Courses science physics 1477268914 2015 Physics Notes PDF

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HSC Physics

Motors and Generators

HSC Physics Notes Motors and Generators 1. 2. 3. 4. 5.

MOTORS GENERATION OF ELECTRIC VOLTAGE GENERATORS TRANSFORMERS AC MOTORS

1. Motors

1.1 Forces on Current-Carrying Conductors • • • •

The stronger the field strength, the stronger the force on the conductor (↑B = ↑F) The stronger the magnitude of the current, the stronger the force on the conductor (↑I = ↑F) The longer the conductor, the stronger the force on the conductor (↑L = ↑F) The larger the angle of the magnetic field lines to the conductor (max 90), the stronger the force on the conductor (↑θ = ↑F)

These relationship can be seen mathematically in the formula for the motor effect [F = BILsinθ]

1.2 Parallel Conductors Consider two parallel current-carrying conductors, such as two wires running next to each other: • When both currents are going in opposite directions, they move away from each other • When both currents move in the same direction, they move closer together We can work these two rules out using our two right-hand rules. Draw two wires next to each other, think of just one of the conductors and the magnetic field it creates (using the right-hand grip rule)." Then look at how this magnetic field effects the other wire (using the right-hand palm rule). The force between two such parallel conductors is given by the formula below: F = Force (N) L = Length of wire (m) K = Magnetic force constant (2x10^-7) I1 = Current in first conductor (A) I2 = Current in second conductor (A) D = Distance between conductors

1.3 Torque Torque: refers to the turning effect of a force. For example, opening a door or undoing a nut with a spanner. The two factors affecting the torque is the distance from the object (m) and the original force applied (N). T = torque (Nm) F = force (N) D = perpendicular distance (m)

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In some questions the force may not be at a right-angle to the turning mechanism and trigonometry with vectors will need to be used to find the perpendicular force.

1.4 The Motor Effect The motor effect is the action of a force experienced by a current-carrying conductor in an external magnetic field. The direction of the force can be determined using the right-hand palm rule. The motor effect: current + magnetic field = force on conductor The force is at a maximum when the magnetic field is perpendicular to the current in the conductor. This can be seen mathematically in the formula for motor effect force, where sin(90) = 1 and hence the force is at a maximum. F = Force on conductor (N) B = Magnetic field strength (T) I = Current (A) L = Length of conductor (m) θ = Conductor/magnetic field angle

1.5 Current Carrying Loop in a Magnetic Field One side of the loop will experience a force upwards, and the other side of the loop will experience a force downwards. These two opposite forces will create torque and cause the loop to twist 90 degrees until the loop is perpendicular to the magnetic field. However, when it reaches this position, the wire will oscillate"to a stop.

1.6 Components of a DC Motor • Magnets to Create Magnetic Field: these can be either permanent magnets, or electromagnets. Creates a magnetic field that interacts with the electric current to create motion via the motor effect. • Armature: cylinder of laminated iron around the axel to hold the copper wire conductors, and enable for movement of the conductors, when the motor effect is started. • Rotor Coils: these are wrapped around the armature to create current. The amount of coils depends on the complexity of the motor. The most complex motors will have many coils, and three poles around the armature. • Split-Ring Commutator: broad ring of metal mounted on the axle at one end of the armature, and cut into an even number of separate bars (two in a simple motor). When the axle turns past a split in the ring, the DC current is reversed and the motor effect continues. Brushes: the brushes connect the power source to the split ring commutator, and are mounted • on either side of the axle at the commutator. Usually compressed carbon blocks.

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1.7 Magnetic Fields in DC Motors The magnetic field in a DC motor can be produced using either a permanent magnet, or an electromagnet (made using a current-carrying coil and an iron core).

1.8 Calculations With Parallel Conductors Remember that parallel conductors carrying current in the same direction will attract each other, while parallel conductors carrying current in the opposite direction will repel each other. F = Force (N) L = Length of wire (m) K = Magnetic force constant (2x10^-7) I1 = Current in first conductor (A) I2 = Current in second conductor (A) D = Distance between conductors

1.9 Motor Effect Investigation The apparatus shown below is set up, where a wire is placed on an electronic balance. The wire is connected to a variable power source. Permanent magnets are placed on either side of the wire as shown. When no current is passed though the wire the electronic balance is zeroed. Now a current is passed though the wire, depending on the direction of the current the electronic balance will measure a positive or negative value. However the value has changed meaning the wire is experiencing a force. This shows the motor effect.

1.10 Calculating Magnitude of the Motor Effect F = Force on conductor (N) B = Magnetic field strength (T) I = Current (A) L = length of conductor (m) θ = Conductor/magnetic field angle The direction of the force can be obtained using the right-hand palm rule.

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1.11 Calculating Torque in Motors T = Torque (Nm) n = Number of coils B = Magnetic field strength I = Current (A) A = Area (m^2) θ = Angle between force and turning mechanism

1.12 Applications of the Motor Effect The motor effect is used in the galvanometer and the loudspeaker. THE GALVANOMETER

A galvanometer is used to measure the magnitude and direction of direct current (DC). When a current is applied the pointer and coil will turn. It has a spring in it so that it will return to zero, and so that the force has something to push against. It can go both ways showing the direction of the current. The magnets used are shaped so that the field is perpendicular to the plane of the coil. This allows a uniform scale. The angle used in the force on a wire formula is constant as the sides of the coil are straight up and down. As the force on the coil = 𝐵𝐼𝑙 sin 𝜃, and as sin 𝜃, 𝑙 and 𝐵 are constant, force is proportional to current. Hence, the force applied to the needle is an accurate measure of current. THE LOUDSPEAKER

Speakers create sound waves from electrical impulses. The motor effect in the speaker is used for movement in one dimension, not a spinning motion like the DC electric motor. When both a current in the coil, and a magnetic field are present, the coil will have a force pushing it out, and hence, pushing the speaker cone. The spring brings it back in to normal position. This movement creates sound waves.

2. Generation of Electrical Voltage

2.1 Electromagnetic Induction (Michael Faraday) Michael Faraday discovered that when a magnet is moved in and around a coil, an electric current will be generated in the coil. This is the opposite of the motor effect, and the process is known as induction, in which relative motion between a conductor and a magnetic field is supplied and a current is produced. In other words, kinetic energy is transferred into electrical energy. The same formula is used as with the motor effect, i.e. 𝐹 = 𝐵𝐼𝑙 sin 𝜃.

2.2 Magnetic Flux Density Magnetic field strength (B measured in teslas) is the same as magnetic flux density.

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Flux: is the rate of flow of a fluid, radiation or particles. In this topic, flux is the name given to the amount of magnetic field passing through an area.

2.3 Magnetic Flux and Magnetic Flux Density • Magnetic flux density: deals with the strength of an overall field and is measured in Teslas (T). • Magnetic flux: deals with the strength of the magnetic field inside a given surface area, measured in Weber (Wb). So to calculate magnetic flux, multiply the normal magnetic flux density component by the area through which the magnetic field lines are passing.

2.4 Generated Potential Difference Generated potential difference: is the rate of change of magnetic flux through a circuit. Change in magnetic flux can be either due to the movement of the magnet, or the movement of the coil within the magnetic field. Either way" magnetic flux lines are “broken,” causing current to be induced. If there are two coils and one of them has AC current running through it, the magnetic field is constantly changing from one direction to another causing induced current.

2.5 Lenz’s Law Lenz’s Law states that an induced current is always in a direction such that its magnetic field opposes the changing field that created it (hence, the current is known as a back EMF). In other words, the current always attempts to oppose the motion. Therefore, we can determine which way the current in a coil travels if we know the direction of the motion causing magnetic flux to change.

2.6 Back EMF in Motors In the coil of a motor, due to Lenz’s law, the supply EMF is opposed by the back EMF. The back EMF opposes the motion of the motor, and hence, back EMF make motors less efficient.

2.7 Eddy Currents Eddy Currents: refer to currents produced by the relative motion between a metal (not necessarily magnetic) and a magnetic field.

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They are small circular paths that create heat due to resistance. Eddy currents produce magnetic fields (the direction of which can be determined using the right-hand grip rule) which, due to Lenz’s Law, oppose the relative motion. This diagram compares the eddy currents produced in a tube with no slit and a tube with a slit in it. This helps explain why laminating the iron core helps to prevent the production of eddy currents. Why we want to reduced Eddy Currents: we want to reduce eddy currents because they oppose the motion of the motor, and hence, decreases its efficiency. The faster the motor spins, the greater the eddy currents and the less efficient the motor becomes. ! How we reduce Eddy Currents: eddy currents can me minimised in a motor by using a soft laminated iron core. This is made up of thin slices of iron separated by an insulating layer of oxide or paper. The insulation disrupts the eddy currents and stops them building up. This diagram shows the production of eddy currents (represented by the red lines) in a metallic pipe, when a magnet is dropped through it. We know from Lenz’s Law that the direction of these eddy currents will be so that they create a magnetic field which opposes the motion of the magnet through the tube. If the north pole of the magnet is facing down—as in the diagram—the eddy currents below the magnet will cause a north pole so as to oppose the falling motion of the magnet.

2.8 Electromagnetic Induction First-Hand Investigation A magnet was moved into a coil of copper wire that was connected to a galvanometer. Due to Faraday’s Law, the relative motion between the coil and the magnet, or rather, the change in magnetic flux, caused a current to be induced in the coil. This induced current was measured and observed using the galvanometer.

2.9 Factors Affecting a Generated Electric Current • Distance between coil and magnet: as the distance between the coil and the magnet is increased, the strength of the magnetic field affecting the coil is decreased, and hence, the induced current is decreased. • Strength of the magnet: as the strength of the magnet is decreased, the change in magnetic flux is less, and hence, the induced current is decreased. • Relative motion between coil and magnet: as the relative motion between the coil and the magnet is increased, the induced current is increased. All three of these variables can be tested in the above investigation (2.8).

2.10 Induction Cooktops There is a coil of copper wire embedded in the cooktop with AC current flowing through it to produce a constantly changing magnetic field. This changing magnetic flux will induce an EMF in the metallic pan. The pan needs an iron based (ferromagnetic) bottom so that the current produces a heating effect from the friction of the molecules in the iron.

2.11 Electromagnetic Braking When a metallic wheel moves into a magnetic field, eddy currents are induced in the wheel. The eddy currents produce a magnetic field which opposes the motion (an application of Lenz's Law which uses the principle of conservation of energy). As the wheel slows down, the eddy currents produced are decreased so the braking effect is reduced. This results is smoother braking.

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Electromagnets are used because, unlike permanent magnets, they can be turned on and off. Also the strength of the magnetic field can be controlled by altering the current in the electromagnet, which allows for soft and hard breaking. Electromagnetic braking is used in trains and fairground rides which involve ‘free-fall’.

3. Generators

3.1 Components of a Generator Generator: is a device that transforms mechanical kinetic energy into electrical energy. A generator is essentially a motor in reverse, and hence, the components of a generator are the same as the components of a motor. The only difference being that a generator needs some mechanism that allows kinetic energy to be used to spin the axle.

In its simplest form, a generator consists of a coil of wire fixed around an armature that is forced to rotate about an axis in a magnetic field, created be either a permanent magnet or electromagnet. As the coil rotates, the magnitude of the magnetic flux passing through the area of the coil changes. The changing magnetic flux produces an induced current in the coil, which flows through the split-ring commutator and through the brushes into the electric circuit.

3.2 Comparing Motors and Generators Generators and electric motors are essentially the same thing, working in reverse to each other. • Motors: use electrical energy to create kinetic energy and need a source of current in the circuit. • Generators: use kinetic energy to create electrical energy and require a source of kinetic energy to spin the axle and hence cause a current to be induced in the coil.

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3.3 AC and DC Generators DC Generators create a current going in one direction using a split-ring commutator.

The diagram to the left shows a DC motor rather than a DC generator, however as we know, they are essentially the same thing. The direction of the current in a DC generator can be determined using the lefthand palm rule, which functions the same as the right-hand palm-rule, however the current flows in the opposite direction.

AC generators create current that constantly changes direction using slip rings or continuous rings. Slip rings: are used to collect AC current from a generator. Each slip ring connects to one end of the coil. As we know, every time the coil rotates 90 degrees, the direction of the current in the coil is reversed. Hence, the direction of the current flowing through each slip ring is reversed each time the coil moves 90 degrees, and the circuit connected to the coil via the brushes receives an alternating current. As in the above description of a DC generator, the left-hand palm rule can also be used to determine the direction of the current in an AC generator.

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3.4 Energy Loss in Generators Line loss: is the loss of electrical energy caused due to resistance in conductors causing the transfer of electrical energy into heat energy. The further you go along the ‘lines’, the greater the resistance and hence, the greater the electrical energy loss. However, energy is only lost when current goes through the lines, having a high voltage in the lines will not cause line loss. Therefore, transformers are used to increase voltage and decrease current to avoid line loss as much as possible while maintaining the same power created. Loss of power can be calculated using the following two formulas:

P = VI or P = (I^2)R P = Power (watts) V = Voltage (V) I = Current (A) R = Resistance (ohms) The second variation of the formula can be derived using Ohm’s Law.

3.5 Effects on Society and the Environment Effects of the development of AC generators on society • Greater leisure time as electric machines do work instead of humans • Decrease in job vacancies for unskilled labour created increase in unemployment • Mass production enabled the manufacture of cheaper products • Communication between people and businesses has increased with development of easily portable low voltage devices such as mobile phones and laptops Transmission of electricity to rural/isolated areas has improved quality of life and communications • for those people " Effects of the development of AC generators on the environment • Vegetation removed under transmission lines removes animal habitat and puts animals in danger when they cross clearings • Transmission lines can affect birds and flying foxes • Power stations produce thermal and chemical pollution • C02 emissions contribute to enhanced greenhouse effect • Transmission lines are not aesthetically pleasing

3.6 Generator Investigation Rose Bay Secondary College AC generator used in the investigation. The slip rings were examined, and it was noted that there was continuous contact between the brushes and the rings, unlike the DC motor. One ring was connected to one end of the coil and the other ring to the other end of the coil (as described in 3.3).

3.7 Advantages and Disadvantages of AC and DC Generators A number of advantages and disadvantages are given for both AC and DC motors. However, the main difference that should be noted for this course is that AC can be transformed, reducing line loss and meaning it is able to be transmitted over long distances. This makes it far more suitable than DC for large scale power distribution.

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AC Advantages

AC Disadvantages

• Can be transformed, meaning there is less power lost in transmission • Slip rings are more efficient than split rings because there is constant contact between the brushes and the ring, meaning less friction relative to the split ring where the brushes are constantly changing surfaces across the split

• Emits electromagnetic radiation, so wires need insulation and shielding. • Frequency must be sent to consumers at 50Hz, this must be monitored and maintained. • Back EMF opposes supply EMF.

DC Advantages

DC Disadvantages

• No shielding from electromagnetic radiation is required in the wiring. • Magnetic field is stable so there is no back EMF. • Many appliances us DC

• Can not be easily transformed • Transmission is limited to short distances only due to line loss (approx. 1.5km) • Split-ring commutators are less efficient than slip rings because split-rings wear more easily and spark as they cross over the split from one surface to the other.

3.8 Westinghouse and Edison • Edison opts for DC electricity: Edison went into business setting up a DC system of distributing electrical power in New York City. Because DC can not be transformed, its transmission distance is severely limited, due to line loss. Edison had hundreds of generators...