HSC PHYSICS NOTES Collins Module 2

The strength of the magnetic field in which it is located-The magnitude of the current in the conductor-The length of the conductor in the external magnetic field-The angle between the direction of the external magnetic field and the direction of the length of the conductor-

1.1 Discuss the effect on the magnitude of the force on a current carrying conductor of variations in:

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

F is proportional to the current in each of the conductors, and the length of the conductor and inversely proportional to the distance between the conductors

Where k = 2.0 x 10-7 NA-2Two current carrying conductors when current in the same direction, force is towards each otherWhen current is in opposite directions, force is away from each other

1.3 Define torque as the turning moment of a force using:

A current carrying conductor in a magnetic field will experience a forceThe direction of the force is determined by the right hand push rule

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

One side of the loop will go in one direction as the current travels along. The other side will move in the opposite direction. At the top of rotation, there is 0 torque as the forces cancel. Inertia keeps the coil moving. To continue moving through the other side, the current must be reversed by a commutator.

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

Commutator: changes the current direction in the rotor/armature every half cycle so as to continue the motion of the motor

Brushes: transfer current from power supply to the commutator - made of a soft material or spring loaded so as not to provide too much friction and slow down the motor

Armature/rotor: moving part of the motor, that has a current flowing through it which produces a magnetic field which interacts with the field magnets

Field magnets (stator): produce a constant magnetic field which interacts with the magnetic field of the rotor

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

1. Motors use the effect of forces on current-carrying conductors in magnetic fieldsSaturday, 23 October 20109:45 AM

9.3 Motors and Generators Page 12

Stator can be either an electromagnet OR a permanent magnet. An electromagnet will generally produce a more powerful, directed magnetic field

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

Circuit set up so a piece of copper wire is free to move and placed on a permanent magnetWhen the power supply is turned on, the wire movesWhen the magnet is reversed, the wire moves in the opposite directionConclusion: a current carrying conductor in a magnetic field experiences a force

1.9 Perform a first-hand investigation to demonstrate the motor effect

The galvanometer-The loudspeaker-

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

Galvanometer: Magnetic field from coil interacts with permanent magnetic field and motor effect force pushes the movable coil and the needle. A spring is also attached that provides a reverse torque. When the reverse torque matches the forwards torque produced by the motor effect, the needle stops.

Loudspeaker: Circuit constructed so AC is at same frequency as sound wave. Current passes through a coil which is free to move. AC current produces a movement in the coil because of motor effect interaction. Coil attached to a speaker cone which produces compressions and rarefactions in the air, which is a sound wave.


Wrapped two coils around a block of wood. One attached to a galvanometer, the other to a battery. When battery was turned off or on, a small current was measured in the galvanometer (i.e. a changing magnetic field produced a current)

Moving a permanent magnet near a coil also generated a current in the coil

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

Magnetic field strength is magnetic flux density, best visualised as the amount of field lines present in a particular area

2.2 Define magnetic field strength B as magnetic flux density

Magnetic flux () measured in Waber's (Wb) is magnetic flux density (measured in Tesla (T)) multiplied by area (m2). That is:

=BA

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

Size of emf is the rate of change of flux over time:

Anything that effects the rate at which field lines are moving through the conductor effects the size of the emf

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

Lenz's Law states: "An induced current is always in such a direction as to oppose the motion or change causing it"

That is, the direction of an induced current is such that the force produced is in the opposite direction to the direction of movement of the conductor.

This is in accordance with the law of conservation of energy in that if the force produced added to the force supplied, the conductor would continue to speed up without stopping, thus violating thermodynamics.

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

As a motors speed increases, the potential difference across it decreases. This is because the rotation of the coil in the magnetic field induces a current in the coil which opposes the direction of movement of the motor (Lenz's Law). This is known as back emf, and always opposes the supply emf, decreasing the measured potential difference

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

When a changing magnetic flux is generated in a chunk of a conductor (e.g. a sheet of metal), small circular currents are induced which oppose this change. These are known as eddy currents, and are produced in accordance with Lenz's Law

2.7 Explain the production of eddy currents in terms of Lenz's Law

When the magnet is placed in the coil a current is induced. Once the movement of the magnet ceases, so too does the current. When the magnet is removed, a current is induced in the opposite direction to the initial current.

2.8 Perform and investigation to model the generation of an electric current by moving a magnet in a coil or a coil near a magnet

2.9 Plan, choose equipment or resources for, and perform a first-hand investigation to predict and

2. The relative motion between a conductor and magnetic field is used to generate an electrical voltageSunday, 13 March 201110:03 AM


The distance between the coil and magnet is varied-The strength of the magnet is varied-The relative motion between the coil and the magnet is varied-As distance between coil and magnet increases, generated electric current size decreasedAs strength of the magnet is increased, generated electric current increasesAs relative motion between coil and magnet increases, generated electric current increases

2.9 Plan, choose equipment or resources for, and perform a first-hand investigation to predict and verify the effect on a generated electric current when:

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

Eddy currents cause an increase in the temperature of the metal due to the collisions between moving charges and the atoms of the metal, as well as agitation of atoms by the changing magnetic field at high density. Thus when they are produced in the saucepan, the saucepan heats up.

2.11 gather secondary information to identify how eddy currents have been utilised in electromagnetic breaking

A moving metal disk in a magnetic field will have eddy currents induced in it. These will oppose the direction of the rotation, thus slowing the disk down. This is known as electromagnetic breaking.


Rotor: a rotating coil in the magnetic field in which the current will be produced inStator: field magnets to interact with the coil and produce a currentBrushes: to allow the transfer of power from the rotor to the terminalsSlip rings/commutator: depending on wether AC or DC is to be produced, these are the link between the brushes and the coil (rotor) itself.

3.1 Describe the main components of a generator

The structure of a generator and an electric motor can be very similar, as they perform reverse functions. A motor uses current to produce rotation, whilst a generator uses rotation to induce current.

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

An AC generator has slip rings, which are permanent contacts between the coil and brushes. A DC generator needs a commutator to reverse the direction of the current every half cycle. (rectified DC is produced)

3.3 Describe the differences between AC and DC generators

As energy is fed through transmission lines, some is lost via heat energy over the vast distances due to the resistance in the lines. Power = Voltage x Current, and Voltage = Current x Resistance. Therefore, Power = Current2 x Resistance. Therefore, by reducing the current flowing through a wire, the power loss is minimal.

Therefore, the energy that is transferred from generator to consumer is usually done at high voltages.

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

3.5 Asses the effects of the development of AC generators on society and the environment

Advantages Disadvantages

Electric engines more efficient and produce less heat into environment than steam engines, less fuel wast pollutants, cleaner air, cleaner clothes

Cleaner environment means better health in peopleBetter health means greater ability to workMore efficient machinery in factories, cheaper produceGreater demand for products ==> more employmentHydroelectric schemes ==> work for thousands of peopleCheaper electricity ==> communications, refrigeration and food storage

Personal comfort improved (air conditioning, electric blankets etc.)

Leisure activitiesComputer revolutionMore opportunities ==> suburban sprawl

Atmospheric pollution has increased enormously as demand for electrical power increase, pollution from burnt fossil fuels

Heat pollution from coolant water

Nuclear power stations ==> nuclear waste disposal

Hydro schemes ==> environmental costs

Ugly power linesTransformers and substations not visually attractive

Harmful effects from high frequency electromagnetic radiations

Long term elimination of unskilled jobs

3.6 Plan, choose equipment or resources for, and perform a first-hand investigation to demonstrate the production of an alternating current

3. Generators are used to provide large scale power productionSunday, 13 March 201110:04 AM


By moving a coil of wire in a magnetic field, an AC can be produceddemonstrate the production of an alternating current

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

Brushes do not wear as fastNo possibility of an electrical shortAC generator therefore requires less maintenanceMuch easier to draw current through a fixed connection in the stator than through a commutator

Better suited to high current demandsThree phase electricityDistribution over a wide area - easily transformed

Advantages of ACBrushes wear downCommutator bars wear downShorts between bars of commutator

Larger the current, heavier the rotor coils

Electric arcs and radio "noise"Limited usefulness in high current applications

Disadvantages of DC

Output not suitable for use with equipment that needs a steady voltage

Disadvantages of AC

Output can be made smootherBetter for use in equipment that needs a steady voltage

Advantages of DC

Westinghouse - AC - Westinghouse ElectricWestinghouse purchased patents off Nikola TeslaEdison - DC - General Electric CompanyEdison adopted a smear campaign to prove AC was dangerousFirst electric chair (recommended by Edison) used ACDidnt work as planned, Edison lost prestige over itWestinghouse got contract for first electric fair, the Chicago World's FairAC eventually won out because it was more cost-efficient, could harness large natural resources (e.g. Niagara falls) due to its ability to be placed a large distance from the city

Both companies morally and financially drained from long battle, Westinghouse begged Tesla to rip up contract giving him large royalties, Tesla agreed

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

Insulated from supporting structures-Protected from lightning strikes-

3.9 gather and analyse information to identify how transmission lines are:

Insulated from supporting structures:


Insulated from supporting structures:

Continuous cable connected through all the towers that allows the electricity from the lightning to be directed into the Earth and prevent damage to transmission lines or substations

Protected from lightning strikes:


A transformer is a device that steps up or steps down the voltage of a circuit. If a particular voltage/current is needed in a circuit, then a transformer must be used

4.1 describe the purpose of transformers in electrical circuits

All transformers consist of two coils of wire connected by an iron core. An AC is fed into the first coil and thus a changing magnetic field is induced. This carries over to the secondary coil, which then in turn induces a current.

The ratio of the turns in the coils determines the voltage produced on the other end. If there are more turns in the primary coil, it is a step-down transformerIf there are more turns in the secondary coil, it is a step-up transformer

4.2 compare step-up and step-down transformers

4.3 identify the relationship between the ratio of the number of turns in the primary and secondary coils and the ration of primary to secondary voltage

Electromotive force (voltage) is proportional to the amount of turns as well as the amount of magnetic flux going through the coil. By changing the amount of turns, the produced voltage is changing. The power in the primary coil must equal the power in the secondary coil (Law of Conservation of Energy) and as such, if the voltage is increased, the current is decreased to compensate.

Thus in a step up transformer, the voltage is greater in the secondary coil, but the current is less and in a step-down transformer, the voltage is less in the secondary coil, but the current is greater

4.4 explain why voltage transformations are related to conservation of energy

To decrease energy lost via heat in wires over long distances, a large voltage and small current must be used. Electricity sub-stations then take this and transform it into 240V AC for domestic use and 415V AC for industrial use.

4.5 explain the role of transformers in electricity sub-stations

Some electrical appliances need a lower voltage to operate due to the capacity of the circuit. Therefore, some appliances use a transformer to achieve this.

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

Without transformers, electricity stations would have to produce exact voltages, resulting in large currents.

Transformers allow the efficient transfer of energyTherefore, power stations can be further awayTherefore, cities suffer less pollution from power stations

4.7 discuss the impact of the development of transformers on society

When the number of turns on the secondary coil is increased closer to the primary coil, the voltage difference is less (i.e. the voltage is closer to the supply)

4.8 perform an investigation to model the structure of a transformer to demonstrate how secondary voltage is produced

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

4. Transformers allow generated voltage to be either increased or decreased before it is usedSunday, 13 March 201110:04 AM


Laminated iron cores, that is, iron sheets separated by insulating glue rather than one piece of metal reduces eddy currents

Use of other materials that have high resistance and therefore reduce eddy currents

eddy currents in transformers may be overcome

Power station - voltages as high as 25kV. Step up transformers increase this further to about 50kV

This is transferred to a substation which reduces the voltageThen a power line transformer reduces it to 240V or 415V

4.11 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


An armatureA field structureIn some AC motors, slip rings that conduct electricity to and from the motor

Two sets of electromagnets and a capacitor (stator)Squirrel cage rotorThe capacitor means that the poles of one set of electromagnet are set up a fraction of a cycle after the other so the rotor 'chases' the changing field

Induction Motor:

5.1 describe the main features of an AC electric motor

Rotating magnetic field establishedEddy currents in aluminium pie dish established which move the pie dish in a way so that the magnetic flux change is minimised, i.e. the pie dish moves with the magnet

5.2 perform an investigation to demonstrate the principle of an AC induction motor

Heating element transforms electrical energy into heat energyLight energy in incandescent lights and TV screensMicrowave energy in a microwaveKinetic energy in fans and food blendersSound energy in loudspeakers (after conversion to kinetic energy)

Home:

Kinetic energy in electric motors

Chemical energy in car batteriesChemical energy in electroplating

Industry:

5.3 gather, process and analyse information to identify some of the energy transfers and transformations involving the conversion of electrical energy into more useful forms in the home and industry

5. Motors are used in industries and the home usually to convert electrical energy into more useful forms of energySunday, 13 March 201110:04 AM


Force on two parallel conductors:

Motor effect force:

Torque:

Torque on a coil in a DC motor:

Voltage to turns ratio in a transformer:

FormulasSunday, 13 March 201110:20 AM


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HSC PHYSICS NOTES Collins Module 2