remember- In accordance with conservation of energy, back emf always opposes supply emf in a motor.

Explain that, in electric motors, back emf opposes the supply emf

As shown before, back emf is induced in a rotating coil as found in a motor. The back emf causes torque that opposes the movement of the armature. However, the initial movement of the armature is caused by the supply emf running through the circuit in a particular direction. As back emf produces force in the opposite direction to the supply emf, which caused the armature to rotate in the original direction, the back emf must act in the opposite direction to the supply emf, therefore opposing it. The effect of back emf opposing supply emf is to reduce the net emf, thereby reducing the current. This means that a motor spinning rapidly uses far less current than a stationary motor, since the spinning motor has induced back emf that reduces the current.

Remember- Motors generate an emf while they operate, which opposes the supply emf

Explain the production of eddy currents in terms of Lenz’s Law

Eddy currents are formed when there is a change in flux and therefore induced emf, but no conducting path in which electrons can flow. In this case, rather then flowing around in a circuit the electrons flow around in a circle. This circular movement of charge is an eddy current. When there is a change in flux, emf is induced, and according to Lenz’s Law this emf must oppose the original movement that created it. This means that the charge experiencing the field produces a force that opposes the original movement of the conductor. As the charge moves, it gradually forms into a circular flow of electrons. This circular flow of electrons produces a magnetic field that will repel the original magnetic field that induced the eddy currents.

Remember- Eddy currents form according to Lenz’s Law.

Gather, analyse and present information to explain how induction is used in cooktops in electric ranges

Induction cooktops use electromagnetic induction to produce heat. Beneath the cooktop is an induction coil consisting of a solenoid placed horizontally (i.e. with circular coils lying horizontally). When alternating current (AC) is passed through the coil, a magnetic field is formed that is aimed straight up through the cooking surface. However, AC is fed into the induction coil, and this causes the magnetic field formed to constantly change. This means that when a saucepan or pot is placed above the induction coil, it experiences changing flux. This causes eddy currents to be formed in the pan, which results in the pan heating up due to electrical resistance. An induction cooktop is much more efficient at converting electrical energy to heat because the pan is heated directly, rather than indirectly.

Remember- Induction cooktops use induction from a coil beneath the cooktop to heat pans.

Gather secondary information to identify how eddy currents have been utilised in electromagnetic braking

Electromagnetic braking is mainly used in trains, where it can silently and efficiently slow down the train without the noise and wear and tear of conventional friction braking. With electromagnetic braking, an electromagnet is placed underneath the train. When turned on while the train is moving, it causes the metal rails below it to experience a change in flux. According to Faraday’s law, this causes eddy currents to be induced in the rails. According to Lenz’s law, these eddy currents must have a magnetic field that opposes that change which produced them. This means that the eddy currents that form repel the electromagnet as it moves, exerting a force opposite to the direction it is moving in. This is therefore a braking force, acting against the movement of the train. By inducing eddy currents in tracks, trains can slow down. The other advantage is that because emf is dependant on rate of change of flux, when the train is travelling quickly there is a greater production of emf than when it is travelling slowly. This means that electromagnetic braking is most efficient at high speeds which are when it is needed most, because it is at high speeds that the most noise and heat are produced by frictional braking, which also results in wear and tear on the braking system. Alternatively, a magnetic field can be applied directly to the wheels of the train by electromagnets placed near the metal wheels. This also provides a braking effect.

Remember- Magnets mounted on a train are used as electromagnetic brakes.

Current-carrying wires and the Motor Effect

Discuss the effect on the magnitude of the force on a current-carrying con- ductor 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 ex- ternal magnetic field and the angle between the direction of the external magnetic field and the direction of the length of the conductor

All of these variables can be summarised by the formula F = BIlsinθ. When the strength of the field increases, so does force. When magnitude of the current is increased, and when the length of the conductor is increased, force increases. As far as angle goes, force is at a maximum when the angle is 90 degrees with the conductor perpendicular to the field, and force is zero when the conductor is parallel to the magnetic field.

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

You may want to model this yourself by using the right-hand grip rule to determine how the magnetic fields in each wire interact to cause them to attract. Essentially, the wire on the right side has a magnetic field running upwards through the other wire. Then use the right hand palm rule to work out the direction of force.

The force between parallel current carrying conductors depends on the direction of current flow. If current flow is in the same direction, then the wires will attract. If it is in opposite directions, then the wires will repel. The formula is \frac {F}{l} = \frac {kI_1 I_2}{d} and shows that as length and currents increase, force increases, and that as distance increases, force decreases in a linear relationship. This follows from the formula B = \frac {kI}{d} which calculates the strength of the magnetic field produced by a conducting wire with current I, at distance d from the wire.

figure 14

Remember- Wires with current flowing in the same direction attract each other.

Describe the forces experienced by a current-carrying loop in a magnetic field and describe the net result of the forces

A current carrying loop will experience force due to the motor effect. Perpendicular sides of the loop have current moving in opposite directions, so they experience opposite forces. If the coil is able to pivot around its centre, one of the sides will experience an upward force, and the other will experience a downwards force. Because of the structure of the loop, each of the sides produces torque, as they experience a force acting tangentially to the pivot point when the loop is horizontal. This causes the loop to rotate.

figure 15

Remember- Opposite sides of the loop with have current flowing in opposite directions, so they experience opposite force, so the coil rotates.

Perform a first-hand investigation to demonstrate the motor effect

In our experiment, we had a wire sitting on a piece of wood. Magnets on either side of the wood set up a magnetic field passing through the wire, with the field lines perpendicular to the wire (in the horizontal plane). When we passed a current through the wire in the correct direction (according to the right-hand palm rule) the wire jumped upwards, due to the motor effect where moving charge in a magnetic field experiences force. The setup could only be used intermittently, because there was no load in the circuit. This meant large currents flowed through the wire, and operation for more than a few seconds caused dangerous overheating and the power supply to shut down.

Remember- The experiment made a wire jump up due to the motor effect.

Define torque as the turning moment of a force using T = Fd

Broadly speaking, there are 2 forms of motion. The first is linear motion, which occurs when an object travels through space. Examples of this include the motion of a ball as it is thrown, or the motion of the Earth around the sun. In both cases, the spatial position of the object changes. The second form of motion is rotational motion, which occurs when an object spins. Examples of this include the spinning of a CD in a drive, or the rotation of a fan. In both these cases, the objects are not moving through space but instead remain stationary while they rotate (the spinning of car wheels is also an example of rotational motion, but in that case the wheels also have a component of linear motion as they are moving through space with the car). Linear motion occurs when a linear force is applied to an object, for example when something is pushed or pulled. Rotational motion occurs when a rotational force is applied to an object, such as when it is twisted. The difference between the two is subtle. Linear force is a vector with both magnitude and direction. When a linear force is applied directly in line with an object’s centre of gravity, then the object will travel through space without rotating. However, if a linear force is applied off centre, some distance away from the object’s centre of gravity, then in addition to moving through space the object will also rotate around its centre of gravity. The rotational force that has been applied to cause the object to rotate is equal to Fd, where F is the magnitude of the linear force, and d is the distance between the point at which the force is applied and the object’s centre of gravity. For simplicity, linear force is referred to simply as force, while rotational force is referred to as torque.

Torque can be considered the turning force on an object. It occurs when a force is applied to an object tangentially rather than straight at it. The torque depends on the force and the perpendicular distance from the pivot point, which is equal to Fd.

figure 16

Identify that the motor effect is due to the force acting on a current-carrying conductor in a magnetic field

The motor effect is caused by the moving of electrons in a magnetic field. When placed in a magnetic field, moving charge experiences a force. This force is dependant on the direction of the field. When charge is moved through a wire, the wire experiences a force. This force is what constitutes the motor effect- the conversion of moving electric charge in a magnetic field into kinetic energy.

Remember- Moving charge in a magnetic field experiences force, and this can be used to move a conductor with electrons moving through it.

Identify data sources, gather and process information to qualitatively describe the application of the motor effect in the galvanometer and the loudspeaker

Make sure you can comprehensively and clearly describe how the galvanometer and loudspeaker work, and ensure that you explain how the motor effect is used in these applications.

The galvanometer and the loudspeaker are two devices which rely on the motor effect for their operation. The motor effect is where moving charge in a magnetic field experiences force, and leads to wires carrying current in a magnetic field experiencing force. In a loudspeaker, a solenoid is immersed in a static magnetic field. The solenoid is free to move as it is alternately attracted and repelled by the permanent magnet. This movement occurs because when current is passed through the solenoid, it causes the solenoid to experience force. This force moves the solenoid in motion that corresponds to the current being fed into it. If the current being fed in corresponds to an audio wave, the solenoid will oscillate in the same way as the audio wave. Because the solenoid is connected to a large cone, the solenoid causes the cone to vibrate as it moves. These vibrations result in the formation of pressure waves in the air, which are heard as sound. So the operation of a loudspeaker depends on the motor effect to move the solenoid, converting electrical energy into sound energy.

figure 17

In a galvanometer, a coil of wire is wrapped around an iron core onto which is attached a needle. This entire assembly is free to rotate. A stator consisting of permanent magnets produces a radial magnetic field immersing the solenoid. The iron core is used to direct and intensify the magnetic field. When current is passed through the solenoid, a force results from the motor effect. Torque is thus produced, the same as in a motor, and causes the solenoid and the needle to rotate. Because a radial magnetic field is used, the torque produced is constant regardless of how far the coil is deflected. However, the galvanometer also incorporates a spring that is attached to the solenoid. The solenoid compresses the spring as it rotates, and in this way the solenoid rotates up until the torque experienced by the solenoid is fully countered by the spring. Because a radial magnetic field is used, the coil experiences constant torque. Further, the force exerted by the spring also increases linearly (using the formula F = −kx which is outside the scope of the syllabus, where k is a constant describing the properties of the spring, and x is the degree to which it is compressed). Because torque and spring force scale linearly with current, a linear scale can be developed linking current to coil rotation. As the coil is connected to a needle, this setup can be used to assess how much current is flowing through a circuit.

Remember- A loudspeaker uses the motor effect to move the cone, and a galvanometer uses the motor effect and a radial magnetic field to measure current through the strength of the motor effect.

Describe the main features of a DC electric motor and the role of each feature

Direct Current (DC) motors consist essentially of 3 parts- the stator, the armature, and the com- mutator. In order for the motor effect to occur, a magnetic field is required. The stator consists of permanent magnets or coils of wire that produce a magnetic field that runs through the centre of the motor. The stator does not move during the operation of the motor, and is arranged around the armature which is at the centre of the motor. The stator provides a magnetic field that immerses the armature. The armature consists of loops of wire wrapped around a ferromagnetic core that experience a force when current is passed through them in the presence of a magnetic field. The armature is free to rotate, and is connected to whatever the motor is driving. The commutator is attached to one end of the armature, and is the means by which electricity is passed into the armature. It consists of a ring shaped conductor that is split into two parts, each part connected to one end of the coil that makes up the armature. Brushes on either side are connected to the circuit, one positive, one negative. Current flows through one of the brushes, into the commutator, then through the armature, back to the commutator, and out into the rest of the circuit via the other brush. The role of the commutator is to reverse the direction of current in the armature every half-turn. This causes the force experienced by the armature to reverse, and ensures that the torque experienced by the armature always occurs in the same direction.

Remember- Stator, armature, commutator, brushes.

Identify that the required magnetic fields in DC motors can be produced by current-carrying coils or permanent magnets

In order for a DC motor to operate, the armature must be immersed in a magnetic field. This allows moving charge in the armature to generate force. How this magnetic field is generated however, is immaterial. Permanent magnets as well as current-carrying coils can provide this field. Moving charge produces a magnetic field, and so current carrying coils can produce magnetic fields similar to those produced by bar magnets. This principle can be used to generate the field in a DC motor, just like with permanent magnets. If electromagnets are used, the wire is usually wrapped around an iron core to increase the strength of the field.

Remember- A DC motor needs a magnetic field, and this can be provided through either current- carrying field coils or through a permanent magnet.