1 Current Carrying Conductors

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

Formula:

F = BILsinθ

Strength of the Magnetic Field: o Force is proportional to the strength of the magnetic fieldo Stronger magnetic field, greater force on conductor

Magnitude of Current in Conductor: o Increasing current means increasing the velocity of the electronso Each moving charged particle experiences a force in proportion to its velocity

Length of Conductor: o The longer the section of conductor in a magnetic field, the more moving electrons simultaneously

experience a forceo Force is proportional to the length within the magnetic fieldo Shorter length, smaller force on conductor

Angle between direction of magnetic field and conductor: o Force is strongest when particle is moving at right angles to the magnetic field (90°)o Force is zero when particle is moving parallel to the magnetic field (0°)o Movement of electrons is along a length of conductor, magnitude of force varies with angle between

conductor and magnetic fieldo As angle increases, force increases

2.1 Describe qualitatively the force between long parallel current carrying conductors:

Force between parallel conductors exists because magnetic fields due to current flowing through the conductors interact with each other.

Direction of Force (Attraction or Repulsion): Depends on relative directions of the two currents Currents flowing in the same direction, attractive force, towards each other Currents flowing in opposite direction, repulsive force, away from each other

Magnitude of Force: Depends on magnitude of current within wire Increases or decreases with the product of the two currents Also depends on distance of separation between the conductors Increasing as the conductors are moved closer together

Relation to Length: Force between conductors depends on length of parallel conductors Larger for longer conductors "Force per unit length" - varies only with magnitude of the two currents and the distance between them.

Formula:

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F = Force (N) B = Magnetic field strength (T) I = Current (A) L = Length of the conductor (m) θ = Angle of the conductor to the magnetic field

F I1I2

l d

F = Force (N) l = Length of parallel conductors (m) I1 & I2 = Currents in the conductors (A) d = Distance between the conductors (m)k = constant (2.0x10-7)

k

Define torque as the turning moment of a force:

Turning force or turning moment of a force Increased by increasing the applied force or perpendicular distance Formula:

τ = Fd

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

Forces on the sides ab and cd: Experience maximum force since the current in them is perpendicular to the magnetic field Magnitude of the force does not change throughout its rotation Using the right hand palm rule, the direction of the force on sides ab and cd can be deduced The net result of these two forces is to produce a torque on the loop about the axis; in the diagram above, the

torque is acting in an anticlockwise direction

Forces on the sides bc and ad: The sides of the loop, ad and bc, experience no force because the current is parallel to the magnetic field

Magnitude of the force varies from zero to maximumo Zero when the plane of the coil is parallel to the magnetic field (i.e. as above)o Maximum when the plane of the coil is perpendicular to the magnetic field

Net torque: Maximum when the plane of the coil is parallel to the magnetic field (i.e. as above) Direction alternates through a complete rotation Current-carrying loop orientated in a plane at right angles to a magnetic field will experience no net force

Formula:

τ = nBIAcosθ

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τ = Torque (Nm) F = Applied force perpendicular to axis of rotation (N) d = Perpendicular distance between ‘line of action’ and pivot (m)

τ = Torque (Nm) n = number of turns/loops of the coilB = Magnetic field strength (T) I = Current flowing through the loop (A) A = Area of the loop (m2) θ = Angle between the plane of the loop and the field

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

Force on a Current-Carrying Conductor – The Motor Effect: Motor effect - The force experienced when a current carrying conductor moves through a magnetic field. This

force is produced because of the interaction between the magnet’s magnetic field and the current’s electromagnetic field.

4.1 Describe the application of the motor effect in the galvanometer and the loudspeaker:

The Galvanometer: Device used to measure magnitude and direction of small DC

currents

The Motor Effect: When current flows through the coil, the coil experiences a

force due to the presence of the external magnetic field The iron core of coil increases the magnitude of this force Needle rotated until magnetic force on the coil is equalled by

a counter-balancing ‘restraining spring’. (torque) Scale of galvanometer is linear, amount of deflection

proportional to current flowing through coil.

The Loudspeaker: Device that transforms electrical energy into sound energy.

They consist of a circular magnet that has one pole on the outside and one on the inside.

The Motor Effect: A current-carrying coil (voice coil) interacting with a permanent magnet

experiences a force as a result of the motor effect when a current is present. The voice coil is connected to the amplifier.

This force causes the coil to vibrate rapidly back-and-forth, in turn making the speaker cone vibrate and send sound waves into the air.

When the magnitude of the current increases, so too does the force on the coil

When the force on the coil increases, it moves more and the produced sound is louder.

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5.1 Describe the main features of a DC electric motor and the role of each feature & identify that the required magnetic fields can be produced either by current-carrying coils or permanent magnets:

An electric motor is a device which converts electrical energy to useful mechanical energy (usually rotation)Part Description Role of partPair of magnets Two permanent magnets on

opposite sides of the motor, with opposite poles facing each other.

The pole faces are curved to fit around the armature.

The magnets supply the magnetic field which interacts with the current in the armature to produce the motor effect. The shape of the pole faces makes the magnetic field almost uniformly radial where the coil passes.

Pair of electromagnetic coils

Each stator coil (or “field” coil) is wound on a soft iron core attached to the casing of the motor.

The coils are shaped to fit around the armature.

Each opposed pair of stator coils produces a magnetic field similar to that provided by a pair of permanent magnets. The iron core concentrates the field.

Armature The armature consists of a cylinder of laminated iron mounted on an axle.

Often there are longitudinal grooves into which the coils are wound.

The armature carries the rotor coils. The iron core greatly concentrates the external magnetic field, increasing the torque on the armature. The laminations reduce eddy currents which might otherwise overheat the armature.

Rotor coils These are several turns of insulated wire, wound onto the armature.

The ends of the coils are connected to bars on the commutator.

provide torque, as the current passing through the coils interacts with the magnetic field.

Split ring commentator The commutator is a 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).

Each opposite pair of bars is connected to one coil.

The commutator provides points of contact between the rotor coils and the external electric circuit.

It serves to reverse the direction of current flow in each coil every half-revolution of the motor. This ensures that the torque on each coil is always in the same direction.

Brushes Compressed carbon blocks, connected to the external circuit, the brushes are the fixed position electrical contacts between the external circuit and the rotor coils.

They are mounted on opposite sides of the commutator and spring-loaded to make close contact with the commutator bars.

Their position brings them into contact with both ends of each coil simultaneously, as each coil is positioned at right angles to the field, to maximise torque.

This maximises torque.

Axle A cylindrical bar of hardened steel passing through the centre of the armature and the commutator.

provides a centre of rotation for the moving parts of the motor.

Useful work can be extracted from the motor via a pulley or cog mounted on the axle.

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where ɛ = Potential difference (V)n = Number of turns in the coilΦ = Magnetic flux (Wb) t = Time (seconds) = Rate of change in magnetic flux

ΔΦ Δt

ΔΦ Δt

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

After discovering that an electric current produces a magnetic field, in 1820, Faraday’s ideas about conservation of energy led him to believe that since an electric current could cause a magnetic field, a moving magnetic field should be able to produce an electric current.

In 1831, Faraday attached two wires through a sliding contact to touch a rotating copper disk located between the poles of a horseshoe magnet. This induced a direct current and was the basis to an electric generator.

Faradays explanation was that an electric current can be induced by moving a conductor within the 2 poles of a magnet as it cuts a number of lines of magnetic force coming from the magnet (the magnetic field). The wires allowed the current to flow in an external circuit where it could be detected.

7.1 Define magnetic field strength (B) as magnetic flux density:

Representing Magnetic Fields: Magnetic flux lines 'flowing' out of the north pole and into the south pole Lines closer together near the poles where magnetic field is strongest Lines further apart at greater distances from the magnet Magnetic field of stronger magnet, larger number of magnetic flux lines Magnetic field of weaker magnet, smaller number of magnetic flux lines

Magnetic Flux Density: Measure of the number of magnetic flux lines passing through a unit area (1m2) Magnetic field strength at a point is the same as the magnetic flux density at that point

7.2 Describe the concept of magnetic flux in terms of magnetic flux density and area:

Magnetic Flux: Magnetic flux - Amount of magnetic field lines passing through a given area Represented diagrammatically as number of flux lines passing through the area The relationship of the magnetic flux is given by (Note: This formula is not required):

Φ = BA

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

The size of an induced EMF is directly proportional to the rate of change in magnetic flux In order to induce an EMF, a changing magnetic flux is essential The change in flux can be changed in an conductor by:

o moving the conductor or the magnetic fieldo Changing the strength of the magnetic field.o The speed of the relative motion between the magnetic field and the conductoro The number of turns of coil or conductorso The change in area that the magnetic field passes through

Formula:

ɛ = n

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where Φ = the total magnetic flux (Wb)B = magnetic field strength (T)

A = perpendicular area through which the flux passes (m2)

8.1 Account for Lenz's Law in terms of conservation of energy and relate it to the production of back EMF in motors & explain that in electric motors, back EMF opposes the supply EMF:

Lenz’s Law: “The direction of any induced EMF will always be such that it opposes the change that caused it” To find the direction of an induced emf (or induced current) we apply the RHPR to the given situation and then

reverse the direction of the current flow

Conservation of Energy: The law of conservation of energy states: Energy

cannot be created or destroyed, it can only be transformed or transferred

Consider a magnet moving into a Cylinder: By Lenz's law, work must be done to move magnet into

coil providing energy to induce EMF If Lenz's Law did not hold true, the magnet would be

accelerated into the coil – i.e. creating mechanical energy with no input energy

Thus, to obey the law of conservation of energy, the induced current must flow to oppose the cause.

Back EMF: When an electric motor is first switched on, the applied

voltage produces a large current in the coils. When the coils begin to rotate, changing flux within coils induces an emf; by Lenz's law, the induced emf is opposite to the emf applied to the motor and this is known as the back emf.

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

Eddy currents – the current produced by the back emf opposing current in the coil from a external source. By Lenz's Law, eddy currents oppose the changing magnetic field producing them. Eddy currents produce its own magnetic field which opposes the relative motion of the magnetic field which

created it.

9.1 Identify how eddy currents have been utilised in electromagnetic braking:

Eddy currents have been utilised in the electromagnetic braking of free fall recreational rides. A copper plate is attached to the bottom of a ride. Near the bottom of the ride, there are permanent magnets.

When the copper plate passes the magnets, eddy currents are induced due to magnetic poles in the copper plate. The eddy currents oppose the direction of the magnet causing the ride to slow down. It slows down slowly as the eddy currents are proportional to the speed of the plate.

Minimising eddy currents

Eddy currents can be minimised by using a lamented soft iron core. This is made up of thin slices of the iron separated by an insulating layer of oxide coating or paper. This disrupts the eddy currents and stops them building up.

9.2 Explain how induction is used in cook tops in electric ranges:

On each cooking area on the cooktops, there are copper coils wrapped around magnetic materials. An alternating current is produced and run through the coils producing a magnetic field.

This magnetic field induces an eddy current in the metal pan above. The resistance in the pan to the current causes heat to be produced at the base of the pan cooking the food. When more current is run through the coils, there are more eddy currents, making the pan hotter.

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An electric generator is one that converts mechanical energy to electrical energy using the principle of electromagnetic induction.

10.1 Describe the main components of a generator:

Part DescriptionRotor Usually consists of several coils wound on an armature which is made to rotate within a

magnetic field.Armature: Cylinder of laminated iron mounted on an axle which is carried in bearings mounted in

the external structure. Torque applied to axle to make the rotor spin.Coil Each coil consists of many turns of copper wire wound on the armature. The two ends of

each coil are connected either to two slip rings (AC Generator) or two opposite bars of a split-ring commutator (DC Generator).

Brushes The brushes are carbon blocks that maintain contact with the ends of the coils via the slip rings (AC) or the split-ring commutator (DC), and conduct electric current from the coils to the external circuit.

Stator Fixed part of the generator which supplies the magnetic field in which the coils rotate.

Magnetic field: The magnetic field can be provided by permanent magnets or electromagnets which are mounted and shaped in such a way that opposite poles face each other and wrap around the rotor.

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

Structure:Similarities: Both have a stator providing the magnetic field, both have a rotor which rotates in this field In both, the magnetic field is supplied by either permanent magnets or electromagnets In both, rotor consists of coils wound on armature connected to brushes. In both, their rotor coils are connected to the external circuit through a split ring commutator.

Differences: DC Generators and electric motors – use a split-ring commutator to connect external circuit AC Generators – use slip rings to connect external circuit

Function: The function of an electric motor is the reverse function of a generator

Electric motors: Converts electrical energy into mechanical energy Rotates when current is supplied

Generators: Converts mechanical energy into electrical energy Supplies current when rotor rotates

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11.1 Describe the differences between AC and DC generators & discuss advantages / disadvantages of AC and DC generators related to their use:

AC generator DC generatorDescription Brushes run on slip rings, constant

connection between coil and external circuit.

Induced EMF changes polarity with every half-turn of the coil

Voltage in the external circuit varies like a sine wave

Current alternates direction

Brushes run on split-ring commutator, which work by reversing the connection between the coil and the external circuit each half-turn

Induced emf does not change polarity Voltage in external circuit fluctuates

between zero and maximum Current flows in one constant direction

Advantages Brushes in AC generator last longer, increasing efficiency.

Less maintenance and more reliable, Uses slip rings which cost less to

manufacture and requires less maintenance

AC voltage can be easily increased/decreased using transformers

Can be used for power distribution. They can be easily designed to

produce 3 phase electricity, meaning it can be generated over a wide area.

DC output can be used for devices which rely solely on DC current to function

DC current is generally more powerful than AC (for a given voltage)

Its output can be made smoother by arranging many coils in a regular pattern around the armature. This means that the more coils, the more smoother it is.

Disadvantages Cannot be used to power some devices which rely solely on DC current to function

AC output in different regions around the country must be synchronised for correct integration of electricity – i.e. have the same frequency and are in-phase

AC output is much more dangerous than the equivalent DC output

Brushes in DC generator do not last as long because they wear quicker

Chance of creating electrical short circuit between segments due to pieces of metal worn from commutator bars

Cannot supply power over long distance.

The larger the current, the heavier the rotor coils causing high demands on structures.

Voltage output

Diagram

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Analyse the competition between Westinghouse and Edison to supply electricity to cities:

Westinghouse was the overall winner, as the AC system was more efficient.Thomas Edison:

Direct Current System DC Generators use commutators, which were a problem – i.e. maintenance, cost, performs poorly at high

speed rotations Could only supply power to areas a few kilometres away. Relied on thick copper cables to carry electric current

George Westinghouse: Alternating Current System Westinghouse saw the advantages of AC, and so he purchased the rights to Tesla's AC motors and generators AC transmissions through the action of transformers were much more energy efficient. Electricity could be transmitted over longer distances with only a small energy loss. The motors needed no brushes or commutator.

Identify how transmission lines are insulated from supporting structures and protected from lightning strikes:

Insulation from Supporting Structures: Insulation chains- Large insulators that consist of stacks of disks made from porcelain are used to separate

transmission lines from metal support towers. They prevent sparks jumping across the gap between the wires and towers. The insulators (commonly porcelain) are strong and retains its high insulating properties even under a very high voltage.

Protection from Lightning Strikes: Shield conductors- A non-current carrying wire runs over and parallel to the transmission wires. If lightning

strikes it will hit the overhead wire first and the wire will conduct the huge current of the lightning into the earth, leaving the transmission wires untouched. The transmission lines do not suffer a sudden surge of voltage.

Distance- The distance between towers is at least 150m to protect each tower from other towers if it is hit. Earth cable – This runs from the top of the pole down into the earth.

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

Energy loss due to resistance: As current flows through the transmission lines that has a resistance, heat will be dissipated The heat lost during transmission can be quantitatively described by using the formula: Formula:

P = I2R

Minimisation: Transmission at highest possible voltage, lowest possible current Careful choice of materials – i.e. using good conductors (e.g. copper), thicker wires = less resistance

Energy loss due to induction of eddy currents: Induction of eddy currents in iron core of transformers Circulation of eddy currents generates heat representing energy loss to the system

Minimisation: Transformer core made of laminated iron - thin layers of iron, separated by thin insulating layers Limiting eddy currents and reducing corresponding heat loss by utilising cooling fins on the outside of the

transformer and cooling oil circulating on the inside

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where P = heat lost during transmission (J)I = the current flow through the wire (A)R = the total resistance of the wire (Ω)

(This equation can be derived by combining the power equation P = IV and Ohm’s law V = IR)

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

Effects on society: Effects on Environment:

Positive Effects:

Development of a wide range of machines, processes and appliances – improving the standard of living

Many tasks once performed by hand now can be accomplished with electrical appliances

Most domestic and industrial work requires less labour

Influencing technology development - tasks such as electronic communication now achieved

NONE

Negative Effects:

Reduction in demand for unskilled labour, thus increasing long-term unemployment

Disruption to supply compromises safety, causes widespread inconvenience and loss of production

Injuries and deaths from electric shocks with the widespread use of AC power

A major electricity failure could cause economic crisis

Transmission lines criss-cross the country, strip through environmentally sensitive areas

Remote wilderness areas tapped for energy resources such as hydroelectricity

Air pollution from burning fossil fuels, cause of acid rain

Global increase of atmospheric C02, long-term global climate change

Radioactive waste from nuclear power stations

Discuss the impact of the development of transformers on society:

More efficient transmission of electricity – power loss during transmission is dramatically reduced Allows the development of devices which run at different voltages Access to high-voltage electricity in remote areas, stepped-down by transformers in order for use in devices Raised living standards in rural communities (e.g. electrical lighting, refrigeration, air-con.) Industry no longer clustered around power stations and can be developed away from residential areas Power stations in remote locations, relocated pollution away from homes

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Discuss the need for transformers in the transfer of electrical energy from a power station to its point of use & explain the role of transformers in electricity sub-stations:

Without transformers electricity would be generated at voltage typically used, resulting in very large energy losses and costly transmission losses.

In large cities, many power stations would be required every few kilometres and each different voltages require separate power stations and distribution systems

Transformers: More efficient to use very high voltages for long distance transmission Transformers step-up voltage for transmission, progressively step-down voltage along transmission lines until it

reaches consumer.

The voltage change during the transmission from the power plant to consumers:

Describe the purpose of transformers in electric circuits:

Transformers are devices that increase or decrease the size of the AC voltage as it passes through them via electromagnetic induction

Step-down transformers are used for appliances containing components requiring lower voltages – e.g. clock radios, hair dryers, CD players, etc.

Step-up transformers are used for appliances which require higher voltages to function – e.g. televisions, air conditioners, etc.

Many appliances contain both step-up and step-down transformers supplying different voltages for different components

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Electricity is usually generated by a three-phase AC generator; generally the voltage generated is as big as 23000V and current output from each set of the coil is almost 10000A

For long distance transmissions, the electricity is then fed into a step-up transformer that increases the voltage to 330000V and correspondingly decreases the size of the current (P=VI)

After this electricity has been transmitted over a long distance, the voltage is stepped down at different regional sub-stations, mainly for safety reasons. Correspondingly, the current increases.

Eventually, the voltage is stepped-down to 240V at the local telegraph pole transformers for domestic uses; industries may use slightly higher voltages

Compare step-up and step-down transformers:

Step-up transformer Step-down transformer

Consists of two inductively coupled coils wound on a laminated iron core

Consists of two inductively coupled coils wound on a laminated iron core

More turns in the secondary coil than the primary coil

Fewer turns in the secondary coil than the primary coil

Higher output voltage than input voltage Lower output voltage than input voltage

Lower output current than input current Higher output current than input current

Used at power stations to increase voltage and reduce current for long-distance transmission

Used at substations and in towns to reduce transmission line voltage for domestic and industrial use

Used in cathode ray television sets to increase voltage to operate the picture tube

Used in computers, radios, and CD players to reduce household electricity to very low voltages for electronic components

Identify the relationship between the ratio of the number of coils in the primary and secondary coils and the ratio of primary to secondary voltage:

Ratio of primary to secondary voltage = ratio of number of turns in the coils Step-up transformers - more turns and higher voltage in secondary coil Step-down transformers - less turns and lower voltage in secondary coilFormula:

Explain why voltage transformations are related to conservation of energy:

Conservation of Energy: Amount of electrical energy entering must equal total energy in all forms leaving Power in = power out Pp = IpVp = IsVs = Ps

where subscript ‘p’ indicates primary coil and subscript ‘s’ indicates secondary coil No power loss – if voltage increases, current correspondingly decreases, and vice versa

Real transformers: Heat due to eddy currents acting in the resistance of iron core Energy is lost from the system in the form of heat – escaping into the air Power output cannot exceed power input, power output is less than power input;

Pinitial > Pfinal due to loss of heat energy

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

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where Vp = voltage input into primary coil (V)Vs = voltage output from secondary coil (V)np = number of turns of the primary coilns = number of turns of the secondary coil

transformer:

Many household appliances function at voltages other than the standard domestic voltage of 240V

Appliances that run on 240V AC: Electricity supplied to homes typically 240 V AC Many domestic appliances designed to run at this voltage Connected directly to the mains supply without a transformer

Running on Lower Voltages: Some appliances contain components operating at lower voltages than supplied For these appliances, a step-down transformer can be used to decrease the voltage to required – e.g. phone

chargers use a transformer to step-down the voltage from 240V to the required voltage (commonly <10V)

Running on Higher Voltages: Appliances such as television receivers and computer monitors contain cathode ray tubes requiring voltages

above supply voltage These appliances have a built-in step-up transformer to provide the necessary voltage

Discuss how difficulties of heating caused by eddy currents in transformers may be overcome:

Technique/ part How it helpsLaminated iron core Stacks of thin iron sheets, each coated with insulation materials.

Lamination effectively increases the resistance of the core to the flow of eddy currents, therefore restricting the circulation of large eddy currents – thus, less heat dissipation

Painting the casing a dark colour

to absorb the heat produce by the transformer more quickly in order to dissipate it to the surroundings

Internal fan to assist air circulation to remove excess heat faster

filling the transformer with a non-conducting oil

which circulates inside the case; transports heat produced in core to outside where heat can be dissipated to environment

Heat-sink fins added to metal transformer case, heat dissipation can occur more quickly over larger surface area

located in well-ventilated areas

E.g. up in the air to maximise air flow around them for cooling

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Describe the main features of an AC electric motor:

Standard AC Electric Motors:

Same features as DC electric motor, except slip rings used instead of split-ring commutator Slip rings – conducts electricity from the power source without interfering with the rotation of the coil The motor spins at 50 revolutions per second, as it is the same frequency as the oscillation of AC current (50Hz)

AC Induction Motors: Stator:

The stationary component of the motor, it contains the electromagnet coils which create the magnetic field and it surrounds the rotor.

o Electromagnet coils: When current flows through the coils, it produces a magnetic field. There are 3 pairs of coils in the stator, which when turned on one after the other, creates a rotating magnetic field.

Rotor: The rotating component of the motor. Induced eddy currents flow in the rotor in such a way that it will rotate in the same direction as the rotating magnetic field created by the stator.

o Squirrel cage: The squirrel cage is made up of parallel aluminium bars that have their ends embedded in a metal ring at each terminal. It is covered by laminated soft iron and embedded in the stator.

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

In the home:o Ovens and kettles create heat energy from electrical energyo Stereo systems create sound energy from electrical energyo Light globes and TVs create light energy from electrical energyo Washing machines create kinetic energy from electrical energy

In the industry:o Mainly turning electrical energy into kinetic energy for drills and other machineryo Light energy in large industrial lights from electrical energy

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