Chapter 2 - Trade Science Flashcards
Why does a millwright need to understand trade sciences?
- sometimes a millwright needs to think a problem through using scientific principles, or to calculate a dimension, a load, or some other quantity using a formula
Define atoms and molecules.
Atoms
- are single units of elements
- elements are materials containing only one type of atom
- the atomic structure of an element determines it chemical behaviour (how it interacts with other elements)
Molecules
- atoms of elements unite to form complex molecules
- most matter is composed of molecules containing two or more atoms.
- the arrangement and structure of molecules determine the characteristics of that material
Define compound and mixtures.
Compounds
- the product of two or more atoms that unite chemically
- substance creates has properties different from those of the combining elements
Mixtures
- two or more elements or compounds may combine so that the molecules in the resulting material retain their original properties
- unlike compounds, mixtures can be made with varying proportions of each ingredient
Is brass a mixture or a compound?
- brass is an alloy of copper and zinc
What are the physical states of matter?
- Solids
- liquids
- gases
- plasma
Can all forms of matter be changed from one physical state to another and then be returned to its original state?
- No, many substances cannot change from one physical state to another without becoming permanently unrecognizable
Define cohesive forces and adhesive forces.
Cohesive forces
- molecules
Define mass and weight of an object.
- mass is a measurement of the quantity of material in a body
- all objects that have mass are attracted to each other. The force of attraction is called gravity.
- the force of gravity acting on an object is called weight
Define volume.
- a measurement of the amount of space an object occupies
= L x W x H
= area x H
Define density.
- indicates the mass of a body in a given volume
= mass / volume
What are the reference materials for a specific gravity of:
- solids and liquids?
- water is the reference substance for liquids and solids
What are the reference materials for a specific gravity of:
- gases?
- air is the reference for gases
What are the main characteristics of a solid?
- maintain their own shape without a container
- compared and selected based on their properties
— mechanical
— physical
— chemical
Define the following terms:
- ultimate strength
- the point at which a material ruptured due to applied forces, which may be:
— tensile
— compressive
— shear - when materials are close to their ultimate strength, they must be reinforced
Define the following terms:
- stress and strain
Stress
- the force per unit area acting on an object to change its dimensions
- pascal (Pa)
- pounds-force per square inch (lbs/in2)
Strain
- the ratio of the extension per unit length when a force is applied
- strain = extension / original length
Define the following terms:
- tensile strength
- the ability of a material to resist being pulled apart by external forces
- Ultimate tensile strength (UTS) is the maximum amount of stress the material can withstand before breaking
Define the following terms:
- compressive strength
- the ability to resist external forces that push into or against the material
- Ultimate compressive strength (UCS) is the maximum compressive strength that a material can withstand before its surface area changes
Define the following terms:
- shear strength
- the ability to resist forces that try to slide part of the object along itself
- Ultimate shear strength (USS) is the maximum stress the material can withstand before its cut apart.
- USS of a material is about 40% of its UTS
Define the following terms:
- fatigue strength
- the ability to withstand repeatedly alternating stresses
Define the following terms:
- elasticity
- the ability of a material to return to its original dimensions after it has been acted on by a force
- all solids have some elasticity
Define the following terms:
- elastic limit (or yield strength)
- the maximum force that a solid can withstand without being permanently deformed
Define the following terms:
- plasticity
- the ability for material to retain a shape permanently after deforming force is removed
Define the following terms:
- ductility
- the ability to stretch and maintain a new shape
Define the following terms:
- malleability
- the ability of a material to be permanently deformed by compression forces
Define the following terms:
- brittleness
- the absence of malleability
- brittle materials break without noticeable plastic deformation
Define the following terms:
- toughness
- the ability of a material to withstand shock loads
Define the following terms:
- hardness
- ability to resist a force that is trying to penetrate it
- hardness is associated with durability and abrasion resistance
What are the physical properties of solids?
- electrical conductivity
- thermal conductivity
- expansion and contraction
What are the chemical properties of solids?
- composition
- corrosion resistance
- electrochemical reaction (electrolysis)
What effect does electrolysis have on metals?
- when two different metals are in contact with one another and there is water present, an electric current is produced
- The electron flow has a chemical effect, corroding the metals. It may even disintegrate on of them.
What is the galvanic series with regard to metals?
- list of metals in which each metal is corroded by all those metals listed after it
Describe the properties of liquids.
- cohesion
- adhesion
- volatility
- viscosity
Describe the properties of gases and compare them to the properties of liquids.
- compressibility
- elasticity
- the combination of compressibility and elasticity in confined gases is called resilience
How is the pressure of gases measured?
- Gas pressure may be measured by means of a gauge
- psig
- psia
- atm
- bar
Describe internal energy.
- a materials molecules are in constant, random motion. This molecular movement is an energy form called thermal energy, or, more correctly, internal energy
Define heat.
- heat is the energy in transit from a body at a higher temperature to a body at a lower temperature
Name the instruments used to measure temperature.
- thermometer scales
- liquid expansion thermometers
- bi-metallic strip thermometers
- pyrometers
- thermocouples
Define a British thermal unit (BTU)
- the amount of heat needed to change the temperature of 1 lb of water by 1 degree Fahrenheit
How many British thermal units in a kilojoule?
- 1 kJ = 0.948 Btu
Define:
- thermal expansion
- material expansion proportional to the change in it’s temperature
Define:
- linear expansion
- a change in the dimension of an object in a particular direction: length, width, height, or diameter
Define:
- coefficient of linear expansion
- the different rates of linear expansion of various materials
- used to predict dimension changes as temperature changes
Define:
- volume expansion
- the expansion of volume of a heated material proportional to a change in its temperature
Define:
- coefficient of volume expansion
- for solids, 3X the coefficient of linear expansion
Describe how heat is transferred by conduction, convection and radiation.
- conduction
— heat travelling directly through a material - convection
— the upward movement of a less dense fluid coupled with the downward movement of a fender fluid - radiation
— special waves produced by thermal radiation that are emitted by an object
Define thermal conductivity.
- A materials ability to conduct heat
Define force.
- an external agent that causes a change in motion or stress in a body
State Newton’s first, second and third laws of motion.
1st
- a body at rest will stay at rest and a body in motion will remain in nation at he same speed and in the same direction unless acted on by some unbalanced force
2nd
- the size of a net force (F) on an object is equal to the product of the mass (m) and acceleration (a) of the object. Also the direction of the force is in the direction of the acceleration
F = m X a
3rd
- for every action, there is an equal and opposite reaction
Define:
- velocity
- the distance a body moves in one direction per unit of time
Velocity = distance / time
V = d/t
Define:
- speed
- linear velocity, rate at which it moves in the direction of acceleration
Define:
- acceleration
- proportional increase/decrease of velocity per unit of time
How are rotational motion and periphery speed expressed?
Rotational
- when an object makes one complete rotation around its axis it is called a REVOLUTION
Periphery
- the outside edge travel at a particular velocity is expressed as SURFACE FEET PER MINUTE
List three classifications of energy.
- potential (static)
- kinetic (dynamic)
- radiant (electromagnetic)
Define:
- potential energy
- occurs when an object is in a position to do work but not work is being done.
- the potential energy of an object is the same as the work required to put it in that situation
Define:
- kinetic energy
- occurs when an object moves
- the total kinetic energy of a moving object is equal to the energy used by the object to reach its velocity from being at rest
What does the law of conservation of energy state?
- energy cannot be created or destroyed, but it may be converted from one form to another
Define work.
- the work (W) done on an object equals the force (F) applied to the object times the distance (d) that the object moves in the direction of that force
W = force X distance W = F X d
- work is done whenever one kind of energy is converted to another type of energy
List and describe the units used to measure work.
Metric
- joule (J), kilojoules (kJ), megajoules (MJ)
Imperial
- foot-pounds-force (ft-lbf)
- foot-pounds (ft.lb)
Define power.
- the rate at which work is done
Power = work / time P = W/T
List and describe the units used to measure power.
Metric
- watts (W); 1W = 1 J/s
- kilowatts (kW); 1 kW = 1000 J/s
Imperial
- horsepower (hp); 1 hp = 550 ft.lbf/s
Conversions
1 hp = 746 W = 0.746 kW
1 kW = 1.34 hp = 737 ft.lbf/s
Define:
- mechanical advantage
- the ratio of work done to effort required
Define:
- efficiency
- when work is done with very little loss of energy to other processes
Refer to figure 3.
How much effort (E) will be required to lift the rock if the Rock (R) is 200 kj and the resistance arm (RA) is 1 metre and the effort arm (EA) is 4 metres long?
E X EA = R X RA
E = (R X RA) / EA
What class of levers are the following: - scissors?
- Class 1
- they have a fulcrum between the resistant force and the effort
What class of levers are the following: - wheelbarrow?
- Class 2
- the resistance is between the fulcrum and the effort
What class of levers are the following: - tweezers
- Class 3
- the effort is applied between the resistance and the fulcrum
What class of levers are the following: - driven axle and wheel of vehicle?
Class 2
- when the wheel is used to apply the effort, the axis of the wheel and the axle act as the fulcrum, the radius of the wheel is the effort arm and the radius of the axle is the resistance arm
Class 3
- when the axle is used to apply the effort, the radius of the axle is now the effort arm and the radius of the wheel is the resistance arm
What is the purpose of a single fixed pulley and load line?
- a single pulley is considered to be s continuous lever with equal effort and resistance arms
- the single, fixed pulley does not increase the applied force, but simply allows the force to change direction
What is the mechanical advantage (MA) of a multiple pulley system if the number of rope parts around the lower pulleys supporting the load is 6?
- MA = 6
- you can quickly find the Ma in a multiple pulley system by counting the number of rope stands supporting the load
What is the mechanical advantage in a belt drive when a 2 inch diameter pulley drives a pulley with a 4 inch diameter?
- MA = driven diameter / drive diameter
- MA = 4 / 2
- MA = 2
What is the mechanical advantage in a chain drive when a sprocket with 10 teeth, drives a sprocket with 30 teeth?
- MA = # teeth driven / # teeth drive
- MA = 30 / 10
- MA = 3
What is the RPM of the driven element in a gear drive when the driving gear has 10 teeth and an RPM of 800 and the driven gear has 40 teeth?
- RPM drive X drive diameter = RPM driven X driven diameter
- RPM driven = (RPM drive X drive diameter) / driven diameter
- RPM driven = (800X10) / 40
- RPM driven = 200
What is the mechanical advantage of the following compound gear drive
- A 10 tooth gear driving a 40 tooth gear which is attached to a shaft that drives a 15 tooth gear that drives a 60 tooth gear.
-
What simple machine is the following mechanical device derived from:
- screws?
- inclined plane wrapped around a cylinder
What simple machine is the following mechanical device derived from:
- cams?
- rotary wedges, using a turning motion to produce an oscillating (linear) force
What simple machine is the following mechanical device derived from:
- wedges?
- inclined plane
How is mechanical advantage achieved in hydraulics?
- input pressure = output pressure
- pressure = force / area
What are compound machines?
- two or more simple machines coupled together
- ex. Hydraulic jack; handlever to input force multiples the Mechanical advantage
Why should a millwright have a basic understanding of electricity?
- many machines and tools are driven by electricity
- on occasion, the millwright installs and aligns electrical machines
- a basic understanding of electricity is necessary to ensure safe maintenance of electrical equipment and to communicate with other workers
What are the subatomic particles that make up an atom?
- protons
- neutrons
- electrons
How are the electrons of an atom held in orbit?
- they are held in the orbit by the force of attraction between opposite electric charges
Define electric current.
- the forced movement of free-electrons in the same general direction
What properties are required for conductors and for insulators?
Conductors
- elements that make available a large number of free-electrons allow relatively large currents to flow
Insulators
- elements that make available relatively few free-electrons allow only incredibly small currents to flow
What effects are produced when an electric current flows through a conductor?
- heating effect, the temperature of the conductor rises
Describe magnetism.
- magnetism is caused by the spin of electrons within a molecule
- if the molecules in a metal can be aligned so that their individual magnetic fields all act in the same direction, then the metal becomes magnetic
- ferrous metals can be magnetized. Some are magnetized easily and others with more difficulty
What materials are used to produce temporary and permanent magnets?
Temporary magnets
- iron; the molecules are easily aligned and easily misaligned
Permanent magnets
- steel; the molecules are difficult to align, but once aligned tend to stay that way
State the fundamental law of magnetism.
- like poles repel, and unlike poles attract
Describe electromagnetism.
- whenever an electric current flows through a conductor, a magnetic field is set up around that conductor
- the strength of the field can be increased by forming the conductor into a coil
- the strength can be increased further if an iron core is inserted into the coil
- such an arrangement is used to create an electromagnet, where current can be used to switch the magnetic properties on and off
Describe an electric circuit.
- the path which an electric current takes is termed a circuit
- the simplest circuit consists of a: — power source — conductors — a circuit element (load) — a switching device
What unit of measurement is used to measure the following in an electric circuit:
- current flow?
- measured in amperes (A)
- 1 ampere (A) = 1000 milliamperes (mA)
- 1 kiloampere (kA) = 1000 A
What unit of measurement is used to measure the following in an electric circuit:
- potential?
- measured in volts (V)
- 1 volt (V) = 1000 millivolts (mV)
- 1 kilovolt (kV) = 1000 V
- 1 megavolt (MV) = 1000000 V
What unit of measurement is used to measure the following in an electric circuit:
- resistance
- measured in ohms (Ω)
- 1 ohm (Ω) = 1000 millohms (mΩ)
- 1 kilohm (kΩ) = 1000 Ω
- 1 megaohm (MΩ) = 1000000 Ω
What is a load in electrical jargon?
- the load uses electrical energy to do useful work
- a load may be:
— motor
— lights
— warning horns
— heaters
— etc
Describe switches.
- used to OPEN or CLOSE the circuit
- OPEN when the other for a current is broken
- CLOSE when the circuit has a continuous path for the current to flow
What type of circuits are used in electrical work and which one is most common?
- series
- parallel (most common)
- series-parallel
- complex
What does Ohm’s Law state?
- at a given temperature, the current flowing through a conductor is directly proportional to the voltage across that conductor
How does temperature affect conductors and insulators?
- as the temperature of most metals (conductors) increase, the value of resistivity increases, acting to increase resistance in the circuit
- as the temperature of most common insulators increase, resistivity decreases, acting to reduce resistance. This explains why overheating is a major cause of insulation failure.
Define a watt.
- 1 watt of power results when 1 volt produces a current of 1 ampere
P = V X 1
1 watt = 1 J/s
What type of current cannot be transformed to a higher or lower voltage?
- direct current
Describe the operating principles of a transformer.
- a transformer comprises two coils, or windings, wound around a common silicon-steel core
- one winding (primary winding) is supplied from the utility’s alternating, high-voltage system. The resulting magnetic field links with a second winding (secondary winding). The changing magnetic field induces an alternating voltage into the second winding.
What are step-down and step-up transformers?
- if the secondary winding has fewer turns than the primary winding, the. The voltage is induced into it is lower. The transformer is then termed a step-down transformer.
- if the secondary winding had more turns than the primary winding, then the voltage is induced into it is higher. The transformer is then termed a step-up transformer.
Are homes generally supplied with single-phase or three-phase electrical?
- single-phase
What voltages are supplied to homes?
- 120V and 240V
Where is three-phase electricity used?
- larger buildings and factories
How is three-phase electricity generated?
- generators work by rotating magnetic fields within conductor coils called armature windings. This induces voltage in the coils. The generators at power stations contain three separate armature windings. They are physically displaced from each other by 120 degrees
How are three-phase systems connected to conserve copper wire?
- wye and delta connections
What are the advantages of three-phase motors in comparison to single-phase motors?
- smaller and more simply constructed
Describe the operating principles of a three-phase induction motor.
- the motor’s field windings are connected to a three-phase supply
- the resulting current causes a natural, smoothly rotating, magnetic field to be set up within the stator
- this rotating field, passes through the rotor’s copper bats, inducing voltages into them. This causes currents to circulate within them.
- these current, in turn, set up magnetic fields which react with the main rotating field. This creates a torque that causes the rotor to be turned by the rotating field
How is the direction of rotation changed in a three-phase induction motor?
- the direction of the rotating field can be reversed by interchanging any two of the three line conductors that supply the motor
What is the purpose of fuses?
- circuit protection devices
- they are connected in series with a load. In the event of excessive current flow, the fuse melts. This opens the circuit, and protects the load device and its supply conductors from overheating
Define inverse time characteristic with regard to a fuse.
- the greater the value of a fault current, the faster the fuse operates
How do high-voltage fuses differ from low-voltage fuses?
- physically larger and more complex
- incorporate some means of extinguishing arcs. These vary from sand-filled cylinders, which suffocate the arc, to spring-loaded elements that stretch and snap the arc
Describe circuit breakers.
- switching devices that can interrupt fault circuits
- Oil circuit breakers
— use the motion and cooling effect of oil to extinguish the arc - Air-blast circuit breakers
— use a blast if compressed air to extinguish the arc
What purposes do motor controllers serve?
- used to control and protect electric motors
Describe a contactor as it applies to a motor controller.
- a heavy-duty electromagnetically controlled switch (contactor) that opens and closes the circuit that supplies the motor
- can be made to — start — stop — reverse immediately — reverse after stopping — move in increments (jog)
What functions do temperature sensors perform in a motor controller?
- monitor the load current
- if the motor stalls, the resulting excessively high current is sensed. The motor controller disconnects the motor from the supply
- these sensors also prevent the motor from being restarted until its temperature had returned to normal
