Topic 2: Motors and Generators

9.3.1.2.1 Discuss the effect on the magnitude of the force on a current carrying conductor of variations in: the strength fo the magnetic field, the magnitude of the current, the length of the conductor in the field, the angle between the direction of the external magnetic field and the direction of the length of the conductor.

So:

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

Two wires carrying currents in the same direction attract

Two wires carrying currents in different directions repel 

F is the force between the conductors

I1 and I2 are the currents in the two conductors, respectively

d is the distance of separation between the conductors, and

l is the length of the parallel conductors

F/l is force per unit length

9.3.1.2.3 Define torque as the turning movement of a force

Torque is a vector which is the turning movement of a force. Can be clockwise or anticlockwise

 where f is perpendicular force to the axis of rotation and d is distance from axis of rotation

Measured in Newton Meters

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

The motor effect is due to the force acting on a current carrying conductor in a magnetic field.

Right hand rule shows us why this works:

Fingers → magnetic field

Thumb → Current

Palm → force

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

As the current moves from the positive terminal to the negative as shown in the diagram above, the conductor will feel a force, the direction of the force can be determined by use of the right hand rule which as shown in the diagram will force the conductor to move up on the left hand side and down on the right hand side. These forces result in a torque or a turning moment.

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

Commutator: Ensures the current is reversed every 180 degrees to keep torque in the same direction.

Brush: Ensure contact between the coils and circuit.

9.3.1.2.7 Identify that the required magnetic field in DC motors can be produced by either current carrying coils or permanent magnets.

Magnets: Provide the external magnetic field which creates torque via the motor effect.

Can be provided either by permanent magnets or electromagnets (current carrying coils)

9.3.1.3.1 Calculations using Ampere’s Law

9.3.1.3.2 Perform a first hand investigation to demonstrate the motor effect

9.3.1.3.3 Solve problems on the force on current carrying conductors in a magnetic field

Must be perpendicular to the field.

9.3.1.3.4 Solve problems and analyse information about simple motor and their torque.

Torque at the top is 0

9.3.1.3.5 Qualitatively describe the application of the motor effect in the galvanometer and loudspeaker

Galvanometer

Loudspeaker

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

Michael Faraday discovered that if a wire was moved relative to a magnetic field, an electric current is induced in the wire.  

9.3.2.2.2 Define magnetic field strength B as magnetic flux density

Magnetic field strength B is equivalent to magnetic flux density

Magnetic flux density over area

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

Magnetic flux is the name given to the amount of magnetic field passing through a given area.

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


EMF = - (lenz’s law - emf opposes change in flux) * rate of change of magnetic flux (Teslas/second)

Faraday’s Law: Whenever the magnetic field in the region of a conductor changes, an emf (potential difference) is induced across the conductor. If the circuit is completed, a current will flow through the circuit.

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

Assuming no Lenz’s law, a changing flux in a coil would create another flux in the same direction leading to a greater change in flux and so forth - inducing greater and greater currents. Energy would be created without doing any work.

As energy cannot be created nor destroyed (via the Principle of Conservation of Energy), the magnetic field created must oppose its motion.

When the coil of a motor rotates, a back emf is induced in the coil due to its motion in the external

magnetic field.

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

Back EMF: An electromagnetic force that opposes the main current flow in a circuit (due to Lenz’s law). When the coil of a motor rotates a back emf is induced in the coil due to its motion in the external magnetic field. The back emf increases as the applied emf does until it is equal to the supplied emf (0 net torque)


Smaller the back emf is, the greater the current flowing through the coil. So if motion of the coil is resisted (by a load) back emf will be less.

Sample Explanation:

9.3.2.2.7 Explain the production of eddy currents in terms of Lenz’s Law
Eddy Currents:
Induced currents set up in a conducting plate of metal which is in a changing magnetic field. Direction of the eddy current is such as to create a field which opposes the change in the magnetic field in the plate (Lenz’s law)

9.3.2.3.1 Perform an investigation to model the generation of an electric current by moving a magnet in a coil
AND

9.3.2.3.2 Perform a first hand investigation to predict and verify the effect on a generated electric current when

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

F - A power supply with a high frequency alternating current is connected to a conducting coil below the cooktop, creating a constantly changing flux above the cooktop.

E - This creates an emf above the cooktop and eddy currents are induced in the metal cookware.

C - Eddy currents are induced in the metal cookware. The moving currents in the metal pan create heat due to resistance. (note: only the pan is heated by the currents, not the cooktop which is made of glass or ceramic)

Temperature is controlled by increasing current to create a faster changing flux. This results in more induced current and so more heat

Advantages of Induction Cooktops:

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

Screen Shot 2016-05-03 at 10.02.33 PM.png

The brake consists of a rotating metal disk placed between the two poles of an electromagnet. When the electromagnet is switched on, braking occurs:


F - the rotation of the metal disk through the external magnetic field creates a change in flux in the disk. Some of the metal is moving out of and some is moving into the magnetic field at all times while it rotates.

E - this induces an emf by Faraday’s law.

C - this induces eddy currents in the conducting metal disk (find direction from opposing magnetic field).

M - this current induces its own magnetic field.

O - the induced magnetic field opposes the initial change in flux by Lenz’s Law. When the disk rotates and a new section of metal enters the magnetic field, it experiences a change in flux (e.g. into the page). The eddy currents produce a magnetic field out of the page to oppose the external magnetic field/change in flux. These eddy currents experience a force within the external magnetic field given by the right hand rule (by the motor effect), and the direction of this force opposes the rotation, creating the braking effect. Since the induced magnetic field opposes the change in flux, it opposes the rotation of the disk which is creating the initial change in flux.

Advantages:

Disadvantages:

9.3.3.2.1 Describe the main components of a generator

AC Generator                                                DC Generator

 

Essentially the armature is made to move by an external force.

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

Essentially a motor in reverse. Uses electromagnetic induction in which a changing magnetic flux through a loop of wire produces an induced emf (Faraday’s law) and thus an induced current if the circuit is complete.

9.3.3.2.3 Describe the differences between AC and DC generators

AC: Slip ring commutators: Does not reverse current of output so output is AC current

DC: Split ring commutators: Reverses direction of output current every half rotation to provide current in only one direction

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

PLoss = I2R
Energy loss can come from:

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

Pros

Cons

Steam engines were replaced by electric engines

  • More efficient
  • Less pollution (air, fuel and heat)

Atmospheric pollution has increased as the demand for electricity has increased.

Cleaner environment meant better general health for people

  • Which leads to greater ability to work
  • Increased productivity

Heat production has affected water life adversely

Hydroelectricity schemes provided work for thousands and took pressure off welfare programs

Hydro schemes has affected natural water flows and required artificial damming

Cheaper electricity

  • Easy transmission meant power stations didn’t need to be in cities, freeing up space
  • Also lead to communications and refrigeration reform.

Possible harmful effects from high frequency electromagnetic radiations associated with AC transmission

Personal comfort improved with air conditioning and electric blankets

Long term effect of elimination of many unskilled jobs, impacting welfare programs

Computer revolution has impacted on everything

Nuclear power plants have negative effect of nuclear waste and accident possibilities

Transmission lines aren’t aesthetically pleasing

9.3.3.3.1 Perform a first hand investigation to demonstrate the production of an alternating current


9.3.3.3.2 Discuss advantages and disadvantages of AC and DC generators and relate these to their use

AC Generators:

Advantages

Disadvantages

Brushes have low wear and tear

  • More reliable
  • No possibility of electrical shorting

Require transformers at plant to step voltage up for long-distance transmission and again to step voltage down at destination

Can produce power efficiently at low voltages

  • Voltage can be stepped up or down to suit consumption

AC power transmission has increased danger of electric shock

  • Power lines must be raised or buried

AC power produced by these generators can be stepped up to high voltage for transmission over long distances with low power loss

A smoothing circuit is required to for equipment requiring steady voltage

Require less and thinner copper wires for transmission - do not need a ‘return’ wire

AC must be rectified to DC for use with many electronic appliances

Current is induced in the stator, simplifying design and increasing efficiency

DC Generators:

Advantages

Disadvantages

Produces smoother current by using many coils and many splits in the commutator

Voltage cannot be changed - power must be produced at desired voltage

Many household appliances use DC electricity

Current is produced in rotor, requiring a commutator and more complicated parts subject to greater wear and tear

Large power losses in long-distance transmission. Power Stations must be built near cities.

Higher wear and tear for brushes from contact and sparking with commutator


9.3.3.3.3 Discuss the competition between Westinghouse and Edison to supply electricity to cities

9.3.3.3.4 Gather and analyse information to identify how transmission lines are


9.3.4.2.1 Describe the purpose of transformers in electrical circuits

Transformers are used in electrical domestic circuits and appliances to change the AC voltage so that appliances can function or perform their task.

9.3.4.2.2 Compare step up and step down transformers

Step ups have more coils and thus higher voltage on the secondary coil.

9.3.4.2.3 Identify the relationship between the ratio of turns in the primary and secondary coils and the ratio of primary and secondary voltage

and

9.3.4.3.2 Solve problems and analyse information about transformers using the transformer equation

More coils = greater voltage and less current

9.3.4.2.4 Explain why voltage transformations are related to conservation of energy

9.3.4.2.5 Explain the role of transformers in electricity substations

AND

9.3.4.3.4 Discuss the need for transformers in the transfer of electrical energy from a power station to its point of use

Transformers are useful to reduce energy loss in transmission of power because heat loss is proportional to the current squared; if voltage is increased for transmission, then current is reduced, reducing the heat loss and increasing the efficiency of transmission. Generally, power is generated around 23 kV, transmitted at 330 kV, down to 66 kV at a city substation, 11 kV at a suburban substation, and 240 V for the home (from a power pole transformer). In addition substations in residential areas step down the voltage to make it suitable for home consumption .

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

The mains power in a home is a 240V alternating current which is far than sufficient to power

numerous household appliances at once. Most electrical appliances only need 12-24V, for this to occur a transformer needs to be used to step down the voltage of from the mains box – otherwise the appliance would short out with too high (maximum) current flowing through the electrical circuit.

Some extremely large appliances which drain large levels of power such as large TVs and other high demand appliances require more than 240V. For this reason, such appliances contain a transformer to step up the voltage, however, lowing the current in the circuit. If these appliances were not able to get the required voltage above 240V, they simply wouldn't have the power to turn on or function.

9.3.4.2.7 Discuss the impact of the development of transformers on society

9.3.4.3.1 Perform an investigation to model the structure of a transformer to demonstrate how secondary voltage is produced

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

Eddy currents reduce the efficiency of transformers since power loss= I²R (currents)

Any current induced in the iron core of the transformer is unwanted energy loss.

Ways to improve efficiency/ reduce eddy currents:

9.3.5.2.1 Describe the main features of an AC electric motor

Universal Motor:

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

A sheet of aluminium foil was placed on top of a pool of water. This allows the aluminium foil to move and spin. When a magnet above the foil is spun the aluminium sheet also spins in the same direction. This is due to Lenz’s Law, where the aluminium foil will induce eddy currents to create its own magnetic field which opposes the original changing magnetic field. The interaction of these two magnetic fields causes the aluminium foil to spin.

9.3.5.3.2 Identify some of the energy transfers and transformations involving the conversion of electrical energy into more useful forms in the home and industry

Energy Conversions (in the home):

Radios: Electrical energy →  Sound energy

Lights: Electrical energy  →  Light energy

Hand-Drill: Electrical energy  →  Kinetic energy

Electric Heater: Electrical energy →  Heat energy

Energy Transfers:

Kettle: Electrical energy  →  Heat energy of kettle  → Heat energy transfer to container and water

Induction Cooktop: Electrical energy  →  Heat energy of pan → Heat energy transfer to food to cook

Oven: Electrical energy  →  Heat energy →  Heat energy transfer to food

Industry:

Electrical energy →  kinetic energy which then drives the machinery used in the production of goods.

Electrical Energy →  EM radiation for X-rays in imaging the interior of motors, foundations. Etc

Electrical energy →  chemical energy in electrolysis